Star Damage Question

[Ken Pendlebury]

Hi Folks,

This has happened in at least 3 images that I have processes with PI.  At Some time during the processing, I seem to be "damaging" larger stars, but only in the cores.  The star halos still look OK.  Tracing this back is a little difficult, because the damage seems to manifest itself as a small "lighter" spot in the star's core during the beginning steps, but is readily apparent when any attempt to saturate the image is made.  HDR Wavelets seems to enhance the core spots, but I am not sure if they are the root cause.

Here are a couple stars from an image of M101 that are representative of the damage I describe above:


If there is any interest or information that can be gleaned by inspecting the full image, the wide field can be found here:
http://www.flickr.com/photos/kenastro/4715393083/sizes/o/

and the crop here:
http://www.flickr.com/photos/kenastro/4715393179/sizes/o/

I have attached my processing steps for M101 and a basic outline is as follows:

• Calibrate (flats and bias)
• Debayer
• Register
• Stack
• BG Neutralization
• DBE x2
• Color Calibration
• SCNR for green
• Masked Stretch (15 iterations)
• HDR Wavelets
• Unsharp Mask
• Curves for Saturation
• ACDNR
• Final Histogram tweak
• Curves for small contrast boost

Thanks for any help,
Ken

[Silvercup]

Hi Ken:

HDRWavelets tends to produce artifacts on bright stars, I would try it with a starmask.

Best, Silvercup.

[Jack Harvey]

I usually do use a inverse star mask to mask my stars when using HDRWT also.

[Niall Saunders]

Hi Ken,

I have been struggling with this same problem recently on a bunch of M82 data - and I had decided that it was down to over-exposure in the RAWs. I might need to just re-think and re-examine the data and, if I see something esle (other than over-exposure) I'll get back to you.

Cheers,

[georg.viehoever]

Ken,

I have seen this in my images with stars that are close to saturation (i.e. have RGB values >0.9). I mainly see this when I boost colors using the V-Transform (discussed in http://pixinsight.com/forum/index.php?topic=1689.0), because the star's halo has a color that is different from the star core. My Canon EOS40D also is no longer linear in this range. Try star masks. Using a HDR combine of different exposures also helps.

Georg

[Ken Pendlebury]

Thanks for all the tips... I really appreciate the input.

Since these posts, I have reprocessed the image using a star mask for processes that are not kind to starts (i.e. HDRWT).  This has not yielded much better results.  In order to trace this issue back to its root, I simplified the processing of M101  for experimental purposes.  This simplified process consisted of the processing of a single uncalibrated frame in order to eliminate any calibration or stacking issues as suspects.  The steps I took, were debayer, Masked Stretch and then adjustment of the saturation curve.  Using only these three steps, my stars still exhibit the damage I showed in my original post.

So...   I am either damaging the stars during capture with inappropriate settings (gain and offset) or I the debayer / masked stretch is the culprit...

More to come as I investigate these remaining items.