Pre processing - Halo's around stars - Why?

[swag72]

Here's a new one on me, so I hope that someone can shed some light on it.

I have got 2 hours of OIII data last night and have run it quickly through a Maxim stack as I do each morning and I am seeing massive halo's around a couple of the bright stars. I've never seen this in pre-processed data (just stacked) and so I dread to think how this particular data will process.

My question is - What may have caused this? As I said, I've never seen it before. Is it simply moisture in the atmosphere? The focusing was OK and Maxim has told me that all of the subs are under FHWM 1.4. Twenty minute subs and nice round stars.

[swag72]

Anyone shed any light on this before I decide whether to keep them or bin them?

I've done OIII before with this filter and scope combo, so it's not the filters.

[RikM]

I've never shot Oiii so can't really offer much advice. No chance there is some nebulosity around the stars? Or how about a few clumps of whispy cloud blowing across during the night?

[davew]

First. Don't bin them yet.

Any chance of a photo ? One persons halo is another persons flare.

Does each sub show exactly the same amount of halo? Did it get worse or better as the night went on ?

Are you sure the filter is fully screwed in and not tilted ?

Has ANYTHING been changed or touched recently ?

Does the temp change much in the fits header ?

So many questions !

Dave.

[swag72]

The halo is *just* visible in each individual sub when stretched in Maxim. They do not get any worse throughout the night. The subs after 0300 were all binned as the clouds rolled in, which made me wonder if it was cloud.

The filters are in the filterwheel carousel and as I only turned it round with the little *knob* from the SII the night before, can I assume that it's all square? This is the first time I've ever had this issue. Also the carousel and filters have not been moved or changed since the last lot of OIII. If it was an off kilter filter that would have showed up in previous outings?

The temperature of the camera stayed the same (0 degrees) all night, and I don't think that the ambient would have dropped more than any other night - It was just as hot here as usual!!

I'll go up to the obs in a bit and stack and stetch the subs so you can see what I have in the unprocessed stack.

No nebulosity around the star that would look like a halo - although there is nebulosity all around it. And wispy cloud? ........ When the clouds rolled in I binned those subs, the ones I kept all looked clear of cloud. Certainly no background brightness or fuzziness to suggest cloud was around.

[davew]

Hmmm. I'll think about it !

Until we see the stars all I can think of so far is moisture somewhere somehow. All things being equal.

If it looks just like reflection off the filter then it probably is.

The reason I asked about your subs and temperature was to check if you have any moisture build up in the CCD chamber. Seems not so far.

Crop into your stars and post. If I don't know ( likely ) then someone else might,

Dave

[swag72]

Here's a screen stretch from Maxim, stretched to show it at it's worse of course and also viewed at 200%. I've not seen these before in my previous OIII stacks.

[davew]

Now that looks like a reflection to me. It doesn't look like I'd expect with atmospheric moisture.

I'm going out now but will have a further think.

Is it a much brighter star than you've imaged before ? Do other stars in the frame show the same effect ?

What ever you do, don't get down about it. Someone may have a simple answer,

Dave.

[swag72]

I've just looked and stacked the only image I have with a brightish star in the field of view - I've then stretched it in Maxim in order to see if I can see a halo - I can.

To me that looks the same, don't know why I've not noticed it before! So, if it's reflection, is there anything I can do about it? It's only this filter that I have noticed anything with.

[davew]

Well, you little fibber

Have a look at this article - http://www.astrodon.com/articles_faq/articles_faq/press_release:391,355,49

In it there is a formula for finding out where the reflection is coming from.

[ O111 ] is a beast. It is for me sometimes anyway. I'm told there isn't much point turning the filter glass round in it's housing but others say it is !

I'll be back,

Dave

[davew]

Have you tried a process on it yet ? It may not be so bad and can be mitigated a touch.

If you keep the background at pretty much the same level as you go, you can try the history brush in PS.

Also, as there aren't many stars around the halo just clone the halo out ?

Let us know how you progress.

Dave.

[swag72]

Some reading?!!! ............... That's not reading, that's a degree in a page!!

I've not processed it yet as I want more OIII, depending on the weather we'll see how long it takes. I suppose at least I've seen it, so I can keep an eye on the halo as I process. There's lots of tricks to process it out I'm sure and I'm not adverse to using the clone tool if all else fails!! I have been known to clone out a whole troublesome star and halo, quite a big one too and no one noticed!!!! If all else fails - Clone it out!!!