Master flat is over...
 
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Master flat is over correcting  

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(@skestergmail-com)
White Dwarf Customer
Joined: 6 months ago
Posts: 17
November 4, 2018 17:03  

I'm having trouble with calibration on the data I captured last night.  I created a master flat, that was calibrated with a master dark, master bias, and bad pixel map.  I then calibrated the lights with all of the above masters.

However, when I view one of the lights  in linear-calibrated mode, the master flat is over correcting the image.  The same is true for the final integration of the lights.  Instead of correcting the vignetting and dust motes, the corners and dust motes go from being darker than the rest of the image, to much lighter than the rest of the image.  It's as if the master flat is being applied more than once.

One thing to note.  The lights were captured via Sequence Generator Pro, and were stored as fits files on the laptop.  The darks, flats and bias frames were capture on camera in Nikon RAW (NEF) format.

Any idea what I am doing wrong here?  Thanks


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(@skestergmail-com)
White Dwarf Customer
Joined: 6 months ago
Posts: 17
November 6, 2018 03:55  

Replying to my own original post.  I had a suspicion that the issue was related to the lights having been taken with SGP and stored on the laptop in .FITS format, while the flats were stored as Nikon .NEF files on the SD card.  To test this theory I converted the flats from NEF format to FITS format before loading them into APP.  I then calibrated the new .FITS flats with the same MB, MD and BMP as before, and generated a new master flat. 

I then calibrated the lights with the MB, MD, BPM and new MF.  The calibration now looks correct, with the flats removing the vignetting and dust mots.  It is now clear to me that the original issue was somehow caused by the flat files being loaded as .NEF while the lights were loaded as .fits.

This would seem to me to be a bug in APP.  I am using version 1.0.68.

Thanks,

Scott

 


(@vincent-mod)
Black Hole Moderator
Joined: 2 years ago
Posts: 272
November 6, 2018 04:08  

Hi Scott,

Thanks for your own research into this! That is indeed very interesting. I'll let Mabula know (@mabula-admin), he should be here tomorrow.


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(@astrogee)
White Dwarf Customer
Joined: 8 months ago
Posts: 20
December 31, 2018 19:26  

Yes, I'm getting this problem in general with version 1.0.70 - but not solved. I've used data from a Canon 6D (CR2 files - flats and lights) and SBIG camera (fits files - flats and lights). Attached is an example of the SBIG mono camera. I used only 5 luminance lights and 5 luminance flats:

Screen Shot 2018 12 31 at 1.19.22 PM

Screen Shot 2018 12 31 at 1.19.37 PM

I put the files in this dropbox folder:  https://www.dropbox.com/sh/ouknnqzqseog5br/AADt1uk7P62o1H-LCAYNz02Ta?dl=0

Thanks

Greg

This post was modified 3 months ago 2 times by astrogee

(@astrogee)
White Dwarf Customer
Joined: 8 months ago
Posts: 20
January 1, 2019 00:59  

Well this is a bit of a surprise but I added bias frames and now the calibrated frames are flat. Looks good.

Screen Shot 2018 12 31 at 6.28.28 PM


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(@vincent-mod)
Black Hole Moderator
Joined: 2 years ago
Posts: 272
January 2, 2019 22:13  

Could it be you didn't use bias frames while making the masterflat?


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(@astrogee)
White Dwarf Customer
Joined: 8 months ago
Posts: 20
January 2, 2019 23:57  
Posted by: Vincent Groenewold - Moderator

Could it be you didn't use bias frames while making the masterflat?

Hi Vincent, thanks for your reply.

Yes, that seems to be the issue. I made the mistake of trying to use a master bias that was generated by another program but that didnt work at all. So I tried without bias and didnt realize it would affect the master flat as much as it did. Finally I regenerated the master bias and master flat with the original bias frames and I'm getting very nice results now.


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(@vincent-mod)
Black Hole Moderator
Joined: 2 years ago
Posts: 272
January 3, 2019 00:30  

Indeed the best workflow in APP is to always use raw data, not touched by any other processor, to go forward. APP uses the entire sensor data which many other programs don't do as they use dcraw to convert the data. This causes the dimensions not to be the same and it fails on that. There's a lot of statistics at play when using calibration data and for that to work correctly, a masterflat needs to be bias and dark-signal free as much as possible (I always use bias, darks and a BPM to correct them). If those signals are still there it'll cause APP to make a wrong estimation of the gradient-signal in the lights.


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