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100 Articles

My first astrophoto was a shot of the constellation of Orion with a DSLR camera on a tripod. It was a little out of focus but still gives me a great sense of pride. The thrill of capturing that shot is something I’ll never forget. But as proud as I was of my first astro image, I soon began to wonder how it could be improved. What could I do to increase the sharpness of the stars, how could I get my photos to show the orange-red colour of Betelgeuse and capture more detail in Orion’s Sword? Trying to answer those questions led to a long period of pestering fellow astrophotographers for advice, reading up on the subject in books and magazines, and poring over catalogues to find more capable…

ASTROPHOTOGRAPHY

Capturing the night sky for posterity is a gripping passion and thanks to digital technology it’s one that has never been easier. That said, it still poses a challenge. The days of wet processing may be long gone, but now there are new camera functions to uncover and much to consider when creating an imaging setup. To make things easier, we’ve split the astrophotography section of this special edition into chapters. We begin with nightscapes – images that are similar to the views of the constellations and the Milky Way that you would see with the naked eye. From there we head out into the Solar System, to the Moon, the Sun and the planets, and finally beyond the influence of our star and into the realm of deep-sky objects,…

ASTROPHOTOGRAPHY

A good place to start with astrophotography is by taking ‘nightscapes’ – wide-field, panoramic images of the heavens encompassing bright stars and the Moon, and perhaps set against a horizon. Nightscapes are particularly good when that great sweep of stars that makes up our own Galaxy, the Milky Way, takes centre stage. Constellations also make great subjects in panoramas. Your astrophotos can also reveal the movement of Earth by capturing star trails and tracking the changing positions of the planets over time. You don’t even have to look beyond Earth’s atmosphere as there are meteor showers to watch out for and, depending on the time of year, ethereal noctilucent clouds or the shimmering greens and reds of the striking aurora borealis. WHAT YOU’LL LEARN 10 Equipment We walk you through…

Many of us use a compact camera or smartphone to take everyday pictures, so why not turn them on the night sky? Smartphones can be used for bright twilight subjects such as planetary conjunctions and there are plenty of apps available to help you get the most out of this sort of photography. Today’s compacts, on the other hand, can take reasonable images in low-light conditions and some even have manual modes that let you take full control of the camera’s functions. The ability to adjust some camera settings for yourself will make for much improved images. A good example is the Canon PowerShot G1 X Mk III compact. It lets you keep the camera’s shutter open to the starlight for up to 30 seconds and its 24.2-megapixel sensor allows…

WHICH DSLR LENS?

There are several DSLR lenses that are useful for astrophotography. Standard DSLR zoom lenses have maximum focal lengths of 50-70mm and can capture the main constellations, brighter sections of the Milky Way and detail within aurorae. To capture the larger constellations or large auroral displays, a wide-angle lens with a focal length of 16-28mm is needed. DSLRs also have high ISO settings compared to film cameras, and you can use these with the higher magnification of 100-500mm focal-length lenses to capture bright stars in asterisms or clusters. If you can go for fixed-focal-length lenses, all the better: they can be easier to focus than zoom lenses and often have wider aperture settings, enabling more light to get to the sensor.…

WHICH DSLR LENS?

Aperture (f/number) This is the diameter of the lens that allows light to reach the imaging chip. The amount of light let in can be increased or reduced with an iris in the lens – this determines the f/number. A low f/number (such as f/1.8) gives a wider aperture and lets more light in. Sensitivity (ISO) The ISO is an international standard for the sensitivity of the chip in digital cameras. Setting this to a low value like 100 gives high-quality images that need more light; high values like 3,200 let you photograph faint targets, but with a decrease in quality. Exposure (Shutter speed) The shutter speed determines how long the imaging chip is exposed to light. The slower the shutter speed, the longer the exposure and the more light…

DSLRs are particularly good cameras for astro imaging because of the amount of manual control they provide. Thanks to interchangeable lenses they offer a wide range of magnifications – from what you can see with the naked eye to views similar to those through binoculars. Their ability to take long exposures at high ISO values means that DSLRs can also capture images of the Milky Way that you couldn't see with your eyes. This is because when light reaches your eye, your retina immediately sends signals to your brain. With a long exposure, however, light can build up on a camera’s sensor before the shutter closes again. But you need to make use of the manual control a DLSR offers to get the best results, as the built-in autofocus settings…

1 Bulb The bulb setting allows you to control the length of exposures. It lets you take shots lasting from one second up to several minutes, using a remote shutter release or the delayed timer. Such a wide range of exposures can be exploited in your shots – stars can be made to look like pinpricks with a short exposure, or trails with a longer one. 2 File format Many cameras come with a range of file format settings for your pictures, including Raw, JPEG and Raw+JPEG. Get to know your camera’s file formats and ideally use the Raw setting. While this will produce the highest quality results, the larger file size means that any given memory card won't be able to hold as many Raw image files as JPEGs.…

While focusing in the daytime is easy, focusing at night is a completely different matter. Autofocus relies on contrast, so the dark sky, coupled with the faint and point-like nature of stars, means that a DSLR’s autofocus will struggle to find anything to lock on to. Squinting through the camera’s viewfinder won't help much either as it’s often only the brightest stars that will be visible and those tiny points won’t be enough to manually focus on. Compact cameras don’t find focusing in the dark any easier. The zoom lenses on some models focus past infinity, making it difficult to bring the view to sharp resolution. Most manual fixed lenses for DSLRs can be set at infinity, but it’s often not where you’ll find the best focus. Here are some…

The night sky is full of patterns, constellations of stars that have helped civilisations from the ancient Greeks and Romans to the Aboriginal Australians find their way around the heavens. Photographing them is a great way to start taking astrophotos – you’ll capture some captivating images and get your bearings at the same time. It’s not just constellations that make recognisable patterns in the sky. Asterisms, such as the Plough in Ursa Major, also make good, indentifiable photo subjects. These familiar groups typically consist of the brightest stars in a constellation and miss out the fainter stars that make up its main shape. Another one to look for is the Sickle in Leo, which looks like a back-to-front question mark. The swarm of stars that is the Milky Way makes…

1 Choose your subject and position Choose the constellation you want to photograph and point your camera to frame the subject nicely. To orientate the constellation in the frame for a better composition, you may need to adjust the tripod legs as well as the tripod head, which the camera sits on. 2 Find the sharpest focus Carefully check the focus using the methods described on page 13, and if you’ve used autofocus recently, turn it off. Take several test shots and then zoom in on them using the live view screen to judge whether the focus setting you’ve used has sharpened the stars. 3 Adjust the camera settings manually Put the camera in manual or ‘M’ mode, and use the menu to set the ISO and exposure, plus any…

To make your nightscapes more interesting, include an object in the foreground to complement the star field or constellation behind it. Good things to go for are lone trees, buildings with lit windows, dramatic terrain, telescopes and even people. Use a red- or white-light torch, or a camera flash, to illuminate the foreground. Combining this with a longer exposure will still reveal the stars. A neat trick is to get one person to appear in several places in the photograph. Give the person a torch, get them to stand still with the torch on and then switch it off while they move to a new position, before turning the torch on again. In the finished photo, they will appear in several places at once. On the other hand, a person…

The stars in the northern hemisphere appear to rotate anticlockwise around one particular star: mag. +2.1 Polaris (Alpha (α) Ursae Minoris). As this star lies almost exactly on the celestial pole, we call it the Pole Star. Over the course of 24 hours the rest of stars in the sky complete a full circle around Polaris. If you take a 10-minute, wide-field exposure that includes the Pole Star, you’ll see that the stars farther from Polaris have longer trails. The arcs get longer as you move from the pole towards the celestial equator until, at the celestial equator, they're at their longest. South of this plane, the arcs begin to decrease in length again towards the south celestial pole, which for those of us in the northern hemisphere lies below…

