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Old 23-04-2010, 12:41 PM
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Astrobserver99 (Rob)
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QHY8 Black Point Clipping

I have only used my QHY8 a few times - due to cloudy skies. FRom what I have obtained so far, I want to get more faint detail for the same length exposure. Should I increase the offset so the minimum value is 1000? Now, it's between 500 and 1000 and I'm not that happy with the results. I know I should experiment, but thought I would ask first - not enough clear skies to experiment...

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Old 23-04-2010, 12:46 PM
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multiweb (Marc)
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Quote:
Originally Posted by Astrobserver99 View Post
I have only used my QHY8 a few times - due to cloudy skies. FRom what I have obtained so far, I want to get more faint detail for the same length exposure. Should I increase the offset so the minimum value is 1000? Now, it's between 500 and 1000 and I'm not that happy with the results. I know I should experiment, but thought I would ask first - not enough clear skies to experiment...

Thanks,
I use Gain :1 Offset: 60. The reason I use gain 1 is that early versions of CCDStack didn't like fits files with zero gain. The lower your offset the lower your sky count (ADU). I aim for 900-1000ADU.

Now black clipping only happens when you save your raws as scaled 16 bits TIFF for example. So just before you save them decrease your background and tweak your DDP so you have enough room on both sides of the histogram. Then you can narrow back down in Photoshop.

PS: the lowest ADU that shows in your raws for a given offset will change with camera and sky conditions. By camera I mean 1 QHY8 might show 500ADU at offset 110 and another 900 at offset 60. Mine has always been a bit high.

Last edited by multiweb; 23-04-2010 at 12:49 PM. Reason: clarification
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Old 23-04-2010, 01:31 PM
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Astrobserver99 (Rob)
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Yeah, I mean't offset between 500 and 1000 ADU. So increasing the offset slightly and ADU should also increase the CCD response (background), and weak signals will not be lost.

I also use gain=1. Increasing the gain will only introduce noise...

OK, I get the Tiff conversion - just have to watch the histogram and adjust accordingly.
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Old 23-04-2010, 02:29 PM
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multiweb (Marc)
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Yes - increased offset = higher background ADU. decreased offset = lower background ADU. Just tweak the offset until you get around 1000 ADU for your background.
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