This article is a brief primer on how you might make your first attempts at astrophotography. It was written by Steve Kimmins in 2010 but remains a good starting point
There are three options:
- Use a simple ‘point and shoot’ camera
- Use a webcam
- Use a DSLR (Digital Single Lens Reflex) camera
Your typical astronomical target will be the Moon or the planets. If you have a special filter for the Sun, or project the Sun’s image onto a card from your telescope then the Sun is an option too. In general fainter objects in your telescope (star clusters, galaxies, etc) aren’t possible because they will require a longer exposure time than that type of camera normally provides – but check how long an exposure you can take – and control!
Using a ‘point and shoot camera’, firstly set up your telescope as you would for a normal visual observing session – with an eyepiece in place to get a magnified image. You now need to have an arrangement to hold the camera in the place your eye would normally be. This is called ‘afocal’ photography (http://en.wikipedia.org/wiki/Afocal_photography). Easier said than done. You can make up some sort of tube to fit over the eye piece and lens, or a ‘meccano’ type of framework with a screw (1/4” thread) to fit into the threaded hole most cameras have on their lower body to fit onto a tripod. Easier is to buy a purpose built frame or adapter from an astronomical supplier or on the internet. An item of no more than £20 (in general) will clamp around the eyepiece and also screw into that tripod thread on your camera – there will be a means for moving the camera around until its lens sits over the eyepiece lens.
Then look through the camera viewer and focus the telescope as normal until you focus the planet/Moon to your desire. Then play around with exposure times (if you have much control on your automated camera) and see what you get, deleting the images that don’t make your grade – though maybe later after you have a good chance to check them indoors. The shorter but fainter exposures, that can be enhanced later, may give the sharper images when you inspect them carefully.
The set up can be very similar to that for option 1 – supporting the webcam over the eyepiece. But another preferable route is to unscrew the plastic lens from the webcam, and support the webcam over the eyepiece tube of your telescope but without the eyepiece (this is called ‘prime focus’ photography). Again there are cheap adaptors available that screw into the webcam lens thread at one end and fit into the eyepiece tube at the other.
You will also need your laptop computer at your side so you can view the webcam image – think about that carefully as it can get pretty damp (dew) as well as cold at night and your laptop may not like it!
You will need to play with your webcam imaging software to get a bright enough image on your laptop using a combination of a long enough ‘frame exposure’ and the gain and brightness. Once you have a focused image (try any planet or the moon – not stars or galaxies) you can take one-off shots but try taking a short video, maybe 30s to 60s to start. You will get several hundred frames in that time – process them with the free software ‘Registax’ that will combine the better exposures to give you an impressive image, free from the ‘shimmering’ that you often see with a normal visual observation of planets. For more info see http://www.astro.shoregalaxy.com/webcam_astro.htm.
This will use a somewhat more expensive camera than above and will allow you to go beyond the brighter objects in the sky – star clusters, nebula and galaxies become possible. The downside is that to record fainter objects you will need exposures of 30s (-ish) and above so your telescope will need to be motorised to allow it to track the stars automatically. That also requires knowledge of how to ‘polar align’ your telescope. You should mount your camera to your telescope in the ‘prime focus’ mode above (remove the camera lens and attach it with an adapter to your telescope eyepiece tube – again with no eyepiece).
Focus the image by viewing through the camera viewfinder. One tip to help focusing (it is almost impossible to focus on the fainter objects directly) is to first focus on a bright star in the vicinity of your target (which is easier) and then go directly without touching the focus knob, or the camera set up, to your target. Take trial exposures of your bright star to check the focus on the image LCD of the camera. Set your camera to 800ASA; if possible trigger the exposure with a remote control or button on the end of a wire to save on shaking; go for a range of exposures (say 10s upwards) and see what you get! Take multiple exposures of the same object and combine (to simulate an effectively longer exposure) with the freeware ‘Deepskystacker’.
An easy alternative use of a DSLR is to keep a lens on your camera and mount the camera on the side of your telescope tube (‘piggybacking’). In effect your telescope and mount act as a super tripod, especially if your telescope is motorised and ‘polar aligned’. Exposures of 20s plus will show far more stars than you see with the eye. A standard 50mm lens will show a large sky area – typically a full constellation can be seen in one image; a telephoto lens (135mm or 200mm for example) will show much smaller sky areas with some galaxies, nebulae and star clusters showing details. Piggybacking is very simple and can give rewarding and beautiful views of large areas of the sky.
Options 1 and 2, on brighter objects, can easily be done in light polluted towns. However, option 3 will show the light pollution (as an orange ‘cast’ to the picture) in towns. Try shorter exposures when there is light pollution and combine them later. In the darker countryside option 3 will work much better and longer individual exposures can be attempted.
Image processing, in your warm house after your outdoor session, is a topic in itself. But there are ‘freeware’ image processing programs on the internet and one will almost certainly come with your digital camera on a utility CD. Play around with contrast and brightness settings and on fainter objects try ‘stretching’ the image. For long exposures with option 3 you should also try to take ‘dark’ frames (at least one image with the telescope mirror/lens covered up; to give the ‘noise’ in the absence of an image – a dark frame is subtracted from the main image when processing) and maybe a ‘flat’ frame (image of a uniformly lit screen or field of view to allow for non-uniform brightness across a field of view produced by most lenses; this image is ‘divided’ into the main image during processing). But these extra steps can wait until you are confident in your abilities in taking shots of the astronomical targets first!
Now give it a go and if you have any queries, or want more detail than included here, pop along for a chat at a lecture or observing session.