MORE INSPIRING TWILIGHT TARGETS

We’ve shown you how to take pictures of constellations and star trails, but there are lots of other interesting targets you can capture using your camera’s basic settings and minimal equipment – for example the Northern Lights and noctilucent clouds can be captured with the same settings you’d use for a constellation by adjusting the exposure and ISO to cater for their specific brightness. Conjunctions of bright planets, when two or more of them appear close together in a twilight sky, are a favourite for many astro imagers. For these scenes, try using your camera’s auto settings to see how well they cope. The Moon can also be used creatively. Thirty-second exposures at ISO 400 and f/2.8 during full Moon will look like daylight, but the blue sky will be…

MORE INSPIRING TWILIGHT TARGETS

The bright, naked-eye planets can also be imaged as points. If you take a photo of the area of sky through which a planet moves over a few months, you can record its motion. With photo-editing software, it’s possible to superimpose images to show the planet’s path over the whole period. 1 Subject Mars, Jupiter, Saturn and Venus make fine targets in a dark sky. To record the movement of the planets over weeks or months you need the planet to be moving steadily, so don’t take shots when the planet’s apparent motion against the sky is slow. 2 Camera settings You don’t want the stars or planets to trail, so take exposures of 10 to 20 seconds, set the aperture to its widest and select ISO 400. If there’s…

1 Equipment Select your lens and mount your camera to a tripod. Adjust the tripod legs so they provide a solid base. Attach the remote shutter release so you don’t vibrate the camera while you’re taking the shot. 2 Camera settings Select the ISO, switch the camera to bulb mode so there’s no limit to the length of the exposure, then focus. Ensure your flash is disabled and switch on the long-exposure noise reduction. 3 Subject Decide whether to shoot short star trails near Polaris, or longer trails with a constellation near the celestial equator. Ursa Minor is good for the former while Orion works well for the latter. 4 Capture and review Take several exposures ranging from five minutes up to half an hour, then review the results. If…

Meteor photography isn’t actually that hard to do. You need plenty of patience but, once set up, the process of catching a meteor on camera is a waiting game – a bit like celestial fishing. There are several prominent showers throughout the year: these include the Lyrids in April, the Perseids in July and August, the Orionids in October, the Leonids in November and the Geminids in December. The first thing to consider is your camera. As you need good sensitivity, a changeable field of view and longish exposures, a DSLR is the best tool for the job. You’ll also need to cover a fairly large part of the sky: opt for a normal photographic lens with a focal length of 18-35mm if you’re using a non-full-frame camera, or a…

1 Narrow or wide-angle? Use an 18-35mm lens for a non-full-frame DSLR or a 28-50mm lens for a full-frame one, and a lens with the smallest f/number. Remember, wide-angle lenses will cover more sky than narrower ones, but the trails will look less impressive. 2 Tilt and pan or equatorial? A simple tilt and pan tripod is the easiest way to mount your camera, though a driven equatorial mount will keep it pointing at the same patch of sky. Alternatively, wedge the camera somewhere so that it points at the sky. 3 Find your focus Set a high ISO – 1,600 or 3,200. The lens should be fully opened (lowest f/number) and pre-focused at infinity with autofocus turned off – the focus needs to be very accurate for meteor work.…

With its shifting phase and the wobbles of libration bringing myriad new features into view each night, the Moon is a wonderful target for imagers of all abilities. As the largest object in the night sky, as seen from Earth, it’s also an obvious place to start when you’re learning to image the heavens. But for all its size, proximity and brightness, it takes great attention to detail to capture top-quality images of Earth’s natural satellite. We show you how to do it in this section – starting with the equipment you need and then moving on to the techniques that will help you to capture better lunar shots, whether you’re using a DSLR or just a smartphone. If you’re stuck for ideas, we also look at six classic lunar…

The great thing about the Moon is that anyone with a telescope and either a smartphone or a compact camera can take a decent image of it – all you need to do is hold your camera up to the telescope’s eyepiece and start snapping away. This technique is known as ‘afocal photography’ and if you already have a camera and telescope, you don’t need anything else to get started. A DSLR provides you with several options. They can be used to take afocal shots, just like smartphone and pointand-shoot cameras. Or they can be turned directly to the night sky to capture nightscapes (see page 8) with the Moon at centre stage. Although, bear in mind that the Moon will appear small in such an image taken with an…

Red filter If you’re using a monochromatic planetary camera you might find it helpful to fit a red filter when capturing lunar images. Using one can help produce a crisper final picture. Barlow lens A must for detailed close-ups of the lunar surface and its craters, whatever camera you use. It increases the focal length and the magnification of your system, typically by two or three times. Smartphone bracket Holding a smartphone squarely over an eyepiece while you take a photo is a delicate art. You can make things easier with a dedicated bracket to do the job for you.…

The first step when setting out to image features on the Moon is to decide what your target is and work out the best time to capture it. There are various apps and programs that can help you to do this – a few of our favourites are listed on the next page (see ‘Planning tools’). To reveal intricate surface textures and the rugged nature of the lunar crust, it’s best to take your shots when your target is lit from an angle. This will be when it’s located near the line between the light and dark sides of the Moon – known as the terminator. This is particularly important when capturing very small craterlets and other subtle lunar features such as rilles; when they’re illuminated from above, they can…

It sounds counter-intuitive, but to end up with a high-resolution close-up of the lunar surface it is better to record a video than take stills. Why is this? The answer doesn’t lie out in space but closer to home, in Earth’s atmosphere. When looking at the Moon at high magnification on an average night, you’ll notice the view undulating, with moments of steady, sharp views and then brief blurred instants. Since they generally only take single still images, the shots from a DSLR or point-and-shoot camera risk being blurred by this atmospheric turbulence at the moment when the shutter fires. A short video taken with a webcam or planetary camera, recorded in the AVI format, has many frames each second. It enables the sharpest moments of clarity to be selected.…

Moon Globe This great firee app can be used on your iPhone, iPod Touch or iPad. It shows what the Moon looks like for a given date and time, complete with shadow details. Via iTunes App Store Stellarium This firee planetarium software is handy for working out where the Moon will be. It’s great for when you’re planning a long night of imaging to create a lunar mosaic. stellarium.org Virtual Moon Atlas An authoritative piece of fireeware for planning your observations in detail using an interactive atlas and feature-rich information panel. ap-i.net/avl/en/start…

Crater Clavius This is a great target if you’re trying to push your equipment to its limit. See how many of the small craterlets on the crater foor you can capture. Rupes Recta Also known as the ‘Straight Wall’, Rupes Recta is an enormous fault on the lunar surface. The trick is to catch it under the right illumination to see it clearly. Crater Copernicus Being one of the most stunning craters on the Moon, Copernicus is a wonderful imaging target. See if you can capture its magnificent terraced walls. Vallis Alpes This valley cuts straight through the lunar Alps. The challenge here is to see if you can capture the elusive rille that runs all the way along its length. Montes Apenninus This chain of peaks is one of…

Many of us carry around smartphones that have cameras built into them. The first of these were good for quick, spur of the moment shots, but as their imaging technology has matured, the capability of smartphone optics has increased to a point where they can rival some quite decent point-and-shoot cameras. We’re going to prove that by showing you how to use a smartphone to take images of the Moon with afocal imaging, the technical term for simply aiming your smartphone’s camera down your telescope’s eyepiece. There are two ways to do this: one is to simply hold your phone’s camera over the eyepiece, the other is to use a smartphone adaptor, which attaches to your telescope’s eyepiece to provide greater stability. You wouldn’t think it would be difficult to…

