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Old 23-08-2011, 11:40 PM
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Bias and Dark Frames clipping signal

I had first light on my QHY10 2 nights back and been processing some of the images.

Will post them shortly and in no rush as they are nothing special.

Thing I notice however is that if I subtract dark frames with QHY10 it destroys good signal. Same with Bias frames.

Its been a long time since I processed anything but I used to allways subtract bias from darks and flats then apply to lights

With the QHY10 doing this destroys good data. I tried without darks and just bias and flats and also had some signal clipped.

Ended up just using flats and lights which seems to give best result.

Anyone else seen this behaviour on QHY camera's ? Could this be something to do with incorrectly set gain/offset ?

Also using nebulosity to do the pre processing. Will try something else shortly to compare in case its software redlate
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Old 24-08-2011, 01:29 PM
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I'm using a QHY-12 which is quite closely related to the QHY-10 and bias, darks and flats are working fine for me.

I set my gain and offset as per Craig Stark's article (http://www.stark-labs.com/blog/files/GainAndOffset.php) and I have done calibration successfully with Maxim, CCDStack and PixInsight.

Are you creating master dark and flat frames? How exactly are you doing it?

Cheers,
Rick.
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Old 24-08-2011, 02:21 PM
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Thaks Rick,

I have taken 15 dark frames, 15 flats and 15 bias.

My flats are showing an average adu of around 23k. maybe a bit high but not enough that it should cause an issue.

I combine then using average combine in Nebulosity to create a master dark, flat and bias

I also tried a sigma combine to make the masters but this made no difference.

if I do a simple test and just subtract the master dark from a single light frame then debayer I can immediately see the histogram is clipped in the dark zone and when i stretch a light frame with dark subtraction vs without there is much more signal in my light frame that has not hard the dark subtraction.

This camera has such low noise that difference between a bias and dark frame is not very obvious.

I will post some links to the raw frames if someone wants to take a look.
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Old 24-08-2011, 03:55 PM
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Doesn't sound right... I've been reading the big fat Berry and Burnell book and I'm keen to test my new knowledge, so I'd be happy to take a look at the raw frames for you. My QHY-12 is also very low noise. Compared to my SXVR-H18 the difference is amazing, but after calibration the H18 is just as good...

Cheers,
Rick.
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