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Old 20-10-2015, 08:44 AM
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yes, it was one of the light subs. The DBE background was then applied just like a normal master flat would be - (you must set downsample under model-image to 1 though). It gets rid of vignetting and even helps with the worst effects of large donuts (if you use enough sampling points). It does not do fixed pattern noise reduction, but dither can take care of most of that - it's not as good as a real flat, but still useful if you have problems getting flat data. You don't get a lot of signal in the synthetic flat, but, being a fitted surface, it is essentially noise free. if you do try it, be interesting to hear if it works well enough - I guess there will be issues with a DSLR due to the Bayer filter, but you should be able to find a way round them.
Must be my Hyperstar flats curse. Or I just keep ballsing it up, Ray. I applied the synthetic flat (downsampled to 1) but it over corrected the vignetting by quite a lot. I tried both the image cal tool and BPP but got the same result.

At work atm, I can attach a reference later today.
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Old 20-10-2015, 04:21 PM
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trying to remember what I did...I think that the following is the philosophy used.

Using DBE to extract the flat from a sub will give a flat that includes the bias and dark current - which have to be removed. With a cooled Sony CCD, it was good enough to just subtract a fixed bias value, but with a DSLR, you will probably need to subtract the master dark (which should include the bias) from the chosen sub before doing the DBE background extraction (could either do a dark-only calibration on the single sub that you want to use or just use pixelmath to do the subtraction). Then you will end up with a flat that has no bias or dark, so will not have "calibrate" checked for the flat when you do the final calibration of the subs.

IF the Master Dark includes the Bias THEN
sub - masterdark
DBE
ELSE
sub - masterdark - master bias
DBE
ENDIF

Last edited by Shiraz; 21-10-2015 at 12:27 PM.
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Old 22-10-2015, 11:10 PM
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Must be my Hyperstar flats curse. Or I just keep ballsing it up, Ray. I applied the synthetic flat (downsampled to 1) but it over corrected the vignetting by quite a lot. I tried both the image cal tool and BPP but got the same result.

At work atm, I can attach a reference later today.
Hi again Rod. went back and found some data where I had used a synthetic flat. The summary image shows an original sub, the synthetic flat from DBE and the calibrated sub, all with similar stretch. Not perfect, but the method does the job OK if you don't have flats and need them - but of course it doesn't correct for pixel-scale sensitivity variations.
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Last edited by Shiraz; 23-10-2015 at 06:37 AM.
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Old 23-10-2015, 08:44 AM
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Hi again Rod. went back and found some data where I had used a synthetic flat. The summary image shows an original sub, the synthetic flat from DBE and the calibrated sub, all with similar stretch. Not perfect, but the method does the job OK if you don't have flats and need them - but of course it doesn't correct for pixel-scale sensitivity variations.
Hi Ray, Thanks for the support! From your images attached I can see it works well. I have been working on it, last night I took a new batch of darks which are a much closer match to the light subs. I'll start again from the calibration of the light sub that I'm using for the flat generation and go from there tonight.

One thing I did notice is when I used the BPP script, the lights were over corrected but when I used the Image Cal tool on a single image the correction of vignetting was much better, although the image seemed to be a lot noisier?

I have 180 light subs which I want to drizzle , so I want to get this right before I put them all in PI. It takes a while on my PC to crunch through the subs.

Thanks again for your assistance

Last edited by Rod771; 23-10-2015 at 11:27 AM.
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Old 23-10-2015, 10:03 AM
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I think that BPP generally needs tuning for specific requirements - it is probably great when everything is working as expected, but I would be inclined to stick with imagecal while sorting out problems - it seems closer to the nuts and bolts.

A good set of darks will probably help - using this method, you cannot get good flat compensation without them, because the flat data includes dark current from a full length sub exposure, not a short exposure.

Last edited by Shiraz; 23-10-2015 at 12:28 PM.
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Old 23-10-2015, 07:35 PM
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It worked, Ray!

Not quite as good as your image sample above but I'll take it and run. DBE should be able to clean up the rest post integration.

Thanks so much , we all benefit from your excellent posts.

Cheers

Rod
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Old 23-10-2015, 09:12 PM
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It worked, Ray!

Not quite as good as your image sample above but I'll take it and run. DBE should be able to clean up the rest post integration.

Thanks so much , we all benefit from your excellent posts.

Cheers

Rod
that's excellent Rod .
did you do anything in particular to get it to work with the DSLR? Also, are the noise levels OK?
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Old 23-10-2015, 09:50 PM
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that's excellent Rod .
did you do anything in particular to get it to work with the DSLR? Also, are the noise levels OK?
When using ImageCal to calibrate the light with the new master dark (which is a much better match ,light was 27c, master dark ranged from 24c - 29c both 60 sec iso 800) I typed " raw cfa" input hints in the Format hits both Input and output. This ensures the data is loaded as pure raw unaltered and save as a grayscale CFA image. This calibrated gray image was use to generate the gray synthetic flat. I then use both imagecal and BPP to test the flat on the light and it worked on both this time? Noise was fine this time around, I think the new master dark help a lot.

PI is now slogging through the 180 subs, here's hoping there are no suprises
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