Regardless of what camera you have, every pixel in a CCD exposure starts out at its bias level. You will always need to subtract the bias whether done using the bias frame or the bias as contained in a dark frame.
When you bias subtract your flats, this only applies to the flat subs to produce the flat master. You can't use the flat master to subtract the bias in light frames, so you must either use a bias or dark to remove the bias from the light. This is the reason why when you only subtract the flat, your light frames are still a mess. You've not removed the bias.
Quote:
Originally Posted by gregbradley
There is a paper by Richard Crisp about flat ADU levels. He advises the opposite here of doing higher ADU values like 30-40K. I am sure its on his website if you look for it. I tend to go for between 20-30K but in the end I check to see what works and adjust. Sometimes I went for lower ADUs with the CDK17 like 20K as 30K sometimes overcorrected and made the corners too bright.
|
Yes, but in order to determine the right value you need a PTC for your camera. As Richard's paper mentions, you don't want to be taking flats in the ADU range where the camera goes non-linear. Its just before that to maximise signal and suppress noise.