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Flats again!
Sorry everyone, I know this issue has been done to death - but I still can't get my flats right.
I'm using a light box for my RC8. I have the sky flat plug in with Maxim which sets the exposure for my flats at the right ADU. All good except...
The flats seem to overcorrect the lights. Rather than excessive illumination in the centre, I have a darknpatch there and overly light areas at the extremities.
This tells me that the flat I am using is not truly flat ie I am applying a flat frame whose illumination doesn't match the actual sensitivity of the camera and filter combination.
I usually take 10 flats. I then dark subtract an equivalent number of darks and apply a bias maters (made up of 3 or so bias frames). I then median combine them to make a master flat.
I've used ADU counts of 20,000 as well as 15,000 and 25,000 and get the same result.
Looking carefully at the thumbnails for the flat frames at once, I see that the frames are not identical. In one shot the centre of the frame is brightest while in the next the brightest area is off centre.
My thinking is that the flats are displaying shutter artifacts and it's the effect of the shutter that shifts the brightness around somehow. The accumulation of these out of kilter frames produces a non-representative flat.
The other point to note is that the bottom centre of the frames display a dark semicircle. I suspect this is shutter artifact as well.
My camera is a SBIG ST8300 binned x2. This give me a flat with exposure of between 0.15 and 0.3 seconds.
So I think I need to include a t-shirt or other material to step down the intensity of the light.
Does this make sense? I am open to suggestions and want to preserve my good humor with this.
Pete
Ps. The unflat nature of the flat is apparent when I set the screen stretch on Maxim to high or medium
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