1 Prepare Anticipate the Moon’s movement and point your telescope just ahead of it in the sky (you’ll take the shot when the Moon becomes centred). Focus the scope as accurately as you can, making sure the features on view are sharp. 2 Line up and shoot Hold the phone squarely to the eyepiece until you get a bright blob on the screen. Carefully alter the distance between camera and eyepiece until the view focuses and isn’t overexposed. With the image on the screen, take the shot. 3 Use a smartphone adaptor A smartphone adaptor takes the strain out of holding your phone and keeping it lined up. Once attached it can be adjusted until the smartphone lens is lined up with the eyepiece. Once aligned, you can take the…

For this masterclass, we’re going to show you how to capture a full-disc shot of the crescent Moon, along with its accompanying earthshine, using a DSLR camera. The image we’re aiming for, like the one above, depicts the Moon with a slim edge illuminated by light directly from the Sun and the rest of its face dimly lit by light reflecting off Earth. As is often the case with astrophotography targets, it involves more than simply pointing your camera up at the sky on the appropriate night and pressing the shutter release. But before we explain how to catch a shot like this, it’s worth explaining how this elegant scene comes about. At the point of new Moon, the lunar disc lines up with the Sun, while appearing slightly above…

1 Attach your camera Attach your DSLR to a telescope or lens with a focal length of at least 300mm – this will give you a large enough image to see the Moon’s disc with some detail on it. A telescope or lens with a focal length of around 1,000mm is ideal. 2 Set the ISO Focus the camera as accurately as possible and set an ISO of 200-400 for the crescent shot. The exposure you’ll need will depend on your equipment. Take a test shot of 0.5 seconds and review the result. 3 Adjust exposure Adjust the exposure so that the crescent comes out bright but without any white in it: check by looking at the image’s histogram on your DSLR. Take several shots increasing the exposure as you…

Before you can create a mosaic, you need detailed pictures. A mono, high frame rate camera is the best tool for the job, as it allows you to record a short video of each ‘pane’ (segment of the lunar surface) as an AVI file, the individual frames of which can be stacked into a still image. If your camera doesn’t have an infrared-blocking filter built in, consider attaching an infraredpass filter. Doing so can reduce the effects of poor seeing. Alternatively, try a red filter. Moving across the lunar surface in overlapping panels is something that sounds easy, but there are pitfalls to avoid. The higher the magnification you use, for instance, the easier it is to get lost and miss a bit out; this is a common frustration when…

1 Load Load a single AVI into AviStack. If you’re processing it manually, select Settings Processing All Manual from the menu. If not, click on All Automatic. Expand the Save Stacked Image section and check Manual processing is set; click on Automatic processing if shown. Expand the Post processing section and select Manual processing. The four options below should be deselected. 2 Save Click on Process File. In the Save stacked image window (right) select PNG as the file type and pick a folder to save it to. Make sure Save Parameters is ticked before you save. Hit Cancel when the Post processing window appears. Check if the result needs wavelet sharpening (see step 5). If not, repeat and adjust parameter settings in a manual run using the help options…

Photographing the planets can, even in today’s enlightened times, lead to surprising discoveries. Often the announcement of an impact on Jupiter or storm on Saturn comes from an amateur, having been recorded by a planetary imager. In this section we’ll consider all of the planets, from Mercury out to Neptune. Some of these worlds, such as Mars and Jupiter, appear more dynamic than others, so we’ll concentrate on these to illustrate some of the techniques needed to image the planets. The best time to image a superior planet (one that has an orbit larger than Earth’s) is around opposition, so we’ve supplied a table to give you prior warning of the next time this is due to occur. WHAT YOU’LL LEARN 34 Equipment Everything you’ll need to take sharp, colourful…

The planets are on our cosmic doorstep, yet you still need a telescope with quite a bit of magnification to see any of their surface details. To see more detail, you need to increase the magnification or image scale (how big an object appears in the image frame). This is determined by the focal length of your telescope. As a telescope’s focal length is fixed, you might think that it can only give one magnification, but this isn’t the case – optical amplifiers such as Barlow and Powermate lenses can effectively increase it, while focal reducers effectively lessen it. Focal ratio (f-ratio) gives an indication of the ‘speed’ of an optical system – the time it takes your telescope to deliver a set amount of light. As the f-ratio goes…

Filters are firequently used in planetary imaging to enhance certain wavelengths, or to prevent some of them from getting to the camera. Imaging chips are often very sensitive to infrared light; unless checked, this can oversaturate and wash out detail. An inexpensive infrared-blocking filter can prevent this. Mono cameras can take full-colour images using red, green and blue (RGB) imaging filters placed one at a time in front of the camera’s sensor. Some of these cameras have an infrared-blocking filter fitted as standard. Other types of filter commonly used are infrared-pass filters such as the popular 742nm ‘planetary filter’. This lets through longer wavelengths of light, which can often appear less prone to atmospheric turbulence, and produces a sharper image. Specialist filters such as methane-band (CH4) or ultraviolet-pass are also…

Smartphone Smartphone cameras have advanced to the point where they can take basic shots of planets afocally, through the eyepiece of a telescope. There are also accessories available to attach them the telescope’s eyepiece. Planetary The first step on the multi-frame imaging ladder is a medium frame rate camera such as the Celestron NexImage. Pre-fitted with a 1.25-inch eyepiece adaptor, they often come bundled with good software. High frame rate Top of the pile are dedicated high frame rate cameras, like those from ZWO, PGR and Lumenera. With high-speed connections such as USB 3.0, rates of several hundred frames per second are common.…

Light from a planet must pass through our atmosphere before it reaches us and as it does pockets of air at different temperatures and densities bend or refract it. This blurs the details that we can see. The state of the atmosphere and its effect on the appearance of celestial objects is known as ‘seeing’. Excellent seeing is something of a rarity in the UK; when it does occur, it’s really something to behold, but for the most part we have to make do with average seeing. An eyepiece view of a planet under average seeing conditions often gives feeting views where everything looks sharp. You can appreciate these brief moments when you’re looking through a telescope, but snap a planet with a DSLR camera under average seeing conditions and…

Planetary imaging is easy if you know what you’re doing. But it’s important to remember that many of the amazing amateur images you see on the pages of BBC Sky at Night Magazine have years of practice and experience behind them. Don’t let that put you off, though, because gaining that experience can be a lot of fun. For basic planetary imaging you should be using a polar-aligned, equatorially mounted telescope fitted with right ascension (RA) and declination (dec.) drives for ease of repositioning. You’ll need to place it outside for two to four hours before you start imaging so the instrument can cool down to match its surroundings. This should be timed so that you can start imaging when your target is close to due south and highest in…

Cool your kit Minimise the risk of air currents inside your telescope affecting image quality by letting it cool down outside before you start imaging. Scopes up to 8 inches in diameter need about two hours to cool; give larger instruments three or four hours. Time the cooling to end when your target planet is close to its highest point. Check collimation All of your scope’s optical components need to be collimated (perfectly aligned) to get the best results. This is especially important with fast scopes that have focal ratios of f/5 or lower, as these are significantly less tolerant of collimation errors. Refer to your scope’s instructions to find out how to check and collimate it properly. Upgrade the focuser High-resolution imaging requires precision focusing. Long focal lengths will…

It is possible to do planetary imaging on a static tripod head with no drive but, frankly, it’s really hard work. Ideally, your telescope needs to be on a sturdy, polar-aligned equatorial mount. Although accurate alignment is desirable, simply pointing the mount’s polar axis at the north celestial pole using a basic polarscope is normally sufficient to achieve the accuracy needed. Motor drives on the RA and dec. axes are also desirable. These can be used to make corrections if the planet drifts, even when you’re in the middle of a live capture run. A slow positional adjustment rate – 2x sidereal speed or less – will normally give you enough fine movement control to compensate for any shifting of the planet within the frame of the imaging chip. There…

