Where do violet star halos come from?

[old_eyes]

A question that has been puzzling me. Where do violet star halos come from (see attached example of the area around Alcyone and Merope in the Pleiades)?

Image taken with an Equinox ED80 and a Canon 1000D. 60mins of 2min subs at ISO 800. The only thing done to the image is stretching the levels in PS.

Is it residual chromatic aberration? Over-exposure? An artifact of the Bayer matrix?

It is obviously a common problem as Noel's Actions provide a specific solution, but I wonder what I need to do to avoid the problem in the first place?

old_eyes

[Glider]

The 2 very brightest stars aren't actually the ones that have the halo.

When you stretched did your curve have a strong S-shape in it. I've seen this when I've greatly over stretched one particular region of my data to get nebuloscity to come out but it has then distorted the stars.

[old_eyes]

The 2 very brightest stars aren't actually the ones that have the halo.

When you stretched did your curve have a strong S-shape in it. I've seen this when I've greatly over stretched one particular region of my data to get nebuloscity to come out but it has then distorted the stars.

I noticed that the two big stars have no halo (indeed all the big bright main stars in the Pleiades have no halo in the full frame) and wondered if that had anything to do with the nebulosity?

I did not adjust the curves at all, just the levels - moving the black level and mid-level adjustments and leaving the white level the same. How can I see the effective cuve in PS?

I did adjust the black levels on the individual RGB channels to remove some light pollution, but that was just to bring the three channels into some kind of alignment.

old_eyes

[ollypenrice]

Hi There!

I have never seen anything like that before but am certain it is a processing or preprocessing artefact and not really optical in origin.

Noel's action is not intended for that so much as for correcting soft star bloat in the short wavelengths (blue and violet) which scatter most easily.

I would start at the beginning with the linear image, unstretched, in Ps. Bring the black point a little to the left of the start of the main peak in Levels and set the white point to the end of the data on the right hand side.

Now in Curves do the standard stretch, lifting the bottom of the curve steeply and then making it straight as it goes towards the top end of the data. ('Flat to fnish.')

Repeat this Levels and Curves iteration, maybe lifting less the second or third time. Stop stretching when you reach the noise limit and the histogram goes feathery.

At no point must there be any negative curve applied. That is, the curve must always be rising. It looks to me as if somewhere along the line a negative curve has crept in but I don't know for sure. It may come from altering the mid point, which I never do until maybe the very end of the whole job. Try not touching that in Levels.

I'm confident you will sort this one.

However, f you didn't touch curves, how come the image is non linear? Did you use a stretch in DSS or some other programme? If so, try doing Curves entirely in Ps from the linear data.

Olly

Oh, another likely candidate, which I should have thought of. Could be the anti-aliasing value used in the debayering process. Where was the dabayering done? I bet that's it!

[old_eyes]

Dear Olly,

Thanks for the (as always) sage advice. I will experiment further with the curves and see if I can prevent it happening.

As to debayering - I am doing the assembly of the subs in Astroart - which seems to do the debayering automatically at some point. I just give it the RAW files from the camera, and out pops the colour version.

One thing that might be a bit odd is that after I prepare a master dark, I stored it as a .fits file. Could there be some confusion betweeen the different file formats? I could try creating an image without dark correction and see if that makes any difference.

Hmmm! Will report back what happens.

old_eyes

[old_eyes]

OK - some interesting and weird stuff.

The purple haloes are seen in all the JPG images I capture at the same time as the RAW images (you can store both with the Canon, and that means you get EXIF info for each frame).

If I add the RAW frames with no dark correction in AstroArt, the haloes are there. If I bring a single RAW frame into AStroArt the haloes are there.

However, if I bring a single RAW frame into PixInsight (which I am trying out at the moment), and use Dynamic Background Extraction to reduce skyglow, I see no haloes. I have tried it on several individual RAW frames and no haloes. I attach an example. Noisy but you can see the difference to the AstroArt processed one.

Problem is I have not yet worked out how to do the preprocessing with darks in PI. That will be the next check.

old_eyes