[peter_4059 (Peter)]

OK - done a lot more testing and here are the results:

1. For the images taken at astrofest adding 28000 to the flats using pixel math gave the best correction. A shorter flat (2s vs 4s made no noticable difference).

2. For an image taken from home under light polluted skies the adding 28000 resulted in very poor calibration. 2s flats were no better than 4s flats. There is a gradient from city light pollution.

I've included the response curves for my qhy8 pro to show the linear region vs exposure for my light box.

The flatstack attachment shows my light box is definetely illuminating the scope evenly - this is a stack of two flats taken with the light box rotated 180 degrees - one as a light and the second as the flat - the pixel values are the same from centre to edge and all edges are the same.