The Andromeda Galaxy, designated M31, is said to be the farthest thing you can see with the unaided eye from a normal dark-sky location – mountaintops do not count.
It’s fairly easy to capture a photograph of the galaxy with a DSLR camera and a good lens, and in this guide we’ll show you how.
But before we get started talking about how to actually take your photo, you’ll need to know how to find the Andromeda Galaxy in the night sky.
More astrophotography guides:
- How to photograph Mars
- How to create a Milky Way mosaic
- Photograph Venus with a digital video camera
How to find the Andromeda Galaxy
There are a number of ways to locate the Andromeda Galaxy, the easiest of which is to start from the Great Square of Pegasus.
Imagine a line from mag. +2.4 Scheat (Beta (β) Pegasi) in the upper-right corner of the square, to mag. +2.1 Alpheratz (Alpha (α) Andromedae) in the upper-left corner. Extend it for the same distance again, then bend the line up slightly to arrive at mag. +2.1 Mirach (Beta (β) Andromedae).
From there, turn by 90º, heading up the sky to reach fainter mag. +3.9 Mu (μ) Andromedae. Just above this is mag. +4.5 Nu (ν) Andromedae. Under darkish skies, M31 can be seen close to this star.
From an urban environment, the galaxy can be quite hard to see due to light pollution, but from a dark site it looks rather tantalising (for more on this, read our guide on how to capture astrophotos from a light-polluted city).
M31 is notably elongated and smudgy in appearance. It also has a reasonable apparent size. And if it looks that good to the naked eye, surely it must look amazing through binoculars or a telescope.
The sad truth is it doesn’t. It looks larger and if you have a sufficiently wide field of view you can also spot its two satellite galaxies, M32 and M110, but M31 itself looks like a larger smudge.
This is to do with the part of the galaxy you’re looking at. M31 is a spiral galaxy tilted towards Earth by 13º. This means that we get to see it as an ellipse in the sky.
If the galaxy were tilted over by 90º, it would appear face on; we would be able to see a bright circular core surrounded by spiral arms.
How to photograph the Andromeda Galaxy
The elongated smudge that we see is just the core of the galaxy. The outer spiral arms are there but they are much fainter and so harder to see.
A large telescope with an aperture of 12 inches or more will start to show the arms, mainly by virtue of the contrast between the arms and the dark dust lanes between them.
The overall size of M31 in the sky is impressive. The bright core ellipse measures 30 arcminutes by 10 arcminutes. That’s an apparent size equal to one full Moon by a third of a full Moon.
A typical mid-exposure image of M31 will reveal more of the outer structure. In an average DSLR shot, this increases the galaxy’s size to 2º by 0.5º, or four full Moons by one full Moon.
The deepest exposures reveal an immense object measuring more than 3º by 1º (six full Moons by two full Moons) in apparent size.
Exploring the outer regions of M31 is fascinating, and not too hard to do as long as you have a driven tracking mount onto which you can mount your camera.
In order to catch the majesty of M31, you’ll need a field of view that can encompass its full size, or slightly larger, to give it a bit of sky context. For an APS-C DSLR this means a focal length in the order of 300mm or less.
For example, on a Canon 60D, a 300mm lens gives a field of view measuring 4.3º by 2.8º, which is about right. As an alternative, you could use a short focal length telescope with a focal reducer.
Whatever arrangement you use, it will need to be attached to a driven mount. If your mount offers autoguiding, then this is even better.
Once you’ve mounted everything, I’d recommend the use of a programmable remote shutter release cable. If you are able to connect your camera to a laptop, there are several programs that can control exposures that way.
One advantage of computer software is that it often provides focus assist functions too.
One basic but very important technique for M31 images is removing the effect of unwanted ‘stuck’ or ‘hot’ pixels. These will appear like stars in your image, but originate from within your camera.
Fortunately, getting rid of them simply entails capping your telescope or lens and taking an exposure of the same length as the main shot.
Once this has been done, it’s a relatively simply job to remove the false stars with image processing software, ensuring that the ones that are left are the real deal.
For more on image processing, read our guide on processing the Andromeda Galaxy in Photoshop.
Follow the steps below and see if you can bag yourself a photo of several hundred billion stars, all in one shot.
You Will Need
- DSLR camera
- 300mm (or less) lens
- Driven mount
- Shutter release cable/laptop with control software
Decide on the lens you are going to use. It could a standard photographic lens or a short focal length telescope, but it needs to have a focal length of 300mm or less. A focal reducer may be useful to bring a slightly longer focal length scope into the required range. Ask your telescope stockist for advice for your particular setup.
Attach the camera to a driven equatorial mount and set the ISO high. Aim at the galaxy and focus roughly. Take a 20-second shot and check the orientation. Rotate the camera so that M31 comfortably fits in the frame. If your focal length is on the longer side, you may need to fit it in diagonally.
Once you’re happy with the orientation, secure everything and focus as accurately as you can, then reduce the ISO. For non-autoguided mounts, try setting the ISO to 800. For autoguided systems, drop it down to 200-400. Lower ISOs will give a better tonal result and reduce digital noise.
The exposure you use depends on many factors, from tracking accuracy to the level of light pollution, so you’ll need to experiment. Start at 30 seconds, then step up in 30-second increments. Use the longest exposure that doesn’t show trailing. Remember to also take a shot with the lens covered. This will be your dark frame.
Open your best image in a layer-based editor. Load the dark frame as the top layer, and set its blend mode to ‘subtract’ or ‘difference’. Reduce orange light pollution in Levels by moving the red channel’s mid-slider right until the image shows a slight green hue. Drag the green mid-slider the same way, with a final tweak on the blue.
Close the Levels window to confirm the change and reopen. Adjust the mid-slider’s position to find a pleasing brightness for the galaxy. If the background is too bright, drag the black slider so it just touches the histogram. Once you’re happy, close Levels. Open the Curves tool, and alter contrast and brightness to suit.
Have you managed to capture an image of the Andromeda Galaxy? We’d love to see it. Find out how to submit your astrophotos to us.
Pete Lawrence is an experienced astrophotographer and a co-host of The Sky at Night.