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Interesting issue with Flats

chuck_e
in PixInsight
I encountered an interesting issue with my Flats not correcting.

I am currently using an old ST10 camera which is non-antiblooming.  It is 16 years old and still works fine other than a few more bright lines.

Since PI doesn't deal with blooms I have been opening my FIT files in CCDStack to correct the blooms then bring the debloomed files into PI.  I have processed several images this way with no problem using the same calibration files.

On my current image I found that the dust donuts and vignetting were not being corrected by calibration.  After spending a few hours trying to figure out what I was doing wrong I finally found a workaround but I have no idea why it works.

If I save the files as TIFF from CCDStack before bringing them into PI the calibration works fine.  It gets wierder.  If I save a file as FIT from CCDStack, open it in PI, save it as a TIFF, then run the calibration on the TIFF file the flats work correctly.

It seems that something in the FIT header is messing with my calibration and the conversion to TIFF strips out whatever is causing the problem..  Any idea what could be going on?  Is there any data in the FIT header that really matters to the final image?

Chuck


Comments

  • BlockHeadBlockHead
    No... it isn't a header problem.
    Here is the issue. When you take files that have been processed in CCDStack- they become 32-bit float images. They are also not normalized between 0 and 1. That second part isn't the killer...it is the first part. When you open a 32-bit float image in PixInsight (or load one) it will try to scale the image based on the highest and lowest values... because the "end points" of the data are not defined. When you save the file as a 16-bit integer file... you are defining these values and the files will appear to be workable in PixInsight.

    However, there is nuance to saving a file from a 32-bit item to a 16-bit integer one. CCDStack or PixInsight needs to decide (or you tell it) which it should scale or clip out of range values. If you do not have many of these... no problem. But if your data does... it is possible that converting to TIFF as you discovered is not *always* going to be perfect... but in most cases it can be made to work.

    -the Blockhead
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