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PixInsight => Tutorials and Processing Examples => Tips & Tricks => Topic started by: STEVE333 on 2016 January 17 21:12:56

Title: How I repair "Rainbow Stars"
Post by: STEVE333 on 2016 January 17 21:12:56

I have a Canon T3 camera used with an apochromatic refractor.  Even though I use a Field Flattener I still have a small amount of Coma that shows up as "Rainbow Stars" near the edges and corners of my images.  I have found a technique that "repairs" these stars. 

The first attachment shows the "Rainbow Stars" that show up near the edges and corners of my images both before and after repair (left image is before repair and right image is after repair).  The technique for repairing these stars is as follows:

• Use the ChannelSeparation tool  with settings as shown in the second attachment. This will produce three images corresponding to the Red, Green, and Blue images.  The Red image will have the same name as the original file but with a _R appended to the filename.  Similarly for the Green (_G) and Blue (_B) images.
• Use the StarAlignment tool with settings as shown in the third attachment.  Notce that the _G image is used as the Reference image and the _R and _B images are used as the Target Images.  This will produce two new images which are the Red and Blue images registered to the Green image.  The new images will have _registered appended to their name.
• Use the ChannelCombination tool with settings as shown in the fourth attachment.  This step combines the _R_registered image with the _G image and the _B_registered image.  This is the repaired image.

So far this technique has produced greatly improved images for me.  Hope this helps someone else too.

I'm very new to PI, so, if there is an easier way to do this I would be glad to be made aware of it.

Thanks in advance for any comments/suggestions.

Steve

Title: Re: How I repair "Rainbow Stars"
Post by: pfile on 2016 January 17 21:38:45

there's some chance the result would be even better if you checked "distortion correction" - give that a try.

rob

Title: Re: How I repair "Rainbow Stars"
Post by: STEVE333 on 2016 January 17 22:16:26

Thanks for the suggestion Rob.  I gave it a try.  Doesn't appear to be any difference in the result to my old eyes.  Was worth a shot.

Title: Re: How I repair "Rainbow Stars"
Post by: russp on 2016 March 24 14:18:30

At what point in your workflow do you do this step?

Title: Re: How I repair "Rainbow Stars"
Post by: STEVE333 on 2016 April 03 17:44:43

Sorry to be so long in answering.

I usually do it right after "stretching" (Histogram or Masked).  There may be a better time, but, this seems to work for me.  Hope this helps.

I've just purchased a Moonlight Focuser for my telescope, and, hope it will make this problem with the colored stars go away.  Won't be able to test it until at least this coming Friday.

Stevde

Title: Re: How I repair "Rainbow Stars"
Post by: fatboy1271 on 2016 April 15 09:37:05

Hey Steve!

I was have problems getting StarAlignment to produce the _R and _B. I believe the issue was that I used the square Apply versus the round Apply Global. Do you know if that's the reason?

fat

Title: Re: How I repair "Rainbow Stars"
Post by: STEVE333 on 2016 December 29 17:44:09

Fat - really sorry it has taken so long to respond to your question.  I do use the round button (Global) to do the StarAlignment. 

I also use the round button to do the ChannelCombination.

Steve