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Use of a bad pixel map?

prusling · 51

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le: septembre 20, 2019, 07:23:19 19
Firstly, many thanks for providing such an awesome software tool that makes use of every bit of resource available to get through so much quality work so quickly in comparison to other well-known alternatives!  To make it perfect for my use I would be grateful if you could answer the following query.

There are some occasions where it has not been practical to capture enough sub-frames of a given filter type to allow the statistical cosmetic correction algorithm to work, but I still need to remove bad pixels and the bias/offset signal.  This is particularly acute where I use dithering and end up with not just bad pixels being present but a little cluster at every bad pixel position...

I have a very low noise camera and normally use a master offset/bias as a master dark which works well in conjunction with the cosmetic correction to remove bad pixels where there are enough sub-frames.  When the cosmetic correction is unable to remove the bad pixels is there an alternative approach that could instead utilise a bad pixel map?

I assume that the normal approach is to use a master dark frame of the appropriate temperature and exposure time, and that if both an offset/bias and the dark are supplied, that the offset/bias is subtracted from the light image, and then that is first subtracted from the dark before the residual dark frame noise is subtracted from the light image thereby avoiding the offset/bias being subtracted twice.  The offset/bias seems unnecessary in this case but I guess that is another topic.

In my case I'd rather subtract just the offset/bias and treat the bad pixels from a bad pixel map instead, to avoid the loss of any other signal that is present in the dark.  Is there an approach that could achieve this - either a 2-stage subtraction or a single subtraction of a combined offset/defect calibration image?  I assume I could not just provide the bad pixel map as a dark as that would have the non-existent offset/bias signal extracted from it.

I use the excellent sirilic front-end and it would be ideal if the solution could work using that as well.

Many thanks in advance for any recommendations you can provide.



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Réponse #1 le: septembre 20, 2019, 08:39:22 20
Hello,
Indeed there is a solution for you.
https://free-astro.org/index.php/Siril:Commands#find_hot

Apply this command on a dark (you don't need to subract the dark ;)), it will produce a text file of all deviant pixels.
Then, the command : https://free-astro.org/index.php/Siril:Commands#cosme_cfa will apply the bad pixel map on the file you have load. Sadly, this method work case by case, not in a sequence.
Also you can add your own bad pixels in the file, or bad line and/or column.



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Réponse #2 le: septembre 21, 2019, 09:45:43 09
Many thanks, will try this out



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Réponse #3 le: septembre 22, 2019, 09:29:04 21
Sorry, it's probably obvious but I'm falling at the first hurdle here... I've selected a master-dark in the Pre-processing tab then entered many combinations around find_hot defects 3 3 in the console but I just get an error message "Command execution failed" . Please can you advise how this should be entered.

Edit: sorry, I've now realised I need to load the master-dark using open!
« Modifié: septembre 22, 2019, 09:40:01 21 par prusling »



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Réponse #4 le: septembre 22, 2019, 10:04:18 22
Citer
Edit: sorry, I've now realised I need to load the master-dark using open!

Indeed ;)