Mars comes to opposition every 2.1 years. This is when the planet is best for imaging since it’s opposite the Sun from our point of view and appears at its largest and brightest. The next oppositions occur in October 2020, December 2022, January 2025. From the UK, the planet’s position will become optimal in terms of altitude and apparent disc size in 2020 but thereafter the disc size will begin to shrink with each successive opposition. Through a telescope there’s plenty to observe both on and above the Martian surface. The reddybrown deserts of Mars are interrupted by dark albedo (shaded) markings that rotate with the planet. Mars has seasons and the effects of these can be seen in the growing and shrinking of the planet’s polar ice caps. There’s…

THE OTHER INNER PLANETS

Mercury The innermost planet is tricky to image because it never strays far from the Sun. This makes it hard and quite dangerous to locate when the Sun is above the horizon. It also keeps the planet’s visibility limited to either before sunrise or after sunset. The best time to image Mercury is when it’s close to maximum elongation, in the evening sky during the spring months or in the morning sky during the autumn months. Venus Venus shows little detail even in high-resolution images. It has the advantage of being bright and easy to locate, and it strays further from the Sun than Mercury. The planet shows phases like Mercury, but its apparent size can get much larger. Specialist filters that pass just ultraviolet light, such as the Schüler…

THE OTHER INNER PLANETS

1 Select a reference frame RegiStax analyses your capture file to assess how sharp each frame is. Load your capture file (typically in AVI format) using the Select option, then adjust the lower slider so that a really sharp frame is displayed. This will be the reference against which the others are compared. 2 Place the alignment box Place an alignment box over an area of high contrast in the reference frame. Typically this will be the main planet’s disc, so choose an appropriate size but don’t worry if some of the disc falls outside the box. Move the cursor over the image and left click to place the box. 3 Align Check the Use Pre-Blurring option and then click on the Align button. RegiStax runs through each frame, comparing…

Jupiter is one of the best planets to image because it’s big, bright and has a dynamic atmosphere full of rapidly changing detail. The planet rotates quickly – features take slightly less than 10 hours to go all the way around. Consequently, if you take too long to image Jupiter you may find its detail is smeared out and lost due to motion blur. Jupiter is an ideal target for all types of telescope. Even a small one will be able to capture the planet’s disc, main belts and the Great Red Spot. If you do have a small telescope, Jupiter is a great object to consider for a ‘family portrait,’ featuring the planet and the four Galilean moons. As telescope aperture increases, so does resolving power, and large telescopes…

THE OTHER OUTER PLANETS

Saturn Saturn’s beautiful rings are a big attraction. Surface detail can be subtle with gentle banding and the occasional white spot representing a storm in the planet’s atmosphere. Saturn is dimmer than Jupiter so exposures typically need to be longer, resulting in reduced frame rates. An 8-inch or larger scope is recommended for detail, aim for f/15-f/25. Keep an eye the planet’s position and on the seeing, then take advantage when the atmosphere appears stable! Uranus and Neptune Uranus and Neptune are both large worlds, but their distance makes them appear small and dim. To get a decent image scale you’ll need to use a long focal length, which reduces the image brightness of both planets. Even so, it is possible to capture them using a high frame rate camera…

THE OTHER OUTER PLANETS

1 Align RGB images Having processed each RGB channel in RegiStax (see page 39), load the images into a layer-based graphics program. Cut and paste the G and B images into the R image as separate layers. Align the G and B layers with the R layer. Crop the image so that there are no gaps at the edge. 2 Create a blank canvas Create a blank RGB image with the same dimensions as the cropped image from Step 1. Select this image’s blank R channel. Copy the R layer from the Step 1 image into this channel. Repeat with the G and B images, pasting them into the blank image’s G and B channels. 3 Turn on the colour Turn on the visibility of all the channels in the…

There are several advantages to solar imaging, not least the opportunity to study a star. It’s done during the day, when the temperature is normally quite pleasant and with plenty of light around, so you can say goodbye to fumbling with red-light torches. The amount of detail you’ll capture depends on your kit, but there’s usually something interesting to see. Of course, there are downsides too. It can be difficult to see a computer screen in sunlight and there’s a real danger from the intensity of light – it’s one of the only times that astronomy can pose a risk of physical injury. Over the next few pages, we’ll look at how to address these issues, what you can expect to see on our star and how to observe it…

When taking images of the Sun it’s important to use the right tools. The danger pointing a telescope at the Sun poses to your eyesight means that you should only ever use proper certified filters. Solar telescopes come in two varieties – those that are dedicated to the purpose and those that have just been temporarily adapted for safe solar viewing. A standard, equatorially driven mount makes a perfect platform for either variety, since the Sun’s motion across the sky is not significantly different from that of the stars. White-light imaging involves removing most of the harmful light that could reach your camera sensor. There are various ways to do this, including projection, using a Herschel wedge inserted into the eyepiece holder or fitting a white-light filter over the front…

At the centre of many hydrogen-alpha telescopes is an optical device known as a Fabry-Pérot etalon. This creates an optical cavity by bringing two optically fat glass plates together, separated by a precise gap. The two parallel facing surfaces are semisilvered. As light enters the cavity through the first plate, it reflects back and forth between the semi-silvered surfaces. The reflecting beams interfere with each other so that certain wavelengths reinforce while others cancel out – this is a process known as ‘constructive and deconstructive interference’. The light that eventually passes through the semi-silvered surface of the second plate contains bright wavelength peaks and dim wavelength troughs. A plot of wavelength versus intensity of this light looks a bit like an upsidedown comb, the peaks being represented by the tips…

1 Gather your materials You’ll need thin, bendy card and thicker (1-2mm) card or mounting board to hold the film; solar film large enough for your scope’s aperture; clear and double-sided tape; a pair of scissors or a craft knife; a compass; and a fat piece of tissue. Work on a fat surface away from draughts. 2 Prepare the card collar Cut a strip of thin card 50mm wide and long enough to wrap around your scope. Use multiple strips if your scope has a very large aperture. Wrap the strip around your scope, fixing it with tape. Use doublesided tape to add a few more layers of strips on top to create a stiff collar. 3 Cut out a filter holder Draw two concentric circles on the thicker card…

DSLR DSLR cameras are good for taking shots of the Sun through white-light filters, but not so good through narrowband filters. Single images are also prone to atmospheric distortion. High frame rate As with other Solar System targets a mono, high frame rate camera will produce the best results, especially when used with narrowband filter systems. The high frame rates these cameras can achieve are ideal for reducing seeing effects. Cooled CCD A monochrome cooled astronomical CCD camera can work well with narrowband-filtered images. The best technique is to take lots of short-exposures and stack them together using a program such as RegiStax.…

The techniques used to image the Sun are similar to those used to capture images of the Moon and planets. The difference is that the intensity of the light from the Sun must be reduced. There are different filter systems to do this and itÕs important only to use certified ones. The best camera for imaging the filtered Sun, especially if you intend to use any magnification, is a monochrome high frame rate camera. Colour devices work for white-light captures, but they may be lacking when using speciality filters as their built-in colour filters can reduce the sensorÕs sensitivity. Solar imaging falls into two categories: white light and narrowband. White-light imaging reduces the amount of sunlight to safe levels, cutting out harmful, invisible wavelengths. With a white-light filter on the…

Solar imaging at low magnification is possible using a fixed mount, but a driven and polar-aligned equatorial mount will make life much easier. While the Sun’s apparent motion across the sky isn’t exactly the same as that of the stars, it is close enough not to have to worry about it during a typical imaging session, and a standard right ascension drive will do a perfectly adequate job. As with any other type of imaging, a stable, heavy-duty mount is highly recommended to help avoid any unwanted wobbles.…

