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Old 03-02-2013, 11:24 AM
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pvelez (Pete)
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Halo reduction in Pixinsight

Can any of the PI users point me to a tutorial on reducing halos? I have some data from a remote obs that has nasty big halos around bright stars - mainly in the Green subs (less in Blue and none to speak of in Red).

There is a script for halo reduction but it and the notes for it on the PI forum are gobbledygook

Cheers

Pete
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Old 03-02-2013, 12:09 PM
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I can't recall seeing a tutorial anywhere but I have had some success with the StarHaloReducer script. The main trick I found is that it takes a while to recompute and redraw the preview after you change settings. If you don't realise that it is easy to keep tweaking while that is going on and get very confused about cause and effect!

I have also had some success building masks and using them to desaturate coloured halos:
  • Create a star mask with parameters set to mask larger stars
  • Clone a copy
  • Use Morphological Transformation to grow the stars in one copy (to cover halos) and shrink them a little in the other
  • Combine with PixelMath to make donut shapes that fit the halos (CloneStamp to remove the ones that don't need halo reduction)
  • Blur, apply and desaturate

Cheers,
Rick.
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Old 03-02-2013, 12:27 PM
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Good to hear it works well for you Rick. Its almost a complete mystery to me.

Here is a screenshot of what I am working with - its a preview of a central portion of my image magnified so you can see the size of the halos.

Do you mind making a few suggestions on how to run this script?

I can centre the mask on the star and halo but where I go from there is a complete mystery to me

Pete
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Old 03-02-2013, 02:30 PM
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Pete,

Start by creating a preview that encloses the star/halo. It's easiest to make sure the preview is centred on the star - there's a little cross displayed at the centre of the preview as you drag it out.

Invoke the StarHaloReducer script. If you move the RGB/K slider way to the right and wait a little while you should see a noticeable effect on the preview displayed by the script. There is a status display to the left of the Cancel and OK buttons which will tell you when the script is computing and when it is finished.

Now try increasing the Star halo radius and Halo reduction radius and wait until you see the effect on the preview.

That should be enough for you to get a feel for how it works. Tweak settings until you're happy. Press the OK button and you'll get a new image with the halo reduction applied to it.

Cheers,
Rick.
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Old 03-02-2013, 02:42 PM
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Here's a very quick attempt at fixing the halo on the central star. All I did was adjust RGB/K and the Star halo radius and Halo reduction radius. Pics are fairly obviously before and after...
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Old 03-02-2013, 09:22 PM
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Great stuff Rick - thanks. I think I am getting the hang of it.

I assume that once you have the position right you slide the RGB/K slider back to bring up the background correct?

I do have odd corners. The preview window is quite small and square. When I set the halo radius - the first slider - I can get it to cover the whole preview. So when I am done, I have black corners being the areas of the preview window that are not covered by the halo.

Am I missing something?

I also assume that you need to apply this script after you have done a STF stretch and applied it to the Histogram Transform. Otherwise I you don't see the detail in the stretched preview. Is that right?

Thanks with your help. I'm a PI novice and I really want to save some good looking data.

Pete
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Old 04-02-2013, 08:13 AM
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Quote:
Originally Posted by pvelez View Post
I assume that once you have the position right you slide the RGB/K slider back to bring up the background correct?
Yes, I just push the slider to the right so I can clearly see the area where halo reduction is happening. Once I have the size right I pull the slider back to adjust the amount of reduction.

Quote:
I also assume that you need to apply this script after you have done a STF stretch and applied it to the Histogram Transform. Otherwise I you don't see the detail in the stretched preview. Is that right?
Yes, I apply the script after stretching.

Quote:
I do have odd corners. The preview window is quite small and square. When I set the halo radius - the first slider - I can get it to cover the whole preview. So when I am done, I have black corners being the areas of the preview window that are not covered by the halo.

Am I missing something?
I'm not sure I understand, Pete. The preview window in the script dialog is usually a wide rectangle with scroll bars. I've never seen the script modify the image outside the selected halo area (assuming that's what you mean by black corners). Are you using PI 1.7 or the new 1.8 RC?

Cheers,
Rick.
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Old 04-02-2013, 08:31 AM
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Rick

thanks for this

I'll post a few screenshots tonight so you can see what I mean. I haven't switched to "Ripley" yet - but should do so soon I guess.

A final question - can I use the same tool on RGB images when combined?

Pete
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Old 04-02-2013, 10:03 AM
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Pete,

I haven't moved to 1.8 yet either. I was asking in case the script behaved differently on 1.8 from what I'm used to.

I've never tried it but I presume the script will work on RGB images since you can apply different amounts of correction to the individual colour channels.

Cheers,
Rick.
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Old 04-02-2013, 08:50 PM
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You are a legend Rick - I managed to get it all rolling nicely this evening. The cut off edges were a result of using too small a preview box.

For a change I stretched each of the R, G and B frames first, the reduced halos in blue and green and combined. Such a difference!

I didn't manage to deal a with a blue halo around 1 star and so ended up with a purple halo. However the halo tool worked a treat on that too.

Here is a before and after shot - the key is to use the the different colour sliders to match the background.

I'm still fiddling with it - its all a learning experience - but its a big step forward for me.

I am a happy camper. Will post the revised image shortly

Pete
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Old 04-02-2013, 08:58 PM
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Here is the cleaned up image

A long way to go but its still a big step for me with PI

Image is IC 417 captured using the iTelescope FSQ106 with STL11k from New Mexico late last year. Subs are 100 mins L, and 30 mins each RGB.

Pete
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Old 04-02-2013, 08:58 PM
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Pete,

Great to see you got it working! A little documentation for some of those tools would be very helpful...

Cheers,
Rick.
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