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Old 07-11-2016, 08:14 PM
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Crushellon (Tim)
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Bias/dark problem

Hi guys,

I'm processing an HDR image of orion that I have taken and I'm having issues with my dark frames but only with the 5 second images. The dark frame totally ruins the image and I'm not sure why. I have attached auto stretched JPG files of the images to see what you think (original 5sec sub, dark subtracted 5sec sub, & 40x stack of 5sec dark frames).

I have tried everythink i can think to do. Just dark subtraction, just bias subtraction, bias & dark subtraction, but they all get the same result.

Thanks, Tim
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Old 08-11-2016, 01:13 PM
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rustigsmed (Russell)
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what program are you using?
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Old 08-11-2016, 01:40 PM
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Shiraz (Ray)
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the problem might well be that there is zero dark current and very little sky in your image with such short subs. when you subtract the bias (or the dark) you end up with an image that has lots of negative values where the data in the sub (overlaid with the read noise) was less than the dark/bias. You maybe need to put in a pedestal to protect the data from going negative - PI allows that and I think that a couple of other packages do as well.
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Old 08-11-2016, 01:59 PM
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Crushellon (Tim)
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I'm using pixinsight.

I was thinking about the possibility of it subtracting into negative values or something like that, but with a similar process done to my flats a perfectly normal result is achieved.

Edit: just realised that even though flats are a similar length they have a lot more signal so therefore they wouldn't go into negative like these 5s subs are.
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Old 08-11-2016, 02:01 PM
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Crushellon (Tim)
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Also, do you have any ideas about what to do in pixelmath to protect the image?
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Old 08-11-2016, 02:46 PM
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under the "output files" part of the imagecalibration process, you can add a pedestal value. Might be worth trying a small value and see what happens.
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Old 08-11-2016, 08:55 PM
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Atmos (Colin)
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Quote:
Originally Posted by Crushellon View Post
Also, do you have any ideas about what to do in pixelmath to protect the image?
In pixel math you can use the expression "$T+0.005" and that'll add about 325ADU.
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Old 09-11-2016, 05:14 PM
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Crushellon (Tim)
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Thanks guys, the pedestal output has done the trick. What kind of adu range should I aim for in a noisy background?
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Old 09-11-2016, 10:52 PM
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have you tried this method? http://www.iceinspace.com.au/forum/s...d.php?t=117010
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