Is this a gradient or reflection?

Discussion in 'Image Processing' started by JoshuaHufford, Nov 12, 2017.

  1. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    I've been working on processing an image of galaxy cluster Abell 1656, and when I stretch the Lum image I'm getting some background variation that I'm not sure if it is just bad gradients or if its maybe a reflection off a star. I've tried stacking smaller sections of my subs and it is always there. I can't really see it in any of my individual subs but no matter how many I stack it is there. I've tried different stacking methods, and 2 different stacking programs, all with the same result. Any ideas? I normally don't have anywhere near this bad of a gradient in any of my images.

    Thanks.

    Fits image here,
    https://www.dropbox.com/s/dak3p9wxfrp93js/Abell 1656 Lum 930 Min stacked stretched.fit?dl=0

    [​IMG]
     
  2. Colin Haig

    Colin Haig Staff Member

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    Nice field.
    In the image above, all the bright stars seem to have a bar at the edge, eg running 8pm and 2pm. For a Mak or Mak Newt, that seems odd to me.

    I pulled the image into MaxIm and played with it a bit, and see these weird triangular features (highlighted one for you). Is that the gradient you are referring to?
    Am wondering if you've baked the dessicant in a while and maybe that is the source of the issue - eg a bit of moisture/frost inside the CCD chamber?

    abellhuff.png
     
  3. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    Thanks Colin. It is a nice field but it is hard to make a good image from my data.

    I've imaged with 2 different Mak-Newts, and they both have the same diffraction (if that is the right term) on the bright stars. I made a post on Cloudy Nights about it a long time ago and was told it was "Off Axis Vignetting". Don't know if that is true or not but it seems to be a trait of these scopes, I have seen it from other scopes but not often.

    Yes the triangular features(s) is what I am referring to. I'm pretty sure it isn't a moisture problem in the chamber as I've imaged other targets at and around the same time without the problem.
     
  4. Colin Haig

    Colin Haig Staff Member

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    Ok, we'll have to wait for Doug or someone else at Diffraction to weigh in with ideas.

    The only thing I could suggest to eliminate the moisture question is to set the cooler to maybe +4C or so, instead of that nice -30C, and see if it doesnt show up if imaging the same field.
     
  5. Doug

    Doug Staff Member

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    That's pretty weird. I notice that one of the "triangles" has an apex near a fairly bright star (HIP63320, mag 6.9) that is right at the edge of the field.

    I'm going to guess that this is some kind of nasty reflection resulting from an unlucky alignment of that star with some structure just outside the FOV of the camera.

    Try repositioning the telescope a little so that the star is either well inside or well outside the field.
     
    Iaffaldano Giuseppe C likes this.
  6. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    Thanks, that is my guess as well since it has never happened on any other object with the exact same equipment. I just wish I had noticed it before I spent HOURS of time gathering images on this:(
     
  7. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    And if anyone has a suggestion on a way to possibly remove these in processing I'd love to hear it.
     
  8. Kevin Morefield

    Kevin Morefield Standard User

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    Seems like DBE in Pixinsight is you best hope on the gradients.

    Good luck!

    Kevin


    Sent from my iPhone using Tapatalk
     
  9. Iaffaldano Giuseppe C

    Iaffaldano Giuseppe C Cyanogen Customer

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    I agree with Doug.... you can rotate the angle of the camera to see if this issue desappears; if yes you have need of a rotator....
     

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