White light A white-light view reveals the Sun’s visible surface, the photosphere. With good seeing, a 4-inch or larger scope will reveal the photosphere’s mottled texture. Called solar granulation, this looks a bit like lots of rice grains packed together. The photosphere looks darker at the edge of the disc due to a phenomenon known as limb darkening and it’s here that you might find bright patches called faculae. If the Sun is active, there may be sunspots on view too. Larger spots typically appear in groups called sunspot groups, pictured, or active regions. A typical sunspot has a dark centre known as the umbra, surrounded by a lighter region known as the penumbra. Hydrogen-alpha Through a hydrogen-alpha filter, the photosphere is hidden under a blanket of hydrogen known as…

An inexpensive way to get into solar photography is to image the Sun in its natural ‘white’ light. When you pay attention to the safety issues, it can be a very rewarding way to monitor our nearest star. One of the most basic methods is to use solar projection. For this you’ll need a small refractor, ideally mounted on a driven equatorial mount. With your scope pointing away from the Sun, fit a non-plastic, low-power eyepiece and ensure the finder is removed or capped. Watching the scope’s shadow on the ground, turn it to point directly at the Sun. A piece of stiff white card held behind the eyepiece will catch the Sun’s image, while a tweak on the focuser will bring it into sharp relief. A card shield taped…

Focusing is a critical skill to master in any form of astronomical imaging. If you’re just starting out, accurate focusing can be quite hard to get to grips with, which can make the whole imaging experience rather frustrating. There’s no reason why this needs to be the case, so here are a few tips on how to get your images as sharp as possible. First, make sure your camera is securely locked into your telescope’s eyepiece holder and that the focuser tension adjustment is set firm. You want to be able to move the camera back and forth quite easily, but you also want it to stay where you’ve put it. Locate a high-contrast part of the Sun. For white-light imaging, the best target is the Sun’s edge. For more…

SEEING AND THE SUN

Earth’s turbulent atmosphere interferes with sunlight on its way to your telescope, making fine detail on the Sun’s surface appear to wobble and distort. This is known as ‘seeing’. The Sun’s own heat can make matters worse, causing heat thermals as the day starts to get going. For white-light imaging, poor seeing means the difference between viewing solar granulation or not, so it’s a good idea to choose periods when the seeing is good. Mid to late morning is often ideal because it’s before the Sun’s heat has had a chance to take full effect.…

SEEING AND THE SUN

1 Fit your filter With the scope pointing away from the Sun, remove its lens cap and fit the solar filter; remove or cap the finderscope too. Make sure everything is securely fastened. If required, use some electrical tape to hold the filter in place securely – this is vitally important on windy days. 2 Line up with the Sun Lining the telescope up with the Sun without using a finderscope isn’t hard. Without looking along the tube towards the Sun, roughly align the scope and then look at its shadow. As the scope approaches the correct alignment, the tube’s shadow will reach minimum size. 3 Keep it in the shade If you find it hard to see detail on your computer screen in sunlight, you’ll need a dark enclosure.…

The Coronado PST has revolutionised hydrogen-alpha solar observing because it’s both cheap and portable. But as far as imaging is concerned there are a few hiccups, mainly due to a lack of inward movement when focusing high frame rate cameras. This results in you being unable to reach prime focus, hindering the scope’s usefulness for solar imaging. But there are solutions. One of the easiest is to take the front lens off a standard 2x Barlow and screw it into your camera. The view isn’t quite prime focus, but it’s not far off and focus can generally be reached. You can also buy a low-profile nosepiece that puts the camera closer to the eyepiece holder. The PST is a low-power instrument. Its 40mm objective works at f/10, giving a focal…

Different camera control programs can have different setting options, but the most common settings are exposure, gain, gamma, contrast, brightness and frame rate. Typically, gamma, contrast and brightness should be left at their zero (default) levels. It might be tempting to adjust gamma down to improve surface detail clarity but, in reality, this can be emulated using a graphics editor after image capture. Exposure, gain and frame rate need to be adjusted so that you get a strong signal at the fastest frame rate possible. If you can keep the gain as low as possible to achieve this, you’ll also reduce the level of noise.…

1 Load in RegiStax Pass your files through a program such as RegiStax or AutoStakkert. In RegiStax 6, select your capture file, click Set Alignpoints and then Align. Drag the bottom slider to include the best frames on the RegiStrationgraph and click Limit. Click Stack to finish. 2 Use wavelets to sharpen Wavelets can sharpen your images. A typical PST result responds well to Wavelet-scheme: Linear and Waveletfilter: Gaussian. Check Use linked wavelets and set the first slider to 30. Once you’ve found a scheme that suits you, save it via the Save Scheme button. 3 Save and load Save your processed result as a 16-bit PNG or TIFF file and load it into a layer-based program such as Photoshop. If necessary, crop the image to remove any white borders…

Deep-sky astrophotography is perhaps the most demanding of all the subjects covered in these pages. What we’ve covered so far involves capturing bright objects that respond well to short exposures, but deep-sky objects tend to be relatively dim. Capturing enough photons to produce clear, detailed and noise-firee images of these far-off targets requires long exposures that will stretch the capabilities of your camera and place great demands on your mount, in terms of its ability to track accurately. In this introduction, we’ll provide tips and techniques to help you get the most from your kit. We’ll show you the best way to set up your gear, capture the data and process it to create stunning images. WHAT YOU’LL LEARN 54 Equipment Find out which cameras you need to image deep-sky…

You might think that the most important piece of equipment you need for deep-sky imaging is the telescope or camera. In fact, it’s the mount. Tracking deep-sky objects and taking long exposures of them as they traverse the night sky places great demands on your mount, so it needs to be up to the job. First of all, you will need an equatorial mount for accurate tracking. Although an altazimuth (altaz) mount is just about suitable for high frame rate lunar and planetary imaging, it doesn’t quite make the grade for deep-sky work. The reason for this is simple: an equatorial mount will follow celestial objects in a smooth arc across the sky, whereas an altaz mount follows the same object in a series of horizontal (azimuth) and vertical (altitude)…

DSLR DSLR cameras are self-contained and equipped with large sensors, and also suit non-astro purposes. Their built-in infrared filter tends to be a little too wide in its coverage, though and this can diminish the amount of important, red hydrogen-alpha (Ha) light you can capture from emission nebulae. Colour CCD One-shot colour CCD cameras are designed for deep-sky imaging, they sport a filter-firee sensor with a Peltier cooling system to reduce ÔnoiseÕ generated by the electronics. They come in a range of sensor sizes and, like DSLRs, capture colour in a single shot – ideal for the UK’s fickle weather. Mono CCD Mono CCD cameras take great greyscale images. If you want to take colour images, things get a little more complicated: this involves taking different sets of exposures using…

All this equipment needs to be connected, and although DSLR camera manufacturers use different bayonet fittings for their lens attachments, there is, luckily, a standard fitting called a T-mount. T-mounts have two sides: a female 42mm-threaded insert (a T-thread) and a camera-specific bayonet. This allows a DSLR camera to be attached to a range of astronomy-related items, including the focus tubes of many telescopes. Most CCD cameras have a T-thread built into them. A simple T to 1.25-inch (or T to 2-inch) nosepiece adaptor allows any camera to connect to the eyepiece holder of any telescope. Achieving focus with a refractor after the star diagonal has been removed from the focuser usually requires a T-extension tube. However you attach your camera, make sure it can’t fop about, as itÕs vital…

Deep-sky imaging is all about preparation. Once the frames start rolling, your work outside is pretty much done and you can keep an eye on the system while you sit back and enjoy the night sky. The first task is to set up your telescope’s tripod and mount so that they’re facing north and the tripod’s top is reasonably level, although this doesn’t have to be perfect. Next you should adjust your mount’s altitude setting to match your own latitude using the two adjustment bolts at the front and rear of the mount, being very careful to release one while tightening the other. You can now carry out the vital polar alignment routine using your mount’s built-in polarscope. Read your mount’s manual, since there is some variation between manufacturers in…

