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.
The light box pictured here is for the AP155 refractor and measures about 220mm x 220mm x 250mm. There is a similar one for the Mak-Newt measuring 350mm x 350mm x 250 mm.
Both boxes use two diffuser screens and are modeled after the excellent design of Greg Pyros. A design drawing is downloadable from his website http://www.gregpyros.com
The only modification I did to Greg's design was putting white light LED's in addition to the incandescent bulbs in order to improve the blue light response.