Mall Astrophotography
home gallery equipment shoot process links

Processing Images

Concepts
Practice

Practice

Some thoughts on some of the practical issues involved when processing images.

Dark Frames

When I take a series of images, I use software that saves them to filenames formed by adding the frame number to a prefix that I give it. A series of 70 Jupiter shots, for instance, might be named jup0.bmp, jup1.bmp, ... jup69.bmp. When I actually start the capture, after focusing and centering, I hold a flat black sheet of foam in front of the telescope and begin the run. After about 5 frames have been taken, I remove the foam and the rest of the exposures are of the target.

Later, when I am processing the images, I start by identifying the first frame that has an image of Jupiter on it, and I discard both it and the frame before it, since either could potentially have been partially blocked by the foam. I then use the remaining dark frames before the change to create an average dark frame. This image is then subtracted from each of the frames that shows the target, and the results are saved in a different directory.

Although it works fine for planetary images for which exposures are very short and dark frames not that important, I have found that simply holding a black sheet of foam or construction paper can cause problems with longer exposures. It turns out that the stray light at my observing site causes the foam to be brighter than the sky background (which is pretty bright, but that's another story...) resulting in dark frames which are lighter than the real images. Subtracting these dark frames results in negative pixel values and generally bad results. I have started taking my dark frames at the end of an exposure run by simply defocusing the telescope - a lot. I suspect this won't work so well if there is anything bright in the frame, but I haven't come to that situation yet.

Also, with the QuickCam, "hot" pixels can sometimes show up or disappear in the middle of a run. There's usually not an awful lot that can be done about this short of manually editing the frames. Sometimes taking dark frames both before and after the image frames can help, but it doesn't work very well with my defocused dark frame technique.


Website © 1999-2000 Emm Graphics