CCD cameras have no controls. Instead they rely on software running on a computer to set exposure times, cooling temperatures, resolution settings and so on. Most CCD cameras come with basic proprietary software to perform these functions, but usually there’s an ASCOM driver supplied as well to allow a range of camera control programs to be used. Sequence Generator Pro, MaxIm DL, AstroArt and Nebulosity are the most popular and have extra features, such as image sequencing to automate the capture process. DSLR cameras can be operated as standalone imagers, most simply by using a plug-in intervalometer to capture a sequence of timed exposures, but it’s increasingly common to use camera control software instead. The software mentioned above works with many DSLR cameras, and BackyardEOS and ImagesPlus are also popular…

Once you have collected all your image data, known as light frames, you’ll need to take three additional special sets of subframes to calibrate your image. These calibration frames remove unwanted artefacts from your image. The first set, bias frames, are very easy to take – simply cap your telescope to exclude any light and take 20 or so exposures at your DSLR camera’s fastest shutter speed, or use the ‘Bias’ option if you have a CCD camera. These frames are subtracted from your light frames and compensate for variances between the output from individual pixels. They also remove any read-out noise generated in the light frames. The next set, dark frames, are also taken with the telescope capped, but with exactly the same exposure length as you used for…

For this masterclass we’ll show you how to capture an image of globular cluster M13, a popular astrophotography target in the constellation of Hercules. You can use a one-shot colour CCD or a DSLR camera for this as you won’t need to worry about hydrogen-alpha emissions. If you’re using your DSLR on its own then ‘Live view’ is particularly useful when focusing. Focus your camera on mag. +2.8 Zeta (z) Herculis, the star at the bottom right of the Keystone asterism in the centre of Hercules. Remove your Bahtinov mask, if you used one, and slew up towards mag. +3.5 Eta (η) Herculis at the top right-hand corner of the Keystone. M13 is found two thirds of the way between these two stars. The cluster’s apparent diameter is 20 arcseconds,…

CAMERA SETTINGS

DSLR If your camera’s hardware allows you to take exposures of a minute or more, use it to set the ISO value to 1,600, white balance to auto, file format to raw, noise reduction (if available) to off and shutter speed to 60 seconds. If not, set these values manually, then set the shutter speed to bulb and use a manual or programmable remote-shutter-release cable to trip the shutter for a 60-second exposure. There’s no aperture setting because telescopes don’t have a variable iris. If your software doesn’t allow a live view image on your laptop, take a series of 10-second exposures using a Bahtinov mask and adjust the focus between shots. CCD CAMERA Although CCD cameras need software to control them, some have a gain control. But use this…

CAMERA SETTINGS

1 Polar alignment To ensure that your mount tracks the sky as accurately as possible and to help your Go-To system to locate objects first time, carry out an accurate polar alignment. Using a polarscope is quick and easy, so don’t compromise – this will be a little time well spent. 2 Getting balanced Your mount will work best if you limit the amount of load it has to move. Getting an accurate balance will go a long way to achieving this. Balance the declination axis first with all of your equipment installed and the camera at approximate focus, then balance the right ascension axis. 3 Lock on Align on a bright star near Hercules such as Vega, fit your Bahtinov mask and adjust focus until the cross is bisected…

Building on the techniques learned in the previous masterclass, we’re going to tackle a very different object – an emission nebula in the Milky Way. Emission nebulae can be quite extensive, so you’ll need a short focal length telescope and a large sensor to capture the whole area of nebulosity. Don’t worry if you don’t have such a setup, though, you can still zero in on smaller sections. NGC 7000, or the North America Nebula, is rich in hydrogen-alpha (Ha) emissions, so getting the best from this object will require a modified DSLR or a CCD camera. DSLR cameras have a built-in infrared filter, which unfortunately filters out the very Ha wavelength that you want to capture from these objects. Many imagers have their camera modified to remove this filter.…

Detail in an image is held in the luminance (L) channel and is embedded in the RGB channels. But you can add extra luminance data to your colour image to produce more fine detail. A good way of doing this with an emission nebula is to use a hydrogenalpha (Ha) image as the luminance channel. Capture the data with a Ha filter using a mono CCD camera, a one-shot colour CCD binned at 2x2 or even a DSLR camera. But simply adding a luminance channel using Ha data will result in muted colours, due to a mismatch in contrast between it and the RGB data. To resolve this, blend the Ha data with the red channel. Start by pasting the Ha image onto the red channel. Use a 50 per…

1 Calibrate, stack and save Apply your calibration frames in the same way as you did for your M13 image, but in the case of a mono CCD camera, you won’t need to de-Bayer your data. Stack your calibrated frames as before and save the resulting image as both a 16-bit FITS and a 16-bit TIFF file. 2 Tweak the shadows Import the TIFF version of your image into either Photoshop or GIMP and select the levels tool. Select each colour channel in turn and slide the left-hand adjustment (shadows) to the right until it touches the start of the histogram to balance the colours across the frame. 3 Apply a gentle 'stretch' Select the curves tool and choose the RGB channel (value in GIMP). Click on the diagonal line…

THE DEEP SKY ADVANCED

Now we’ll help you take your deep-sky imaging a step further. In the previous section we looked at the minimum equipment needed to produce deep-sky images, with the emphasis on DSLRs and one-shot colour CCD cameras. Wonderful images of deep-sky objects can be captured using these tools and for many imagers this is the only type of camera they will ever need. But there are advantages to using a ‘more advanced’ setup with a mono camera and filters to produce your images, and we’ll be concentrating on working with this kit here. We’ll also be exploring the dark art of autoguiding, which enables longer exposures to give you deeper, smoother images and the ability to capture dimmer objects. WHAT YOU’LL LEARN 64 Equipment Introducing the dark art of autoguiding and…

THE DEEP SKY ADVANCED

Autoguiding is a very powerful technique, but it’s seen as something of a ‘dark art’. Many imagers are put off doing it for fear that it will be too much hassle to implement. It isn’t a technique that works straight out of the box, so to speak, but the rewards for mastering it – well-shaped stars and deeper images thanks to longer exposures – make the effort worthwhile. Autoguiding has one simple purpose: to detect any unwanted movement in a ‘guide’ star and to apply a correction to the mount to keep it pointing in exactly the right direction. Movement of the guide star is monitored with a guiding camera. If the software controlling the guiding camera notices that the guide star has changed position, it must follow that the…

Mono CCD camera with LRGB filters A mono CCD camera can produce naturally coloured images of a lot of deep-sky objects by using a mix of red, green and blue filters. Higher quality colour images use an extra component: the luminance channel. Our eyes detect fine detail in an image through brightness and contrast, rather than the colour components, which are simply ‘painted’ on top of this fine ‘canvas’. The luminance channel is the most important, so capture this data at the highest resolution. You don’t need a specific filter, but an infrared filter is normally used to stop stars appearing bloated through light splatter. If you’re short of time, you can capture the RGB data at a lower resolution than the luminance with little drop in quality. You can…

A ccurate polar alignment is a very important preparatory step for deep-sky imaging. Indeed, the longer an exposure lasts, the more accurate the alignment needs to be. If your polar alignment is not sufficiently accurate, long exposures will start to show trailing in the stars towards the edge of the field of view, caused by field rotation. Using a mount’s built-in polarscope is fine for shorter exposures of up to about 180 seconds, but it doesn’t really cut the mustard when it comes to the longer exposures that you need for imaging deep-sky objects. You’re far more likely to get a better shot using drift alignment – a much more accurate method of alignment the should be used whenever possible. Drifting away Drift alignment relies on a process of detecting…

Guidescope and camera There are two ways to set up a guidescope and camera combination – they can either be piggybacked on top of your imaging telescope’s tube rings, or attached alongside it on a dual mounting bar. The guiding camera should then be attached to the focuser using an extension tube in place of the diagonal, which should be removed as this can be a source of fexure. The mount should be carefully balanced in right ascension (RA) and declination (dec.) to ensure that no undue strain is placed on the mount’s motor drives. Unbalance the mount, as we explained on page 57, once you have slewed to your chosen subject. Connect the guide camera to your computer’s USB port, then plug the ST-4 guiding cable between the camera…

