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Old 14-09-2017, 08:34 AM
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OneCosmos (Chris)
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What are your QHY12 Gain and Offset settings?

Hi,

A quick question for anyone with a QHY 12. Would you mind sharing your GAIN and Offset settings for this camera? It would be interesting to see how much consistency there is out there. Currently mine are set to Gain of 10 and Offset of 123 but I think both may be wrong because my minimum value is always 1. QHY seem to say the minimum should be above 200. There are all sorts of ways to work it out but I'm just interested to see if consensus is reached across owners.

Chris

Last edited by OneCosmos; 14-09-2017 at 11:35 AM.
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Old 14-09-2017, 12:43 PM
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What's their logic for saying the minimum should always be over 200?

The spread of values you get will always be a factor of both the gain and offset...
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Old 14-09-2017, 03:05 PM
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Quote:
Originally Posted by Camelopardalis View Post
What's their logic for saying the minimum should always be over 200?

The spread of values you get will always be a factor of both the gain and offset...
not sure but I think the minimum value will determine where the histogram starts and your data may be clipped before you even start the processing. Perhaps they reason a little room to move on the left is a good thing?

Chris
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Old 14-09-2017, 04:01 PM
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If the histogram starts at value 1 then that's fine, no clipping going on.

What you need to be concerned with is any accumulation of pixels that appear to have value 1 that could represent a bunch of clipped pixels.

The offset just shifts the curve along the x-axis.
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Old 14-09-2017, 10:52 PM
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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

Last edited by OneCosmos; 15-09-2017 at 09:54 AM.
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Old 15-09-2017, 07:33 AM
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I had one further thought too. The process described must be performed at all binning levels if you use them. Personally I never bother to bin and only use 1x1. My focus routine however does take 2x2 frames so presumably the GAIN and Offset are wrong for that! No idea how much difference it would make - perhaps none as I'm not trying to process that data.
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Old 15-09-2017, 10:26 AM
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Chris, your CLS won't be affected, it just cuts some light pollution out, so in theory you should need to take longer exposures to get sky limited subs.

It's important to understand that you don't gain (pun intended) anything shifting the signal further right because the first "object" adjacent to the left edge is the bias curve. If you are exposing your subs sufficiently, your background sky values need to be clear of the bias curve i.e. further to the right in the histogram...this is how you get the real signal (even if that is black background sky) to swamp the read noise, and thus your background sky shouldn't be plagued with any pattern noise or similar. With the Sony sensors, the read noise is the largest source of noise at reasonably exposure lengths, as the thermal noise is minuscule.

Incidentally, how I learned to do this was guided by docos by folk like Craig Stark on how to measure read noise. The BasicCCDParameters script in PI is really informative in this case, as you take an ensemble of bias, flats and a couple of darks (of differing lengths) to calculate the gain, read noise, dynamic range, etc. This is invaluable IMO for those of us with variable gain cameras, and it helped me understand how my CMOS cameras respond.

Step 1 in your process is key to understanding why linearly your CCD responds to photons. This is also important as during the course of imaging you might want to avoid straying into the non-linear response zone as it will clip your highlights. Fortunately for me, the 1600 chip is almost perfectly linear for exposures greater than 0.5s
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Old 15-09-2017, 10:29 AM
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Quote:
Originally Posted by OneCosmos View Post
I had one further thought too. The process described must be performed at all binning levels if you use them. Personally I never bother to bin and only use 1x1. My focus routine however does take 2x2 frames so presumably the GAIN and Offset are wrong for that! No idea how much difference it would make - perhaps none as I'm not trying to process that data.
It won't matter.

What might matter is focusing at 2x2 given you've got a relatively high resolution sensor on a sharp scope you might want to calculate how much of a difference that might make with respect to your critical focus zone...
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Old 15-09-2017, 11:27 AM
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I own a QHY12 and when I first got it and did the calibration, I came up with a Gain of 10 and offset 123.
3 years later after it came back from China with a new mainboard, the calibration produced a gain of 12 and an offset of 125.
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Old 15-09-2017, 06:22 PM
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Quote:
Originally Posted by Camelopardalis View Post
It won't matter.

What might matter is focusing at 2x2 given you've got a relatively high resolution sensor on a sharp scope you might want to calculate how much of a difference that might make with respect to your critical focus zone...
Are you saying autofocus should be using 1x1? I did try but thought 2x2 gave just as good results and quicker
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Old 15-09-2017, 06:23 PM
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Quote:
Originally Posted by billdan View Post
I own a QHY12 and when I first got it and did the calibration, I came up with a Gain of 10 and offset 123.
3 years later after it came back from China with a new mainboard, the calibration produced a gain of 12 and an offset of 125.
Thanks. I'm guessing each camera is different. I'm going to run it again tonight and hopefully get the same resiult. Mine is a brand new camera to replace one that stopped working.
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Old 15-09-2017, 09:27 PM
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Quote:
Originally Posted by OneCosmos View Post
Are you saying autofocus should be using 1x1? I did try but thought 2x2 gave just as good results and quicker
I'm saying you should crunch some numbers and see if it makes a difference with your setup.

My scope is relatively short focal length so my pixel resolution is 1.4"/px...IMO the risk of a star getting caught between 1.4" and 2.8" (or other low multiples) is too great.
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