M13 dark halo?

[ollypenrice]

After putting my recent M13 down for a while I went back to it and thought, damn, there's an artifact I hadn't noticed, a dark and slightly reddish halo around the outer part of the cluster. I went back to the data, re-stretched the luminance and blow me, there it was. I was sure it was a colour issue. Then it got odder; I measured the dark looking halo area and compared it with the lighter looking background sky away from the glob. And the dark looking halo is actually slightly lighter!

What do you think is going on? Is it an optical illusion based on the contrast created by the bright globular or could it be a camera or processing artifact, or is it something real? Here's the pic again;

Olly

[Psychobilly]

The pixel values reported back by the info palette in CS would give you some idea if it was an optical illusion...

Peter...

[NickH]

Hmmm/...interesting indeed. I took some long exposure shots on M13 with the 314L a while back and notived what I thought were thing clouds passing near it...

Be good to see what you find Olly

[FraserClarke]

I'm not quite sure I can see the effect you mean in the image?? However, you would expect globular clusters to have a halo of fainter redder stars, and that shows in your image I think -- is that what you mean??

Very nice image in any case

[riklaunim]

Some blue stragglers and rest should be old red, orange stars.

[ollypenrice]

Hi.

I don't mean the stars but the background sky. When measured by the Ps sampler it is slightly lighter amongst the outlying stars but looks distinctly darker to me.

Olly

[johnh]

Olly, I remember Noel commenting when examining Niks Hi-Res Faulkes image of M13 noticing anomalies within outer structure showing possible intervening dust around the cluster. May not be what you mean but interesting anyway showing what can be seen if you look a little harder.

Nik's M13 Mosaic Revisited

Regards, John.

[Psychobilly]

Hi.

I don't mean the stars but the background sky. When measured by the Ps sampler it is slightly lighter amongst the outlying stars but looks distinctly darker to me.

Olly

If its measuring like that then I guess your seeing an optical illusion...

Need to find a linky.. BIAB with an edit if i find it...

Not the one i was really looking for but will do for starters...

http://www.michaelbach.de/ot/

Peter...

[ollypenrice]

John and Peter, great links and contribution, thanks.

John, it sounds just as if Noel was seeing what I see in my image but the odd thing is that I can't see it in his reprocess of Nik's! (However, I was quietly pleased to see how well the 5.5 inch TEC compared with the Faulkes - on a small size image, that is!)

Peter, great link. This is the one to look at I think;

Induced Grating

Theory of visual perception was my father's field of research for much of his life so I have come across many interesting examples of illusory effects and suspected that maybe the sky just looked darker because of the contrasting presence of outlying stars. It may well just be that.

Cheers,

Olly

[Psychobilly]

I still cant find the site I was after... I think I might have it bookmarked at home...

Have you got any way to generate an intensity plot across a line in the image...?

Peter...