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Old 18-03-2011, 09:12 AM
Afro Boy (Carl)
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Removing "hot" pixels

Hi everyone,

My 450D appears to have quite a number of persistent hot pixels which show as "red" points on my lights and darks.

How would I go about removing them from the image?

Say I align and stack 5 light frames. Because of the alignment, each hot pixel actually creates 4 or 5 points on the image. As my darks are stacked on the same co-ordinates (0,0) it can't remove the hot pixel. It's like I need to align my darks to each of the individual flats. Would that be right?

I'm currently using DeepSkyStacker.

Thanks,
Af.
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Old 18-03-2011, 11:17 AM
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Octane (Humayun)
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In IRIS, there is a find_hot function. I am almost certain that DeepSkyStacker would have a similar function, probably referred to as a cosmetic function or menu item of some kind.

H
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Old 18-03-2011, 12:46 PM
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troypiggo (Troy)
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Firstly we should point out that you should be applying your dark subtraction before you align and stack any images.

Once you've done that and align and stack your light frames, it's possible that you end up with a dark trail of pixels instead of the red ones you first described. You can treat this a little in Photoshop by copying the layer, apply a slight median blur, and set the layer's blend mode to "lighten only". Might soften the image a little, but all noise reduction techniques will. Try it and see.
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Old 18-03-2011, 12:58 PM
Afro Boy (Carl)
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Quote:
Originally Posted by troypiggo View Post
Firstly we should point out that you should be applying your dark subtraction before you align and stack any images.
I thought the order of my processing might be wrong. So I stack my darks then subtract the master dark from each of my light frames (before align/stacking the lights).

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Old 18-03-2011, 01:13 PM
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Quote:
Originally Posted by Afro Boy View Post
I thought the order of my processing might be wrong. So I stack my darks then subtract the master dark from each of my light frames (before align/stacking the lights).

Or simply use DSS (Deep Sky Stacker).
With better polar alignment and tracking, of course
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Old 18-03-2011, 01:15 PM
Afro Boy (Carl)
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Quote:
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With better polar alignment and tracking, of course
Yeah I know. I gotta practice that some more me thinks!
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Old 18-03-2011, 02:39 PM
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Octane (Humayun)
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That's right. You combine your dark frames into a master dark. Then, subtract the master dark from each light frame.

You do exactly the same with flat frames, as well. You have a bunch of flat lights, and, you take a bunch of flat dark frames. You then combine the flat dark frames to make a flat master dark, then, subtract that flat master dark from each of your flat lights. Same concept.

H
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