help! images have white strips at the top and bottom

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VicPunch
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help! images have white strips at the top and bottom

Post by VicPunch » Fri Dec 07, 2018 11:44 am

I have been unable to solve the problem of very light areas at the top and bottom of each image when stretched.
Using a Plainwave 14" and a QHY 367C camera.
Also unable to complete the sensor analysis. It ceases to work when the gain gets to 21 and the offset gets to around 50.
Any help would be much appreciated.

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Re: help! images have white strips at the top and bottom

Post by admin » Mon Dec 10, 2018 7:52 pm

Hi,

unfortunately I don't have a QHY 367 camera to test and debug within SharpCap so I can only give you generic advice. I do know from other reports that some of the newer QHY were cameras do have difficulty with the central analysis. This is because their gain and offset controls interact in a way that I still do not understand since I've only got second-hand reports of how it works. I think that the offset is applied before the gain which means that if you set an offset (alternatively black level/brightness) value you will see it has a much bigger effect at high gains than it has at low gains.

It's entirely possible that the light areas at the top and bottom of an image when stretched are what is commonly called amp glow (although it's not entirely clear that an amp is the true cause of this on CMOS sensors). This is often strongest at the edges or corners of the frame and can usually be removed by subtracting dark frames. If you were able to post an image showing the problem we be able to confirm whether this is indeed an amp glow issue.

Cheers, Robin

VicPunch
Posts: 6
Joined: Sun Nov 04, 2018 11:12 am

Re: help! images have white strips at the top and bottom

Post by VicPunch » Tue Dec 11, 2018 4:11 pm

Hi Robin,

I am really grateful that you have looked at this for me.
I tried to upload a URl within the post but it did not work although the URL was Ok
The image is at http://www.vicpunch.com/duf.html
It is enhanced to show the problem.I have tried the amp and it does seem to help.
Vic

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Re: help! images have white strips at the top and bottom

Post by admin » Tue Dec 11, 2018 8:36 pm

Hi Vic,

that definitely looks like amp glow to me. There are some controls on the Q8 cameras which may reduce but not eliminate this. Your best bet is to use dark frame subtraction since the dark frames will also contain the amp glow meaning that the subtraction process should remove most of its effect.

Hope this helps, Robin

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