For the masterclass in advanced deep-sky-object imaging, we’re going to show you how to produce a full-colour image using LRGB filters and a mono CCD camera. A mono CCD camera is a versatile piece of equipment: it lets you capture data using a wide range of filters to achieve different results. Add to this the ability to take long exposures with reduced electronic noise, thanks to built-in Peltier cooling, and you can see why a mono CCD is such a powerful tool. Your choice of filter size will be dictated by the size of your sensor, as well as your pocket. Although it is possible to remove the camera and replace each filter in turn, doing so is really not recommended! Without a filter wheel, it’s a tortuous task to…

FOCAL REDUCERS AND FIELD FLATTENERS

Many deep-sky objects, particularly nebulae, are quite large, so a large sensor and a short focal length telescope are required in order to fit them into the field of view. But large sensors tend to accentuate a problem inherent in most telescopes – that of field curvature. Most telescopes produce a curved focal plane yet a CCD’s sensor is fat, so if accurate focus is achieved with stars at the centre of the field of view, those towards the field edges will be out of focus and will take on a stretched appearance. This effect is made worse by using ‘fast’ optics, yet fast telescopes with their small focal ratios of around f/5 and f/4 are desirable for capturing the dim light from deep-sky objects. Luckily, there’s an elegant solution…

FOCAL REDUCERS AND FIELD FLATTENERS

1 Luminance With the luminance (infrared) filter selected, good focus and the autoguider running, start imaging. It pays to capture luminance first so that if the clouds roll in during the session, you’ll have something worthwhile for your trouble. Take at least 10 480-second exposures. 2 Red channel Once you’ve got the luminance data capture under your belt you’ll have a pretty good idea of what your image is going to look like, so change the filter to red and carefully re-check the focus, adjusting it if necessary. Take six 240-second exposures, binned 2x2. 3 Blue channel With NGC 7000 high in the sky, change to your blue filter and check focus. If you’re using an ED doublet refractor and parfocal filters, it’s likely the focus will match the red…

We’ve shown you how to make natural-colour images using a mono CCD camera and broadband red, green and blue filters, but narrowband filters can also be used to produce colour images with a twist. Although narrowband data can be combined in much the same way as RGB data to produce images with fine detail, these images will show false colours. RGB data is assembled in order of decreasing wavelength, with red (740nm-625nm), green (565-520nm) and blue (500-435nm) assembled in that order. If you follow the same ordering procedure using the popular emission lines of Ha (656nm), OIII (501 and 496nm) and SII (673 and 672nm), you can see that SII is ‘redder’ than Ha. So SII would be mapped to the red channel, Ha would be mapped to the green…

Narrowband filters aren’t limited to depicting objects in unusual ways. Not only can they be used to produce natural colour images, but also there are sometimes advantages to using them over broadband filters. The OIII emission line’s place in the spectrum is right between green and blue, so it could be used to produce either a green or a blue channel. Even better, the same data can be used to produce both the green and blue channels in the same image. This has the advantage of reducing imagecapture time if the conditions are likely to deteriorate, which is why we recommended that you record it immediately after capturing your Ha data. Simply map your Ha data to red and your OIII data to both green and blue, and you’ll be…

1 Combine and reduce star hues With your masters aligned and combined, you’ll see a lot of green. But take note of the magenta stars too: these must be dealt with first. Select Filter > Noise > Reduce noise > Reduce colour noise. Set this to 100 per cent, which will remove the unwanted star colour. 2 Turn greens yellow The first step in transforming your image to the palette’s gold and turquoise is to convert the green colours to warmer ones. Select Image > Adjustments > Selective colour, then Greens from the menu. Reduce the cyan level to -100 per cent and the magenta to -30 per cent. 3 Add golds The next step is to enhance the fairly light yellow detail further by making it more golden. Select…

TAKE YOUR ASTRO IMAGING TO THE NEXT LEVEL

A strophotographers constantly strive to improve their images by whatever means they can – it’s in their nature to do so! These improvements normally come about in small steps made possible by using longer exposures, improved focus, more precise tracking, more sensitive cameras, exotic filters and better optics. Each small change adds to the whole, so that better contrast is captured in dim galaxies and nebulae, sharper images with greater colour saturation are produced, star shapes are improved and hitherto unseen features are revealed. So what do you need to do if you really want to take it up a notch? Here we detail some of the kit you can use to help take your astro imaging to the next level. > OUR EXPERT Steve Richards is a seasoned deep-sky…

TAKE YOUR ASTRO IMAGING TO THE NEXT LEVEL

GRAB AN AUTOGUIDER

One of the most obvious ways of improving your deep-sky images is to take longer exposures to capture more detail. The trouble is, doing so puts greater demands on your mount, because it has to accurately track your target for longer periods of time. But with an autoguider you can give your mount a helping hand. Autoguiding uses a second camera to take a continuous series of short exposures of a ‘guide star’ near to the object being imaged. The position of the guide star on the camera’s sensor is analysed, and if the star starts to move in any direction the autoguider issues a command to the mount instructing it to follow the star’s movement. This second camera can either be connected to a second telescope attached to the…

GRAB AN AUTOGUIDER

FINE FOCUS WITH A BAHTINOV MASK

Deep-sky objects are relatively dim and so are very difficult to focus on. But using a simple device called a Bahtinov mask can simplify the task enormously. To use a Bahtinov mask, simply place it over the front of your imaging telescope and then point the instrument your at a bright star near to the object that you’re intending to image. The numerous cut-outs in the Bahtinov mask produce a very specific set of diffraction spikes, in the same way that the spider vanes of a Newtonian reflector cause a simple cross to appear on images of bright stars. But instead of a simple cross, the mask produces an X-shaped cross bisected by a horizontal line. Adjusting the telescope’s focus while taking a continuous series of short exposures of the…

FINE FOCUS WITH A BAHTINOV MASK

MAX OUT YOUR MOUNT

Without doubt, the most important piece of an astro imager’s kit is the mount. The mount supports the weight of the rest of your imaging equipment and moves it with precision so that it tracks the apparent movement of the sky, which is crucial for crisp astro images. It’s important that the mount’s payload capacity is not exceeded – the best tracking results are achieved with the mount operating at well under its rated capacity. If you’re close to your mount’s payload capacity, then you probably need to upgrade to a larger mount. But it’s not all about capacity. An upgraded mount could also introduce some valuable new features, such as periodic error correction (PEC). PEC is a process in which the movements required to correct a mount’s periodic tracking…

MAX OUT YOUR MOUNT

UPGRADE OPTIONS

MESU 200 Price: £5,399.00 Payload: 100kg Supplier: www.modernastronomy.com The unusual look of the PC-controlled Mesu 200 is a triumph of design over art. The Mesu 200 uses a friction drive that results in a backlash-firee mount capable of handling a huge payload. SKY-WATCHER NEQ6 PRO Price: £1,179 Payload: 18kg Supplier: www.opticalvision.co.uk This is the workhorse mount for many advanced imagers, as it provides unbeatable payload capacity for its price, responds very well to autoguiding and can be fully laptop-controlled. PARAMOUNT MYT Price: £6,550 Payload: 23kg Supplier: www.iankingimaging.com The Paramount MYT combines portability with a high level of accuracy thanks to automated sky modelling and ‘Pro-Track’ software that eliminates errors from refraction and fexure.…

