[ozstronomer (Geoff)]

I have made up a Flats light box for my ED80 and Canon 20d following some threads on hear and other websites.

Could someone tell me how to calculate the correct shutter speed for flats. I understand it varies with each camera and is around 1/3?? the full well capacity of the camera.

Attached is a jpeg of a flat at 0.035 sec The image looks ok to my untrained eye but the histogram doesn't look correct. I believe it should be a single spike somewhere in the centre. If I reduce the shutter speed I start to get a secondary curve on the right hand end of the histogram.

Any help appreciated

Thanks Geoff