A flat field frame or "flat" is an image of an evenly illuminated field with the CCD camera's shutter open. It shows the optical imperfections in the imaging system (vignetting, dust particles, reflections, etc) and is applied to the light frame (image of the object) in order to remove these imperfections.
After experimenting with a number of different methods and materials to obtain quality flats I decided upon a light box, mainly for these reasons:
- constant light level: independent of variable reflections coming off of external surfaces.
- repeatibility: the same performance every time
- ease of use: no guesswork or need to time and rush as with sky flats.
- easy set-up and take-down: slides on and off dew cap within seconds.
I made two light boxes, one for my refractors and the other one for the 10" Maksutov-Newton. When I received the 14.5" Ritchey-Chrétien it quickly became clear that handling and storing an even bigger light box in the dome was not going to be practical. Still, I wanted to keep the advantages listed above, so I opted for an "open box" design with the diffuser and light sources on the telescope and the reflector on the dome roof panel - much like professional observatories shoot their flats.