UPGRADE OPTIONS

UP YOUR SENSITIVITY

The sensors in CCD cameras have a grid of electronic photoreceptors. Photons falling on these receptors are converted into electronic charges, which are then read by the camera. More photons means more charges, meaning greater light levels. Sensors with more receptors are able to record the varying light levels that fall on them and record these for processing into an image. DSLR cameras use large sensors (typically APS-C type, 22.2 x 14.8mm in size) and because of economies of scale in their production, they’re great value for money, hence they’re a popular choice for deep-sky imaging. But the sensors in DSLR cameras are very sensitive to infrared wavelengths of light, so manufacturers fit an infrared blocking filter to tame this, as it’s rarely wanted by photographers. Unfortunately, these filters cut…

UP YOUR SENSITIVITY

UPGRADE OPTIONS

ATIK HORIZON Price: £1,549.00 (mono) £1,219.00 (colour) Pixels: 4,644 x 3,506 (17.6 x 13.3mm) Supplier: www.widescreen-centre.co.uk The Atik Horizon houses a cooled, 16MP sensor with low read noise and fast read speeds. It makes a great option if you’re looking to upgrade from a DSLR and comes in monochrome or one-shot-colour versions. STARLIGHT XPRESS TRIUS-SX25C Price: £2,525 Pixels: 3,024 x 2,016 (23.4 x 15.6mm) Supplier: www.widescreen-centre.co.uk Matching the size of a typical DSLR camera sensor, this one-shot colour CCD camera is perfect for wide-field imaging and can be brought down to 35°C below ambient temperature with its cooler. QSI 683 WSG-8 Price: £3,510 Pixels: 3,326 x 2,507 (36 x 24.7mm) Supplier: www.iankingimaging.com The QSI 683 WSG-8 combines a large monochrome sensor with an eight-slot filter wheel and a built-in off-axis…

UPGRADE OPTIONS

INVEST IN NARROWBAND FILTERS

There are two ways of capturing colour images with modern cameras. The first is to use a one-shot-colour CCD or DSLR to capture a full-colour image in a single shot. The other involves a monochrome CCD camera, but this requires a different approach. With a monochrome CCD you capture three sets of images through red, green and blue (RGB) coloured filters, then combine them afterwards into an RGB colour image using processing software. The main advantage of a mono CCD camera is that you can also image through a range of other types of filters to capture all sorts of extra detail. Standard RGB filters are known as ‘broadband’ filters but there are also various ‘narrowband’ filters available, the most popular being hydrogen-alpha (Ha), oxygen III (OIII) and sulphur II…

INVEST IN NARROWBAND FILTERS

OPTIMISE YOUR OPTICS

Good quality optics are needed if you want to avoid chromatic aberration, where coloured fringes appear around bright objects (the result of different wavelengths of light being focused at Þ The PrimaLuce Lab different distances from the Airy APO 65F is one example of a high-quality objective lens of refractors). Other apochromatic refractor issues include field curvature in refractors and coma in reflectors, both of which result in poorly shaped stars towards the edges of the field of view. Modern ‘ED’ refractors use special low-dispersion glass elements to focus two wavelengths of light to the same point, avoiding chromatic aberration. But the best results are achieved with a threeelement apochromatic (apo) lens, which offers even better correction. Typical high-quality apo refractors include the Astro-Physics Starfire EDF, William Optics FLT 98,…

OPTIMISE YOUR OPTICS

MODIFY YOUR DSLR

Although DSLR cameras can capture great deep-sky images, the lack of red sensitivity caused by their infrared filters will eventually become a limiting factor. Fear not, these cameras can be modified for astrophotography. Common modifications include removing the infrared filter completely or replacing it with a filter that passes the hydrogen-alpha wavelength but cuts the unwanted infrared wavelengths. You can make either modification yourself, but there are risks involved as a complete strip-down of the camera is required, invalidating any warranty. Various online resources explain the process in detail if you enjoy a challenge and accept the potential risks, which include destroying the camera. Canon and Nikon used to produce suitably modified cameras – the 60Da (APS-C) and D810A (full frame) respectively – but these have been discontinued. However, several…

MODIFY YOUR DSLR

IMAGE PROCESSING

A very important part of modern digital astrophotography is image processing. In fact, it can come as quite a shock to realise that the amount of time spent processing and tweaking the most successful astrophotos completely outweighs the time spent capturing them. The range of processing functions available in image-editing software is quite bewildering and knowing which ones to use to achieve the results you want can be quite daunting. But don’t worry – this is what we’ll be covering in this section of The Complete Guide to Astrophotography. In this image processing section, you’ll find clear, useful explanations of which processes to use when – such as how to rid your night-sky images of the orange cast caused by light pollution, as well as step-by-step walkthroughs on essential processing…

IMAGE PROCESSING

ESSENTIAL SOFTWARE

Layer-based photoediting programs Layer-based photoediting programs, such as GIMP (fireeware) or Photoshop will allow you to manipulate the content of an image. The layers allow you to make fine adjustments to certain aspects of an image before combining the layers together into the final image. Software like this is great for final image tweaks. Deep-sky processing programs Specialist programs, such as AstroArt, ImagesPlus, Astro Pixels Processor, Star Tools, as well as fireeware DeepSky Stacker and Iris, can help you get the best out of long-exposure deep-sky images. PixInsight is a powerful image-processing system, removing the need for other programs such as photoediting software. Registration and stacking software The process of registration and stacking is relevant to both Solar System and deep-sky imaging. It’s used to reduce image noise by averaging…

ESSENTIAL SOFTWARE

SOLAR SYSTEM OBJECTS BASIC TECHNIQUES

To get you started in image processing we’ll be looking at how to handle Solar System objects – the Sun, the Moon and the brighter planets. After going through a basic image building method we’ll look at two important adjustment tools: levels and curves. Both allow you to alter specific tonal regions of your shots, which is useful for extracting detail from deep shadow or for pulling back bright exposed regions. Levels is the simpler of two, with only three adjustment points, while curves offers complete fireedom of adjustment. Mastering both is vital. We’ll also cover the tricky issue of planetary colour and introduce some simple techniques to help get filtered imaging colours right. Finally, we’ll discuss some of the related techniques that can be used to enhance planetary detail.…

SOLAR SYSTEM OBJECTS BASIC TECHNIQUES

The best full-colour planetary images are achieved using a high frame rate monochrome camera fitted with swappable red (R), green (G) and blue (B) imaging filters. But in order to get to a final colour image, we need to perform a bit of construction work. High frame rate cameras record movie sequences for each filter as AVI files, and these initially need to be processed with a registration and stacking application such as RegiStax or AutoStakkert. Programs like these will select the best individual frames from an AVI movie file and stack them on top of each other. At the end of this process you get a sharper, brighter, more detailed image for each of the RGB filters, which you can then use to build your final image. We’re going…

1 Red is your master Open the red (R) image and resize it to 200 per cent to make alignment easier. This will be your master alignment image so save it under a new filename to reflect this. 2 Green, then blue Open the green (G) image and resize to 200 per cent, then select all and copy. Paste the G image into the R image from step 1 as a new layer. Repeat with the blue (B) image. 3 Nudge to align Hide the B layer, then nudge the G layer so that it aligns with the R image – toggle the G layer’s visibility to check its position. When aligned, repeat the process with the B layer. 4 Crop the edges Nudging the layers may reveal gaps around…

USING LEVELS AND CURVES

The levels and curves adjustment tools let you target shadows, highlights and mid-tones in an image. Both have similar functionality, but the levels tool is simpler and easier to use than the curves tool. For the best quality, work with 16-bit rather than 8-bit images, if your software allows. Levels shows a histogram of the number of pixels for each tone in the image and restricts you to just three sliders that alter the black, white and mid-points for an image. The black point defines a threshold pixel value below which all pixels are regarded as pure black; the white point defines the threshold above which all pixels will be regarded as pure white. The mid-point defines the mid-value between black and white, altering the intermediate tones to fit your…

USING LEVELS AND CURVES

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