Guest post: Miller’s guide to astrophotography September 1, 2010

It’s time for our second guest post here at the Gutter. Today Miller Crawford from The Fourth Circle is sharing his tips for easily capturing lovely images of the night sky.

Spend a few minutes surfing NASA’s Astronomy Picture of the Day archive and the overwhelming beauty of our universe might just bowl you over. The floral fronds of the Orion Nebula; the delicate wisps of a supernova remnant; the icy blue hue of the gas-strewn Pleiades; the perfect mathematical trace of a spiral galaxy: space is dead pretty, frankly.

Sadly, our eyes aren’t sensitive enough to capture all that beauty. This is where astrophotography comes in. Long exposures let the light from faint astronomical objects build up on a sensor or photographic film, allowing a camera to capture details completely invisible to the naked eye. Cutting-edge astrophotography is more than a hobby – it is a serious, demanding science that requires a bucketload of skill, concentration and patience, not to mention a telescope with a solid and accurate motorised mount and a beefed-up computer with complicated image processing software. Looking at some of the photographs in the Astronomy Picture of the Year galleries (like the ones here, here and here) can be simultaneously awe-inspiring and daunting. Sure, you’d love to take such incredible photos yourself, but how on earth do you get started in such a daunting field?

Thankfully, astrophotography isn’t just for the gadget-heavy expert; you can take satisfying astrophotos with a more modest camera setup. All you need is some knowledge of the sky, a little patience and realistic expectations. A stunningly detailed image of the Orion Nebula will be beyond you at first, but soon you’ll be able to capture stunning vistas of our galaxy with surprising ease. So here it is, a guide to astrophotography for beginners, by a beginner.

First of all, you’ll need a digital single-lens reflex (DSLR) camera. There are many brands out there to choose from, but the most popular are Canon and Nikon. Entry-level models are fine; the more expensive models are more complicated to use, and their weight can cause stability problems. A compact digital camera is less useful; you need the ability to take arbitrarily long exposures, for example. Apparently they are excellent for lunar and planetary photography, but that’s another story. An old film SLR is another option you might choose; the growing scarcity of film is a problem, and there are other technical issues that must be addressed. I’ve tried both film and digital astrophotography, and the latter is much easier.

What about lenses? The lens that comes with the camera should do you at first – you’ll be able to take some nice wide-angle shots with the 18-55mm zoom lens that comes with a Canon DSLR, for example. A telephoto lens can be useful to play with as well – I have a 55-250mm zoom lens I use to capture the Moon and various deep sky objects.

Last of all, you’ll need a tripod – as tall and stable a model as you can afford. Astronomical objects are faint, and long exposures are needed to capture them properly. Shaky hands and astrophotography don’t mix! A good solid spot to rest your camera will do at a pinch, but you’ll be very limited as to where you can point it.

Start by taking a simple photograph of the night sky. Wait for a clear, moonless night; get away from city lights if you can. Put your camera on a tripod and aim it at your favourite constellation (Orion is a great place to start) using a wide-angle lens. Scenery or trees can add interest to composition. Set your camera’s ISO number to 400 or above and the f-number to the lowest your camera will allow. Then set your exposure to “bulb” so you can control exactly how long the camera shutter stays open for. Manual focus on infinity, then take a 30-60 second exposure. Your photo should show the colours of the stars; it will also show stars too faint to see with the naked eye and maybe even a few deep sky objects. The photo below is a 60-second exposure of Perseus using an ISO of 1600. Many faint stars are visible and the famous “Double Cluster” is lurking behind the tree to the centre-right of the image.

Alternatively, your photo might be a fuzzy mess. Don’t worry: focussing can be a tricky business in astrophotography, and that simple statement “manual focus on infinity” gets entire chapters devoted to it in astrophotography books. Using your camera’s autofocus won’t work on the sky, hence the need to use manual focus. Trouble is, stars are faint and difficult to see through the viewfinder, making precise focussing tricky. If you’ve got a DSLR that offers a “Live View” LCD screen function life gets much easier. Aim at a bright star, planet or distant light, view it onscreen using Live View’s highest magnification setting and focus until the image is sharp. If your camera doesn’t have this function, you’ll have to focus as best you can through the viewfinder. Take a short test exposure and zoom in on stars in the resulting photo. Adjust the focus if they look blurry and repeat until you’re satisfied.

Camera shake is another problem that can spoil your astrophotos. Touching the shutter button vibrates the camera, meaning light from a star briefly gets spread out across the camera sensor. The longer focal length of lens you use, the more obvious camera shake becomes. You can avoid it by using the timer mode on the camera or a remote control that triggers the shutter from a distance. I use the latter method – the £20 spent on the infrared remote was money well spent!

That’s not enough, though. A mirror inside a DSLR diverts the light coming through the lens to the viewfinder. When the shutter is released, this mirror flips up and lets the light land directly on the camera sensor. This flipping action causes internal vibrations, again smearing starlight across the sensor. Many DSLRs have a function called “mirror-lock” to help alleviate this. Switch this function on and the shutter opens a few seconds after the mirror flips up. By then the vibrations have died down, giving you a shake-free photo!

