Astrophotography with a DSLR and Tripod (Part One)

Also on Flickr YouTube Facebook Twitter

Most people, on hearing the word "Astrophotography", will immediately form a mental picture of a telescope. That's understandable, as most people tend to think that Astronomy itself revolves around the use of a telescope. The latter is partly true, although you can get a lot of pleasure from the night sky just by using your eyes, and even more so if you have a pair of binoculars.

Astrophotography, however, is not solely the domain of the telescope enthusiast. Very good images of deep sky objects can be obtained with just a camera. You'd be amazed at how much is up there, and how wide an area of the sky some of this stuff covers, but which we can't see purely because it's all too faint. For example, the Andromeda Galaxy, the nearest spiral galaxy to our own, is about six times the width of the full moon in the sky - quite amazing (I wonder if nocturnal animals can see it?). The core of Andromeda, the bright bit in the centre, can actually be seem with the naked eye if the skies are dark enough (some lucky people actually have skies that dark!), and can certainly be seen with binoculars, although it will be nothing more than a faint smudge.

Although the image below was taken with the aid of a tracking mount (not a basic tripod), it gives you an idea of what can be done with just a camera and 200mm kit zoom lens:

M31 The Andromeda Galaxy

This image is actually made up of thirty 4 minute exposures, so it was necessary to use a tracking mount - a mount that follows the apparent movement of the stars across the sky - otherwise the stars would have been long streaks. Andromeda isn't an ideal target for a 200mm lens - 300mm would be better, but I don't own one of those ;)

But - you don't need a tracking mount to get some very nice images of the night sky. If you have a camera and a tripod, you're adequately equipped to take some decent images. A DSLR (Digital Single Lens Reflex) camera is preferable, rather than a compact, as ideally you need full manual control of the camera. If you've never done any form of night sky photography before, and don't know where to start, try this simple exercise:

First of all, make sure you are familiar with your camera, not just how to use the thing but the menu settings and options that are available to you. Different makes of camera have different settings but all should have the following options somewhere in the menus:

1. CSM/Setup Menu: Set this to Full, so that all the menu options are visible.
2. White Balance: Set this to Daylight, which seems to work OK. There isn't a setting for Pitch Black in the Middle of the Night!
3. ISO Auto: Off. You need to set the ISO manually.
4. ISO: Set this to at least 1600. If you have a modern camera (modern as at 2012) then you may have ISO settings much higher than this available to you. If you do, you could try 3200, or even higher, but the higher the ISO setting, the noisier the image, so don't overdo it. Try 1600 or 3200 first. 
5. Auto Image Rotation: Off, definitely.
6. Image Quality: Set this to RAW. RAW files hold more information, but the file size is larger. That shouldn't be a problem with a decent sized memory card in your camera.
7. Long Exposure Noise Reduction: If this is available, turn it on. This will double the time it takes the camera to dump the image to the memory card, but that won't be a problem for this exercise.
8. You want to be using remote shutter release, either infra red or cable (or whatever system your camera uses), so set this option, whatever it may be called in your camera. This assumes you are equipped with the necessary bit of kit to remotely control the shutter release - if not, don't worry too much.

Constellation of Orion, 20mm, 15sec, ISO 3200 by Dave DeHetre

Now, a couple of things that you probably already know, but I'm going to mention just in case: A Prime Lens is a fixed focal length lens i.e. not a zoom. A Fast Lens is a lens with a maximum aperture below f4 (some consider f4 to be fast - it's a matter of opinion). Generally speaking, a fast, prime lens is best for astrophotography, but if you don't own one of those (like me), a slow, zoom lens will do just fine (but not too slow!).

