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Flat mystery - Central obstruction on Rasa 8

Dean Wierman
in PixInsight
Hi all,

I am having an issue with what looks like the central obstruction showing up in calibrated and stacked subs and I am not sure what is causing it.  I believe my flats and bias files are ok, but with the processing for a reflection nebula it appears I am pulling out something so I am doing something wrong.  (Witchhead in this case).  I have a Rasa 8 with the 2600MM Duo.  Any ideas?  I used the Elumiglow flat panel.  I attached the stacked subs (shows on 30s and 120s) this picture is from Blue filter (Baader) though I also see it on the Antilia rgb tri band I used for lum.  I also attached the master flat and a single one. (snippet, let me know if I should post the original files). I used Nina Flat wizard and set the target at 40% for exposure.

processing wise,, I am in a Bortle 6 and working on the CN challenge which is a reflection nebula so the steps leading to the attached is: spectophotometricfluxcalibation, multiscalegradientcorrection, starxterm and then ghs.

Any ideas?

Thanks,

Dean


central obstruction.jpg
1115 x 767 - 115K
master flat.jpg
1255 x 846 - 64K
single flat.jpg
1278 x 862 - 171K

Comments

  • Dean Wierman
    Note: the line through the image is utility cables with the target low for me.
  • BlockHeadBlockHead
    Basically this is just saying your flats are different in some way than your images when you exposing on the sky. The difference can be in degree because flats are bright compared to the sky.

    Generally an annulus or circular over/under correction is almost always due to scattered light caused by some cylindrical surface. It could be shiny adapter tubes, dew shields, and other similar things. Corrector lenses also cause this kind of behavior due to a combination of internal reflections and back surface geometry.

    These items are easy to test by removing the possibly offending culprit. 
    It is also possible that the position of the flat isns't optimal and too many off-axis rays are scattering around your system. This is commonly because the panel is too closed and the baffling is poor. 

    You can prove everything I am saying with Night Sky Flats. See my video on this subject. You can convince yourself that a night-sky flat works perfectly well..but your panel flats do not. This means the way in which the panel is creating the flat or the other things I mentioned are the issue. 


    -the Blockhead
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