Blogging about my astronomical endeavors - astrophoto, outreach and solar eclipse expeditions.
Wednesday, February 22, 2012
Good seeing, bad calibration
Details of my (flawed) flat fielding in MaxIm (click for full size version)
Seeing last time out, Feb. 19th, was the best I have experienced so far while working a long focal lengths. Typical stellar FWHM on long exposures was 2.2 arcsec - on most nights I get ~3 arcsec. That was great! Not so great was the calibration problems I encountered in MaxIm DL Pro. I use a SBIG ST10-XE camera and in the past I have used Mira for calibration. I take a bunch of flat fields using the twilight sky and a white cloth in front of the scope. All light and flat frames are followed up with dark exposures of equal length, so there is no need for bias frames. The sky flats vary quite a lot in their brightness due to the increasing sky brightness, but this is normally of no concern since the frames are scaled after dark subtraction to match each other before the images are combined. However, when using the calibration features of MaxIm the flat fielding failed to remove dust doughnuts. When I processed the same frames in Mira the flat fielding went just fine - hence I suspect some problem in MaxIm, either due to my ignorance or due to a bug. I really want to switch to MaxIm since it has many good features that work well. The image below details what I am doing. A few of the original raw files can be downloaded here (28MB) - can someone with MaxIm do a good calibration with this and send me some advice? My email address is email@example.com - thanks!