Cosmetic hot pixel removal in autostacks?

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admin
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Re: Cosmetic hot pixel removal in autostacks?

Post by admin »

Hi Tim,

thanks for the additional files – I've been looking into this further and I have found a problem with the hot pixel handling that leads to the dark pixels/trails in the output - this was related to a fix I did some time back where the starburst noise on some of the earlier high resolution CMOS cameras was being interpreted as hot pixels. Unfortunately the fix is stopping hot pixel detection from working correctly for your images. I'm now trying to work out a way to fix both problems at the same time :-)

The bright pixels (coloured pixel/coloured trails) are different. In the most recent set of data I can see that the dark frame was captured at a temperature of -4.9 and the light frames were captured at a temperature of 1.7 C. Looking at one specific hot pixel, in the light frame, the dark frame and the final stacked, I see this:

The Pixel
p1.PNG
p1.PNG (140.08 KiB) Viewed 297 times
The Light Frame
p2.PNG
p2.PNG (98.2 KiB) Viewed 297 times
The Dark Frame
p3.PNG
p3.PNG (105.75 KiB) Viewed 297 times
As you can see I get a pixel readout of about 7000 in the light frame, but only about 4000 in the dark frame. I'm fairly sure that a big chunk of this difference is down to the temperature difference between the dark capture and the light capture, which highlights the need to get the temperature the same between light frames dark frames. Mind you, if I get the hot pixel code working as it should, it will remove this particular blemish – even at the lower temperature the pixel is about 80 standard deviations above the mean!

I will try to get a fix for the hot pixel removal into the next update of SharpCap 3.3.

Cheers, Robin
timh
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Re: Cosmetic hot pixel removal in autostacks?

Post by timh »

Thanks Robin,

That is a lot of analysis very quickly !

I do think that the temperature problem was only related to the raw files that I sent you. I sent you the first few when I had just spotted the temp problem so the camera was still in the process of cooling down. The later frames and the ones at the end and certainly those in the second set were all down at the right temp I think.

I did check and when I takes all the same raw files and stack them in DSS I don't see any hot pixels in the final stack.

I can send these later files too if useful.

best wishes
Tim
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Re: Cosmetic hot pixel removal in autostacks?

Post by admin »

Hi Tim,

I'm hoping that I am nearly on top of this problem. My previous approach to try to avoid treating the starburst has hot pixels was to set an arbitrary threshold on the value that could be used for the hot pixel limit. I've removed that and instead I am only allowing hot pixels if they are isolated (i.e. not adjacent to another pixel that counts as hot). I think I've also managed to simplify and improve the performance of the dark subtraction code while I'm about it, so I'm quite pleased (assuming that it all works properly!).

Look out for this in the next update to SharpCap 3.3 – I'd be very interested in getting feedback.

Cheers, Robin
timh
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Re: Cosmetic hot pixel removal in autostacks?

Post by timh »

Thanks Robin,

Thankyou. That is good news and I will look out for this and beta test it if that is helpful? I do really value SC for it's autostacking and livestack interface. It is a big advantage to be able to monitor and tweak things in real time. I also like the fact that it has turned my old dob into really quite a capable long focus astrophotography instrument - without the cost and complexity of a large new mount and guiding etc.

best wishes
Tim
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