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
- easy set-up and take-down: slides on and off dew cap
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
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.