bright "halo" around all images of DSO's

Discussion in 'Image Capture' started by steve peters, Oct 18, 2018.

  1. steve peters

    steve peters Standard User

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    m33 stack 23 300 -25.jpg Hi all, I am hoping someone can tell me what causes this halo around all my deep sky images.

    I am shooting with an 8" RC telescope with an STF-8300C camera.

    I process in Maxim, using at least 15 darks and bias to calibrate and I cool to -25 C.

    First time posting so I am probably missing some vital details. I am happy to reply with any other info needed.

    Attached image is stack of 23 images, 300 second exposure. Calibrated and auto color converted via stack in Maxim. Unfortunately due to size restrictions the image posted is a small .jpg. I can share the fit files if anyone would like to see them.
     
  2. steve peters

    steve peters Standard User

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    M101 with the same issue (I see that I have a bad sub in there causing the streak, but even after removing the glow remains)


    m101 to send.jpg
     
  3. Doug

    Doug Staff Member

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    That looks like vignetting. I'm sure you'd get the same glow if you had no DSO in the field.

    Your best bet is to use flat fields. But you could probably improve the result using the File menu Flatten Background command.
     
  4. steve peters

    steve peters Standard User

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    Thanks Doug, I've always struggled a bit with flat fields. What is the best exposure time to use?
     
  5. Doug

    Doug Staff Member

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    Mileage may vary. Aim for an exposure that gives you 50% to 70% of saturation in the brightest parts of the image.
     
  6. steve peters

    steve peters Standard User

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    So i think I took some good flat images and I have added them to my calibration and don't see much difference. The image on the left below is including 20 flat frames, the right is just dark and bias.



    flat.jpg
    compare.jpg
     
  7. Doug

    Doug Staff Member

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    I'd have to see your raw data to come to any conclusion. Can you upload sets of bias, dark, flat and image data? (One plane required only.)
     
  8. steve peters

    steve peters Standard User

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  9. Doug

    Doug Staff Member

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    I don't see any files in that folder.
     
  10. steve peters

    steve peters Standard User

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  11. Doug

    Doug Staff Member

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    Since this is a Bayer color camera, for flat field groups you need to set "Flat Norm." to either Boxcar Filter (averages the planes) or Bayer Planes (handles the planes separately). The option is at the lower right corner of the Set Calibration dialog box.

    You should perform all of the calibrations together in one step, before doing color conversion.

    I got the following results with and without flat:

    With Flat.jpg
    With Flat

    No Flat.jpg
    No Flat

    The flat is definitely helping. However, the flat isn't perfect as you can see from the background non-uniformity to the right and upper left. So you need to improve your method for taking flat-fields.
     
  12. steve peters

    steve peters Standard User

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    Thanks for your help Doug! I learned a lot. I think I've got some good flats now and a much better understanding of how they work. I'll post a finished image soon.
     

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