Pesky Blue Halos....

[Simon Pepper]

Hi all,

I was wondering if anyone knows how to get rid of the halos on larger stars? I have tried several ways in post. I use PS but when I stretch the image they appear I have tried the astronomy tools plug in as well as running a minimal filter but I think as they are so big the software doesnt even recognise them as stars! 

Am I over or under sampling causing it? I have only noticed it since switching to dedicated camera from DSLR. Is there any other tricks I can do in post or is there something out in the field I can try?  I have attached my image over Bodes Galaxy so you can see what I mean (not finished)

 

Thanks

Edited April 13, 2021 by Simon Pepper
typo

[Seelive]

You could try removing the stars then applying different stretches before recombining in the final image?

[Simon Pepper]

Hi Seelive

i use starnet to remove the stars to wok on the dso but how do I remove the stars and leave the dso?

thanks 

 

[The Lazy Astronomer]

Bloating in the blue channel is likey caused by unfocused deep blue and/or UV light. There are ways to reduce it in post, but my preferred option is to correct it at capture by using a filter which cuts the lower part of the spectrum. The Astronomik L3 filter worked well for me. 

That's really nice image you've got so far though, I'd be well pleased with it. What's your equipment and capture details?

[Simon Pepper]

Thanks Lazy I will give that a go. Well It was an old Celestron 127 SLT scope (F12) that I used for visual when I got into Astro its been gathering dust for about 10 years and thought I would get it out and see what could be done with it. Paired with ZWO 294 MC pro on HEQ 5 mount. I think it was 4 hours of 120s subs at unity gain plus calibration frames. No filters or flattener reducer, never collimated it either I was surprised myself by its capability. I am usually a widefield Redcat Nebula kind, but as it was galaxy season and didnt want to blow another 2k on a Edge HD thought I would give it a go!  Just need to sort that bloating Ill see what I can do 

Edited April 13, 2021 by Simon Pepper

[The Lazy Astronomer]

Hmm, you've got me tempted to dust off the (non-edge HD) SCT now!

[Davey-T]

If it's only a few stars you can sort them manually in Photoshop using a custom cloning brush as recommended by Nik Szymanek IIRC, haven't used it for a while so can't remember the exact process.

Dave

[ollypenrice]

What I do is avoid messing about with the big stars and just let them be. Once the image is fully stretched I make a copy layer and then the following, which I call reverse-processing:

Top layer. Take a well feathered eraser large enough to remove, fully, the star and its bloated halo. (This doesn't show because the bottom layer is what you now see.)

Bottom Layer. Curves. Pin the curve at the level of the background sky just outside the halo. (Place cursor and ALt Click.)  Add a fixing point below the background sky point. Now pull down the curve above the background sky point, experimenting with a shape that gives you a smaller but still natural star. You can see the effect in real time and look for artificial-looking transitions. The same technique will work for stars set against a nebulous background.

Olly

[Simon Pepper]

7 hours ago, Davey-T said:

If it's only a few stars you can sort them manually in Photoshop using a custom cloning brush as recommended by Nik Szymanek IIRC, haven't used it for a while so can't remember the exact process.

Dave

Thanks Dave not heard of Nik or IIRC is there a YT tutorial or point me in the right direction? Thanks 

[Simon Pepper]

55 minutes ago, ollypenrice said:

What I do is avoid messing about with the big stars and just let them be. Once the image is fully stretched I make a copy layer and then the following, which I call reverse-processing:

Top layer. Take a well feathered eraser large enough to remove, fully, the star and its bloated halo. (This doesn't show because the bottom layer is what you now see.)

Bottom Layer. Curves. Pin the curve at the level of the background sky just outside the halo. (Place cursor and ALt Click.)  Add a fixing point below the background sky point. Now pull down the curve above the background sky point, experimenting with a shape that gives you a smaller but still natural star. You can see the effect in real time and look for artificial-looking transitions. The same technique will work for stars set against a nebulous background.

Olly

Thanks Olly will give this one a go!

[ollypenrice]

6 hours ago, Simon Pepper said:

Thanks Dave not heard of Nik or IIRC is there a YT tutorial or point me in the right direction? Thanks 

Nik has been writing a series called Masterclass in Astronomy Now for quite a few years. I don't know if they've been collected together at any stage but it would be a good idea.

Olly

[Davey-T]

45 minutes ago, ollypenrice said:

Nik has been writing a series called Masterclass in Astronomy Now for quite a few years. I don't know if they've been collected together at any stage but it would be a good idea.

Olly

"Not heard of Nik" hope he doesn't read this 😂

I probably read his technique for sorting stars in one of his articles but haven't got time to trawl through years of ANs.

Dave

[ollypenrice]

48 minutes ago, Davey-T said:

"Not heard of Nik" hope he doesn't read this 😂

I probably read his technique for sorting stars in one of his articles but haven't got time to trawl through years of ANs.

Dave

He must be a young 'un!!!  🤣

Olly