NGC925

An experiment with very long (30min) Lum subs on a faint galaxy.

Since I'm fortunate to image from a relatively dark location, I've often wondered if I should increase my Lum sub exposure lengths from my 10mins "standard" to something greater. Given the poor UK weather, I decided that 30mins would be the practical maximum amount, which would align with my "standard" for my NB exposures.

After taking the 1800s Lum subs and 600s RGB, I noticed that some of the star cores with the Lum subs had ADU readings in the non linear portion of my camera, indicating that I was starting to loose information eg white clipping was starting. Since I'd previously measured the linearity of my CCD and determined that the non-linear portion starts at just over 47k ADU, I got CCD stack to reject all the pixels above 47k and set these to a Missing Value. In CCDStack a Missing Value is a special type of number that has the property: Missing Value + X = X. I then created a pseudo lum from the RGB information and replaced the Missing Values with the pseudo Lum information.

LIGHTS: L:18 x 1800s, R:9, G:9, B:9 x 600s. DARKS:30, BIAS:100, FLATS:40 all at -20C.