Wab, I did take a few other shots if you want to try to fiddle with and stack them.
Sure, just put them on your dropbox. I'll give it a try.
How do I work the dark frames? I know I have to put the cover on the scope, but what do I do after that?
Darks provide a calibration to account for system artifacts like hot pixels and amp glow. (You camera looks pretty clean but with longer exposures might start showing more. My guess your biggest immediate bang for the buck would be from stacking.)
You don't even to have the camera attached. Ambient temp is important. Put the cap on your camera and take a series of images at the same ISO and duration as you "Light" or image frames are going to be. If you expect a large temp drop during the session I'd take half the darks before and half after you image frames to average out the temp drop. Ambient temp greatly effects the amount of noise in the image.
Just a rough example:
I'm planning to take 20 x 5min exposures of M42 at ISO800.
So before my session I turn on the camera and let it warm up a little then take maybe 5 darks (cap on) at ISO800 at 5min.
I then set up and take 20 x 5min @ ISO800 images of my target (light frames).
When I'm done I take off the camera, put the cap on, and take another 5 x 5min @ ISO800 dark frames.
(For the kind of image I do I usually don't need to mess with flat frames but you might want to get to that someday.)
Later, with software, I stack and average the 10 dark frames toget a master dark frame to use for calibration.
I then align, stack, and average the light frames to get a master light frame.
The software will then subtact the master dark frame from the master light frame to remove systemic noise.
You can find this stuff on the net. However I can recommend "A Guide to Astrophotography with Digital SLR Cameras" by Jerry Lodriguss. Its a great resource to get started with.
The idea behaind stacking the light frames is that the noise is distributed randomly in each frame. The image should be consistent between each frame. So averaging them together tends to reinforce the image and weaken the noise. Noise will decrease to the inverse square Root of the number of frames. stacking 4 images cuts the noise in half compared to a single image.
There is a trade off between taking a long exposure versus a shorter exposure. A long exposure tends have worse tracking errors creep in. Many bad things like a passing plane or bumping the scope might ruin the frame. On some cameras you start to get amp glow. On the other hand a shorter exposure has more noise and might not be long enough to separate data from the background noise. ON the other hand if you are taking 20 5min images and bump the scope you only ruin 1 sub-frame and still have 95 min good data. If you were taking a single 100min exposure, you would be hosed.
However, digital cameras have a magical property. Assuming the exposures are at least be long enough to separate the image data from the systemic and background noise, then their combined exposure time is virtually equal to a single exposure of the total. With my camera and skies, a 5min sub-frame exposure is ideal. So 20 x 5min exposures is virtually equal to a single 100 min exposure without all the other problems. Also, stacking allows you to use a much higher ISO setting that normal because the stack can overcome the increased noise due to the higher ISO settings. Its worth the extra sensetivity. I often use ISO1600 if I'm taking 20 or so sub-frames.
Regards,
Wab
