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Old 31-05-2016, 08:22 AM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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Quote:
Originally Posted by glend View Post
I have done some Dark & Bias comparisons for different Gain and Offset settings, there is some stretched histrogram differences but really not much at least in the 60" lengths for Darks.

For example, using the SGP stretched histogram feature, a series of 10 x 60" Darks (with camera at -25C) all come in almost identical for the same Gain and Offset setting. Using the Unity Gain and Offset preset (which is Gain = 139 and Offset = 21) the histrogram shows B 15, W 612. When the same Dark sequence is run with Gain at 71, and Offset at 12 (which are the Jon Rista recommendations), I get B 15, W 388.

Moving to Bias Frames, using the same sequence setup & temperature, ten bias frames yield:
For Unity Presets (G 139, O 21) I get B15, W 548. For Rista settings (G 71, O 12) I get B 15, W 356.

So you can see that Gain and Offset settings do have some marginal impact on the Dark and Bias frames but I don't know if this warrants efforts to capture and maintain Dark library files for different camera settings.

Opinions?
Looks generally similar to what I am finding.

At 100 gain, the bias is about 290 and quite clean looking.
At 200 gain, the bias is about 220 with small flecks of noise (2 pixel spots, I guess due to pixels sharing the amps?) are starting to show. It is only a few electrons in amplitude, but possibly worth getting rid of in some circumstances.

On the basis that the bias changes in average value with gain change (even though it is a very small change - divide it by 16 to get down to the 12 bit data), I think that it might be worth doing a bias run for each power up - at least until we can establish the importance of the bias change. My guess is that a single dark set (bias removed) will be OK for each exposure and temperature, but that we may need to do a separate bias for each power up for the highest sensitivity (bias runs are delightfully quick for this chip).

In any case, it clearly will be best to choose a couple of set points to work at and then forget about varying the gain or temp. For now, I am going to use 100 and 200 gain at a temperature of -15 and see what sort of images that can produce.

good fun am looking forward to the clouds getting out of the way for a little while so that I can see if the camera changeover was optically OK. We have done enough tests to be reasonably confident that the camera works OK - time for some imaging - you need some competition

Last edited by Shiraz; 31-05-2016 at 09:24 AM.
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