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Old 26-08-2015, 02:59 PM
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multiweb (Marc)
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Image scale and stellar profiles

As you know I've acquired a FSQ106N last year and I've been imaging with it quite a lot lately. One thing I have noticed is that I tend to burn star cores a lot more since I've used that scope.

Check the attachment below zoomed at 300%. That's a 5min sub (debayered to LUM) in the area of the veil nebula. The veil itself is quite faint. I did a bit of 10min subs but mostly 5min which is not long by any standard. Even though the "brightish" stars are burnt.

At first I thought something had changed in my processing flow but when I checked the raw files the stars are already saturated.

Is this a side effect of under sampling? Or should I go for shorter subs? Or do I need a camera with smaller pixels? This is a QHY8 btw.
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Old 27-08-2015, 07:41 AM
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Shiraz (Ray)
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hi Marc.
The QHY8 has quite shallow wells for the pixel size. I guess you already know this, but to maximise the dynamic range, make sure that the gain is set to optimum (ie the saturation point is a bit below 65,000 ADU) and the offset is as low as possible without dark clipping. Reducing the exposure time will improve dynamic range, but the read noise is fairly high, so you will run into problems with low SNR if you go too short. Best might be to consider an HDR approach with normal exposures for faint detail and some very short exposures to capture the star core info.

Last edited by Shiraz; 27-08-2015 at 08:05 AM.
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Old 27-08-2015, 07:45 AM
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multiweb (Marc)
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Thanks for your insight in the matter Ray.

Quote:
Originally Posted by Shiraz View Post
The QHY8 has quite shallow wells for the pixel size. I guess you already know this, but to maximise the dynamic range, make sure that the gain is set to optimum (ie the saturation point is a bit below 65,000 ADU) and the offset is as low as possible without dark clipping.
My gain is set to 1 and offset 121. Might have to revisit these values.
Quote:
Originally Posted by Shiraz View Post
Reducing the exposure time will improve dynamic range, but the read noise is fairly high, so you will run into problems with low SNR.
Yes that is the issue. Read noise.
Quote:
Originally Posted by Shiraz View Post
Best might be to consider a HDR approach with normal exposures for faint detail and some very short exposures to capture the star core info.
I thought about that but I though it's a bit extreme to have to do separate takes for the stars.
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