[jase (Jason)]

Originally Posted by rogerg

Your flat image needs to be taken in the exact imaging configuration that your image is taken.

I'd like to also add to Roger's great explanation that "exact imaging configuration" also includes maintaining correct focus. If your focus is different between light and flat frames the dust donuts and other optical anomalies will be different.

A good starting point is a flat should be approximately 30% of your well capacity. For example, the STL11000 has a full well capacity value of 50,000 so an exposure should be long enough for the CCD pixel well count to reach 15,000. A too short exposure will not reduce the light frame efficiently; too long will have the opposite effect. There are no hard and fast rules, simply experiment with what works best for your imaging train and processing techniques.