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Old 30-01-2014, 08:37 PM
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RickS (Rick)
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Join Date: Apr 2010
Location: Brisbane
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Quote:
Originally Posted by gregbradley View Post
How do you measure bias drift? Do you check background ADU of a series of bias images taken at the same time and same temp?

I get the idea from Richard that it may also tend to shift when ambient temps change.
I measure drift by looking at the mean value of each bias frame (or a smaller section of each bias frame if you prefer - Dave and I did some tests on a 100x100 pixel area at the centre of each frame). The time and ambient temp (did you mean ambient or sensor temp?) shouldn't matter. If your bias is not stable regardless of these then overscan calibration is likely to be of benefit.

Ambient temperature is definitely a possible cause of bias drift by affecting the electronics external to the temperature regulated sensor. I didn't ever see an obvious correlation in my results but that's not conclusive.

Both Dave and I did tests that demonstrated a significant benefit to overscan calibration with our cameras - a SBIG STF-8300M in his case and an Apogee U16M in mine. With overscan calibration we got a much more linear looking growth of dark current over time and could also measure a reduction in effective read noise (bias drift can't be distinguished from read noise). YMMV of course, and as I said before: if you don't have bias drift then there's no value in overscan calibration.

I'm pretty busy with work stuff at present but once I dig my way out from under that I'd be happy to post some numbers and graphs. Dave might weigh in at some stage too but he's in the middle of a significant lifestyle change and may be too busy to spend time on IIS for a while.

Cheers,
Rick.
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