Alistair (Stravaiger) asked me how I got that recent pic of Andromeda, so I promised him this "how-to" post. Now I'm aware that there are other and better ways of acquiring and processing frames, so please bear in mind that I'm fairly new to this astrophotography malarkey, and that this is an account of the techniques used for this image only.
My basic kit:
- Camera: Nikon D50 body with remote IR shutter-release
- Telescope: Celestron C8-N Newtonian reflector 200mm (8.0") aperture, focal Length = 1000mm, focal ratio: f/5
- Tripod and Mount: Celestron CG-5 Equatorial with retro-fitted Skywatcher Synscan motor-drives and control
The purpose of this little lot is quite simple - the mount is set up so that its axis of rotation is aligned with the axis of rotation of the Earth. The motors drive the telescope around the mount-axis at the same rate as the rotation of the Earth, but in the opposite direction, so that the telescope stays pointing at the same place. The telescope is there to grab the light and direct it into the camera. The camera needs no lens attached, as the telescope, attached to the camera by a T-ring adapter, is acting as a lens of 1000mm focal-length and 200mm aperture.
The basic method is to find the target and take long exposures, these are "light" frames. For this first attempt at Andromeda I took 22 RAW exposures, each 30 seconds long, at ISO 200. The number and duration of exposures required isn't set in stone, but the general rule is "more is better" - longer exposures capture more light, more exposures allow better processing, more about that later. Lower ISO settings give a better the "signal-to-noise" ratio, and hence cleaner, finer images. Other folk use higher ISO settings, and rely on image-processing to get rid of the excess noise. RAW exposures allow post-processing with minimal data-loss, but almost any format will do at a push. The other important exposure is a "dark" frame - taken with the same settings as the others, but with the telescope cap on, so that the only signal detected by the CCD is that from the camera itself. Take it just after you've taken the "lights", the camera should have warmed-up by then. This should provide a black frame which shows up any "hot pixels" and other camera-induced noise, the effects of which need to be negated. I should have taken more "darks".
This method provided 22 light frames like this:
Single light frame, originally in Nikon NEF format, converted to .jpg format to allow uploading
and one dark frame like this:
Single dark frame, originally in Nikon NEF format, converted to .jpg to allow uploading
Most folk then plug these frames into image-stacking software "as is", but I prefer to enhance the light frames beforehand, using Photoshop to tweak the brightness, contrast and saturation before stacking. It's very much a "suck it and see" type of adjustment, I like to see if I'm going to be able to brighten the background stars without burning-out (overexposing) the bright areas. Don't worry if the background gets a bit "noisy", the stacking process deals with that. After tweaking, the light frame above ended up like this:
Single ENHANCED light frame, originally in Nikon NEF format, converted to .jpg format to allow uploading
Now that dark frame - it just looks black, but buried deep within the RAW file data is a lot of info about the performance of your CCD. There's no call for this step during the processing, but this is what it looks like when enhanced to the max - lots of digital noise from "hot pixels" and conditions within the camera, stuff that's there in all of the other frames too, stuff that we want to get rid of in the final image :
Single OVER-ENHANCED dark frame, originally in Nikon NEF format, converted to .jpg format to allow uploading
The next trick is to use image-stacking software to process the frames. For Andromeda I used Deep Sky Stacker because it's simpler to use, but there are others including Registax (lots of tweaking options) and K3CCDTools (great when you get used to it), all of which have free versions available. Just open the program, load the light frames, load the dark frame, tell it to check all of the input frames then tell it to stack them. Then it gets on with the stacking process, finding the stars, subtracting the CCD noise (that you found on the dark frame) from each light frame, and stacking the resulting frames together. This process looks at each frame, and, broadly speaking, averages the data for each pixel across all of the light frames, so if there's the same data in a particular pixel on every frame, it's kept, as the average equals the original, but if it's random noise (in that pixel on one or just a few frames) it gets averaged out and hence is less noticeable. That's why having more frames is good. There's a better explanation here. Cool stuff, eh?
Anyway, after a while (it depends on the processing power of your PC/laptop), it produces a cleaned, stacked image, which it autosaves. This is the one to work with, not the one that it puts up on screen, which has all sorts of auto-adjustments done to it, and which can be dumped. From now on, save your work in a low-loss format, I saved as a .tif file. It's not quite the finished article, it looks fair, but it's full of good data that needs bringing out with a little post-processing:
Stacked image, originally in .tif format, converted to .jpg format to allow uploading
I do the final tweaks Photoshop CS3, but I suppose any decent equivalent will suffice. Small adjustments to contrast, exposure, brightness and saturation are what are required to achieve a pleasing result, the aim being to bring out as much fine detail as possible without overdoing the larger features - it's all too easy to end up with nice bright stars but an overcooked galaxy-core and a grainy background. Colour-casts can be removed, but this can lead to problems with false-colours. Experts tend to use the Levels and Curves functions to tweak the result, (they call it "stretching the histogram") but I only resort to that when all else fails, and I didn't use them on Andromeda. The final action is a touch of sharpening, PS CS3 can do this but I prefer to use the Focus Magic plugin. You'll find the results in the next couple of related posts (or just click the "Andromeda" tag in the post header above).
And that's it. Simple, really!
The good news is that decent results can be obtained with lesser kit. I've stacked pics taken with just camera+zoomlens+tripod, i.e. without a telescope or a tracking mount, and so long as the field of view is wide and the exposures are kept short, any star-trails are minimal and the result can be quite pleasing.
- Make a note of what you do - it's a pain in the ass when you find a method that gives a good result, then can't remember how to replicate it next time out
- Progress slowly, learn from everything you do
- Experiment a lot
- Try using a CCD webcam instead of a dSLR - it's a cheap way to get into imaging planets and the Moon
- Don't go out and spend a fortune - there's no end to the amount of money that you can throw at this game. Second-hand gear is usually very good, especially scopes, which can be amazingly cheap
- Get yourself onto an astronomy forum, they're great repositories of information imparted by folk that have usually learned the hard way themselves. I recommend Stargazers' Lounge, even though I'm banned from there (don't mention my name there - you might regret it!)
- Make full use of the free software available on the web - there are some great packages out there, such as WinStars, Cartes du Ciel, Celestia, Starry Night. There are many other options out there, but you might have to pay for them.