Once you’ve taken a few constellation shots and become used to manual focussing, you could try to capture some “star trails”. The Earth’s rotation causes the stars to move across the sky in circles around the celestial poles. In long exposures, stars become streaks of light instead of point sources. Set your camera’s ISO number to a slightly smaller number (200 or 400 perhaps) and set the exposure time to “bulb”. Press the shutter button once, wait 10 minutes or so and then press the shutter button again. The resulting photo will show colourful trails instead of point like stars. Nearby sources of light pollution should be obvious too! Longer exposures will give you longer trails, and if you shoot near the North celestial pole you’ll see the stars forming concentric circular arcs around an unmoving Polaris. Below you’ll see a 5-minute exposure of the Winter Triangle at ISO 100. This photo was taken near the centre of Edinburgh, and the light pollution is all too apparent!

If you want to photograph the sky without star trails you’ll have to limit your exposure time. The bigger the focal length of your lens, the less time you have before the stars start trailing. With my 18mm lens, a 60 second exposure yields stars that look pretty much like points; at 250mm even a 5 second exposure gives noticeable star trails.

Astrophotos often look a little dark and muted; how can you make them look better? Entire books are devoted to the intricacies of image processing, but adjusting the exposure levels, contrast and histograms of your photos using simple software can produce great results. There are lots of programs out there that let you do this; even the basic features offered by the likes of iPhoto really improves contrast and brings out hidden details. Most of the photos in this post have been adjusted slightly in this manner.

Capturing the Milky Way is a simple route to excellent astrophotos. Consult a star atlas and pick a constellation the Milky Way passes through. Good summer bets are Cygnus and Aquila for those in British latitudes; if you’re far enough south, Sagittarius and Scorpius house the spectacular central region of the Milky Way. In autumn aim for Perseus and Cassiopeia, and Orion and Gemini make great winter targets. Use the highest ISO setting and the shortest focal length you can and expose for 60 seconds (or even 90, 120 or 180 seconds if you don’t mind small star trails). The richness of the Milky Way should be obvious in the resulting photo; you’ll notice brighter patches and dark dust lanes cutting across the band of light. Depending on where you aimed, you might even capture star clusters, globular clusters or nebulae.

Below are a few Milky Way photos I took on holiday in Greece this year. Despite the hideous light pollution to the south, I still managed to capture the Milky Way, some open clusters, a globular cluster and some famous nebulae.

If you have a telephoto lens with a focal length of 200mm or above you might want to try photographing the moon. You can let the camera adjust the exposure automatically if you make sure it takes the reading from a spot centred on the moon rather than an average reading over the whole frame. Alternatively, exposure times for a variety of moon phases, f-numbers and ISO settings are available in Michael Covington’s book Astrophotography for the Amateur or at http://home.hiwaay.net/~krcool/Astro/moon/howtophoto/ . Here’s an example I took earlier this year – a zoomed-in portion of a shot taken with a lens set at 250mm. The exposure time was 1/250th of a second. It’s a little blurry, but craters and seas are plainly visible.

If you’re photographing the crescent moon, a longer exposure (1 – 2 seconds or so, depending on the camera settings) will capture Earthshine, the faint glow on the unlit portion of the Moon caused by light reflected from Earth. The lit crescent will be horribly overexposed, but seas and craters on the unlit side will be dimly visible, as shown in the 2-second exposure below.

What else can you do with a telephoto lens? Capturing Jupiter’s moons is easy enough, and Sky and Telescope’s excellent javascript utility can help you identify the moons. In the photo below I used a focal length of 250mm and a 2.5 second exposure; from left to right we have Ganymede, Jupiter, Io, Europa and Callisto.

You could also aim for objects like the Pleiades, the Orion Nebula or the Andromeda Galaxy. Just experiment with different ISO settings and exposure times! A 20-second exposure of the Orion Nebula at ISO 1600 shows trailed stars but captures the glowing gas very well.

So what comes next? Piggyback astrophotography is the next big step; here you put your camera on a motorised mount (often the side of a telescope!) that follows the sky’s motion, allowing you to capture long exposures with pinpoint star images. This really helps you capture faint deep-sky objects; virtually all of the stunning astrophotos out there will have used guiding. The ultimate thing to aim for is prime focus astrophotography, where you attach your camera to a telescope on a motorised mount and image at magnifications a telephoto lens can only dream of. You can immerse yourself in sophisticated image processing techniques such as unsharp masking, deconvolution and other arcanely named things. Then you can start image stacking, where you combine multiple images of the same object to increase detail, dynamic range and contrast.

All of this is another story though, and it’s one I can’t tell. I’m still a beginner, but with any luck I’ll keep getting better. If you’re eager for more information, there are loads of useful websites out there devoted to astrophotography, and Michael Covington has written two excellent books on the subject – Astrophotography for the Amateur and the more recent Digital SLR Astrophotography. Hopefully I’ve convinced you that even a rank amateur can take satisfying astrophotos and have a lot of fun doing it. Go on, have a try – the results may surprise you!