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QHY8 Offset Setting Tip


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For anyone wondering what to do about the offset setting on the QHY8, try reading this PDF which I found, it worked well for me

Before I did this I was getting black frames quite a lot and my resulting images were very poor!!

I've ended up at GAIN = 50, OFFSET = 118 in MaximDL


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In Nebulosity2 :-


So, to determine the gain and offset to use:

1) Take a bias frame and look for the minimum value in it. Is it at least, say 100 and less than a

thousand or a few thousand? If so, your offset is fine. If it's too low, boost the offset. If it's high,


drop it. Repeat until you have a bias frame with an offset in, roughly 100 - 1000. Don't worry

about precision here as it won't matter at all in the end. You now know your offset. Set it and

forget it. Never change it.

2) Aim the camera at something bright or just put it on your desk with no lens or lenscap on and

take a picture. Look at the max value in the image. Is it well below 65k? If so, boost the gain. Is

it at 65k? If so drop the gain. Now, if you're on a real target (daylight ones are great for this) you

can look at the histogram and see the bunching up at the top end as the camera is hitting fullwell.

Having that bunch-up roughly at 65,535 plus or minus a bit is where you want to be. If you

pull up just shy, you'll get the "most out of your chip" but you'll also have non-linearity up there.

You've got more of a chance of having odd color casts on saturated areas, for example, as a

result. If you let that just clip off, you've lost a touch but what you've lost is very non-linear data

anyway (all this assumes, BTW, an ABG chip which all of these cams in question are). Record

that gain and set it and forget it. Never change it."

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  • 2 years later...

I've been running my QHy8 at a gain setting of zero since I got it because I'd read that any higher setting wasn't needed.

Couldn't remember where I'd read it until this afternoon when I was looking at the Wiki for the camera here: http://qhy8.wikispaces.com/What+gain+and+offset+should+I+use%3F

Here's the text I read (by Craig Stark) the Wiki author states the Q8-HR is identical to the QHY8

"On a CCD Labs Q8-HR I have, even at gains of 0 and 1 (on its 0-63 scale), the camera would hit 65535 on bright objects (like the ceiling above my desk). There's no point in running this camera at gains higher than 0 or 1. Why is there no point? The camera only holds 25k e-. If a gain of 0 or 1 gets me to 0.38 e-/ADU (so that those 25k e- become 65535), running at 0.1 e-/ADU will only serve to limit my dynamic range. Each single electron already comes out to more than 2 ADU."

Hope that helps - it seems to work for me :-)


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