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Hi Allan
I have not used a QHY8, and advice from those that own the camera is probably best, but I do see that its a 16-bit camera. You should aim for an exposure length that results in an ADU count of between 35-50% of the SATURATION level of your camera. On an SBIG ST10, full well depth is 77,000, but saturation occurs at around 56,000. So I shoot for an ADU count of between 18,000 – and 28,000.
That said, if you want to test the effectiveness of a flat field you have created, do not apply it to an image containing nebulosity. Test your flat exposures and techniques on just a plain star field. If your resultant background ADU values at the 4 corners and centre of the image are within just a few counts of one another, then you've cracked it and you know what exposure length fits. The fact that you are using a light box is great..as long as you are happy it is supplying a uniform light source.
You can also play with a flat, just for test purposes. Take a master flat you have created, and inrease/reduce its value using pixel math. Apply the resultant image to your light frame to see what happens.
By the way, a perfect flat will not remove a gradient.
HTH
Martin
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