In this second post on astrophotography, I’m going to look at what we can photograph and how to get started on photographing constellations and star trails.
Taking Photographs
We’ve talked a little so far about choice of focal lengths for camera lenses and telescopes and the need for a mount. Now we need to bring this together and work out what we need to do to take various types of astronomical photograph. I’m going to break this into various subject areas as the requirements vary.
| Subject | Equipment | Difficulty |
| Star trails | Camera or phone, tripod | Anyone can do this with something to support the camera |
| Constellations | Camera or phone, tripod | Anyone can do this with something to support the camera |
| Milky Way | Camera. Phone will need an app to help. Tripod, possibly an equatorial drive | Easy enough to do but requires a bit more effort. You can stack images to improve results. You can also use different focal lengths for different results. This is the point where more equipment such as a GEM becomes necessary equipment |
| Large star clusters, nebulae | Camera with longer focal length lens or short focal length telescope / rich field telescope. Tripod and mount with equatorial drive | Technically more demanding. More equipment needed: a star tracker or GEM is required. Longer focal length lenses or telescopes needed. The ability to process calibration frames will give better results |
| Deep Sky Objects: globular clusters, galaxies, planetary nebulae | Telescope of relevant focal length for target. GEM essential, guiding optional | Technically demanding. Calibration frames essential. |
| Moon | Camera with lens in 400mm+ focal length or telescope. Tripod essential. Driven mount optional, depending on focal length | The Moon is oddly easy and difficult. Close-up images require very long focal lengths and is demanding |
| Planets | Telescope with long focal length and substantial mount with drive | The best way to get good quality images of planets is to stack hundreds of images taken with video. This is highly specialist. You can use a camera, but results with video are better |
| Solar | Requires very specialist equipment | Do not attempt without expert advice and assistance |
I’m going to take these areas in turn and go through what is required. At the simpler end, you will just need your camera and its basic lenses or a phone. You will need some form of support. At the other end of difficulty you will need a lot of specialist equipment.
Star Trails & Constellations
In this section I’m going to look at the first two groups as one since they are closely related and need more-or-less the same equipment.
Anyone with a smartphone with a camera, or a basic DSLR on which you can alter the exposure time, will do. Ideally you will need a tripod and phone users will need one of those phone holders that attaches to a tripod.
You need to use your camera / phone on fully manual. For phone users that means going into the camera app. If it’s not obvious, there are specialist apps for astrophotography with your phone such as Night Cap.
There are several things we need to do: we need to keep the shutter open for quite a long time; we need to have the lens as wide open as possible, to help let in more light; and we need to increase the sensitivity of the sensor b boosting the ISO.
You will need to experiment as what works for you depends in various things like how dark your skies are, how much light pollution there is in our area, and so on.
The difference between a ‘constellation’ image and star trails is how long the shutter is open. If the shutter is open for a short enough time, then the trailing of the stars caused by the Earth’s rotation is not apparent. If the exposure is longer, then we will start to see the stars trailing.
Working Out Exposure Times
To avoid trailing of the images, we use the ‘500 rule’ to work out the limit of the longest exposure we can use which does not trail. We talked about it earlier. The formula is: (500) / (focal length x crop factor) in mm. Here’s a guide to crop factors:
Full-frame: 1
Nikon DX: 1.5x
Canon: 1.6x
4Thirds: 2x
So, for a 50mm lens on a full-frame camera: (500 * 1) / 50 = 10 seconds.
Here are some worked examples:
| Focal Length / Exposure | Full-frame | 1.5x | 1.6x | 2x |
| 24mm | 21s | 14s | 13s | 10s |
| 35mm | 14s | 10s | 9s | 7s |
| 50mm | 10s | 7s | 6s | 5s |
| 100mm | 5s | 3s | 3s | 2s |
| 200mm | 2.5s | 1s | 1s | <1s |
These are indicative – the real world will vary. You can get away with slightly longer exposures if pointed near the celestial poles and shorter nearer to the celestial equator.
What is very obvious, is that exposure times with anything other than a wide lens get to be very short. There are things we can do and the obvious one is to take say 20 exposures of the same target and then stack them together. That will bring out more detail and help to reduce noise too, but there is no substitute for the basic exposure being longer.
For the lenses we’ve looked at above, none will challenge your camera. The maximum exposure length we’ve shown is 21 seconds, and most cameras have built-in shutter exposure times of up to 30 seconds which can be selected.
This is fine for constellations as they have bright stars and your 10 – 20 second exposure will show your constellations quite nicely.
If you want longer exposures for star trails, then you need to either use ‘Bulb’ setting (‘B’ on your exposure dial) or attach an intervalometer or use an app to control the camera if there is one with the needed features.
Aperture
We want as much light as we can get, so we want the lens to be ‘wide open’. Faster lenses allow in more light. A focal ratio of f/2.8 is double the aperture of f/5.6 and lets in 4x the amount of light.
You will need to experiment with your lenses. Some lenses give their best results stopped down one or two clicks from wide open. Just remember that if you do stop down, you will accumulate less light.
ISO
The ISO setting is again an area for experimentation. A lot depends on our individual camera’s sensor and its capabilities and on your local sky conditions. I use a Nikon Z6 and an ISO of 3200 is usable in a dark sky for a 10 second exposure.
My general advice would be to start with ISO 400 and see how you get on. If light pollution is not an issue, then try ISO 800, 1600 etc.
Be aware, that if you are aiming for star trails and an exposure of say 5 or 10 minutes, then you will almost certainly need to drop the ISO to 400, since any background light pollution will just overwhelm the data otherwise.