Shooting Flat Frames with MaximDL

Using MaximDL to take Flat Frames

In order to shoot flats I normally use a light box. Alternatively, I wait until late afternoon (when the sun has just disappeared behind the trees) and, with my OTA pointed straight up, place a 11” x 11” square piece of milky opaque Perspex on the end of my scope.

I make sure that the telescope / camera setup is exactly the same to take flats as it is when I'm imaging the skies (focus position, camera orientation, focal reducers in place, etc).

I then use MaximDL to take a series of exposures and adjust the exposure duration so that the maximum pixel value (as displayed in the Information window) in the brightest part of the image (ie, the centre) is between 1/3 and ½ the maximum achievable value (see below).

The maximum achievable value for a 16 bit QHY8 is 2^16 = 65,536

The maximum achievable value for a 12 bit DSLR = 2^12 = 4,096

QHY8 Flat Frame = 250ms

DSLR (Canon 400D) flat frame = 300ms

Once I have worked out the correct exposure length, I instruct MaximDL to take a series of exposures (20) at that duration.