Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 01/22/2015 - 23:12
I have a Canon DSLR with a few lenses and a RC 8" on a GoTo GEM.
What I don't have is a handy way to generate Flat images. I understand about twilight flats but taking them is not always convient. EL panels seem rather expensive for a beginner like myself. Could you point me to another option?
Thanks,
One option for flats is to take pictures of the twilight sky (just after sunset or just before dawn), when the sky is bright enough to overwhelm stars, but not so bright that it saturates your camera. Point the telescope sort of near the zenith around sunset, and start taking a series of images with an exposure time which is long enough to avoid shutter-motion issues -- say, 1 second. At first, the images will be saturated. That's fine -- just keep taking images as the sky darkens. At some point, the sky will reach the right brightness to give you a good signal, without saturating the detector and without showing (too many) stars.
Take a series of images during this brief period -- it will only last a few minutes before the sky grows too dark. After each image, move the telescope a small amount in any direction, so that if there are stars, they will be shifted to different pixels in each image.
Afterwards, you can combine the images via a median technique to get rid of any stars.
This method isn't the easiest: it requires that you be ready to go during a short time window. It will probably be easier to use some sort of "taking pictures of a blank card" method.
You must be careful of wide field instruments and sky flats. What focal length/fov are you talking about? I have no doubt that twilight flats can be done properly with your 8 inch RC.
Gary