[OneCosmos (Chris)]

Tonight I spent a few hours trying to set the Gain and offset scientifically based on following the steps here: www.astropixel.gr/uploads/7/8/.../scientifically_determining_ccd_gain _and_offset.pdf


I have ended up with a GAIN of 16 and an Offset of 128 (originally I had 10 and 123 - but the Gain in particular was plucked out of thin air).

The process to calculate the GAIN is quite interesting: ( I can't see a formatting option to number these points or use bullets)

1) Used my lightbox on max brightness and started taking subs at 1x 1 binning until the MEAN pixel value topped out (and it did this at lower than 65535).
2) Using the exposure arrived at above I took bias frames, raising the offset gradually until the minimum value was between 500 and 1000 and that took me to 128.
3) Repeated the GAIN process and this time a GAIN of 16 took the MEAN pixel value close to 65535.

*NEW* I forgot to read that the author of the document suggested then adding 5% to the GAIN figure to ensure that saturation is achieved around 95% of the GAIN. That would take my GAIN to 19.2. What a shame I have just completed a whole bunch of calibration files based on 16

Does a Gain of 16 and Offset of 128 match any other QHY12 owner's values? I may consider running it again to see if I get the same results, but it is quite time consuming.

I tried originally with the CLS filter on but that was hopeless and I just couldn't get meaningful figures. I just have to assume the figures I have arrived at will still work ok with the CLS.

Proof of the pudding will be RAW data using the new figures and comparing them to RAW data without (and I have plenty using the 'incorrect' values from Astrofest!). Hopefully I will get a chance to try it out very soon. At the very least the new offset will mean my histogram is no longer a gnat's whisker from the left edge. Duncan, perhaps I will 'lose' some data but sometimes a little clipping to darken the sky background is a good thing but with it hard left you can't clip without losing real data from the histogram.

Anyway, I hope this post is useful for someone now or an obscure google search in years to come. setting the GAIN and Offset can be a time-consuming and painful process and you don't ever really know you have got it right either!

Chris