OK. Set your camera to full manual mode (this may require a setting on the lens as well). Put your camera on your tripod and go outside, preferably when its dark and moonless, and preferably when there are no clouds in the sky! Make sure you can see some stars - some nights are cloudless but the visibility is terrible. The more stars you can see (and the less they twinkle), the better. Make sure there are no lights on in the house shining at you - you want it as dark as possible. This also apples to neighbour's lights, but I wouldn't suggest you knock on their door and ask them to turn them off! If lights are a problem, at least make sure you can point your camera away from the lights. Use a focal length of about 50mm or less (wider), select the widest aperture (lowest f number), set the focus to infinity, and select a shutter speed of about 10 seconds. Now point your camera at the sky - anywhere will do for this exercise, but stay away from any areas of light pollution if you can (the sky will be orangy where there is light pollution). Make sure the tripod is nice and steady, and release the shutter. Now keep still until the shutter closes - no really, keep still; cameras on tripods are very sensitive to vibration.

Now take a look at the result. You may be surprised at how many stars are in the image compared to how many you can see in the sky - the image should show quite a few more. That is your first astro image (well, it may not be, but I'm assuming you've never done this before!) Zoom in as as far as you can on the LCD screen and examine the stars. If you are using a good quality prime lens, they may be in focus, but the chances are they are slightly out of focus. You'll know this because they will be slightly "blobby" as opposed to nice sharp points of light. If setting the camera to infinity was all that was required to achieve focus, a DSLR astrophotographer's life would be a lot simpler, believe me! Unfortunately, it doesn't work like that - most lenses go slightly past infinity (which doesn't sound possible, does it?) so you need to do a bit of work to get perfect focus. If you have a distant street light visible, you may be able to switch your camera to Auto Focus and focus on that, or the Moon if it's out (best to do this on a moonless night though - the Moon's inclined to be a little bright!). If this isn't an option, you'll need to make a tiny adjustment to your focus ring (and I mean really, really, tiny) and give it another go. Zoom in again - do they look better? If not, keep at it, making very tiny adjustments, until you've got the stars as small as possible. This is very much a trial and error thing, and can be quite frustrating (welcome to the world of astrophotography), but keep at it and, as is the case with most things, with practice you'll get better. If you have an M and A switch on your lens (Manual and Autofocus), or something similar, then after each adjustment set it to A. This will lock the focus and stop it slipping, but remember that if you need to make another adjustment you'll need to switch it back to M, otherwise you won't be able to move the focus ring. If you don't have this switch, and you're having problems with the focus slipping (this can be the case with inexpensive zoom lenses), then a small blob of blu-tack conveniently placed will help to hold the focus ring in position, but be aware that this may leave a residue (there, you can't sue me now!)

Orion again, 50mm, f1.8, 5sec, ISO 3200 by Fred Agustin. Very dark skies I would assume :)

You may also find that the stars are slightly sausage shaped, which is caused by the apparent movement of the stars across the sky - this will depend upon the focal length you used and the shutter speed. There is something called The Rule of 600 which you can use as a guide: just divide 600 by the focal length you are using and this will give you the longest shutter speed you can use without getting star trails e.g. 50mm focal length = 600/50 = 12 seconds. Very useful.

You may want to target a constellation you are familiar with, in which case you'll need to use an appropriate focal length to get it all in the frame. You could end up with a portfolio of images of all the major constellations, or perhaps all those in the Zodiac - the possibilities are endless! Good luck, and don't get frustrated and give up - this hobby takes practice and perseverance.

I've reproduced throughout this blog a few images I've discovered on Flickr that were taken using this method, with appropriate credits. In the next blog, I'll cover the stacking of images to reduce the noise and enhance the signal, which should give you a result you can be even more proud of - so watch this space for Part Two :)

The Plough, or Big Dipper. 20mm, f2.0, 29sec, ISO 400 by Paul Richards

OK guys, have fun out there. Feel free to post any images you take using this technique (or any technique for that matter) to my Facebook page - there's a link at the top of this post. It would be good to see them, and at least then I'll know I'm not talking to myself here. :)

As always, may your skies be forever cloudless :)


Doug