Is this CA or focus (or both)?

[pmrid]

I had about 30 minutes tonight in between clouds and showers but I was determined to at least have a crack at something with my new rig. I'm trying a new F5 8" Newtonian ( a GSO) for the first time and grabbed a few short subs of 5128 and 5139 while I could. These are both DSS stacks of 4 x 30sec subs at ISO800 with a 1000D unguided on an EQ6Pro. I focused by eye to save time.
I see an artifact in the brighter stars and it looks to me like a small pimple with a blackhead in the middle. Is this just poor focus and a bit of an airy ring or is it CA. Being a Newt, I thought I'd escaped CA and merely had to worry about coma.
Any thoughts please?
Peter

[scopemankit]

I'd say it's out of focus and the pimple in the middle is the shadow of your diagonal.

[Geoff45]

Definitely out (way out!) of focus.
Geoff
BTW, you absolutely can't get CA from a Newt at prime focus without violating the laws of physics, no matter how poor the mirrors may be. Any CA in a Newt can only come from the refractive elements (eg eyepieces, coma corrector etc) in the optical path.
Geoff

[bmitchell82]

3 x confirmation of out of focusedness.... I have a 10" newt i use the live view to hone in but then use Nebulosity to return a FWHM value. this is pretty good unless your seeing is "crud" in which a mask would be easier

[bojan]

Those artefacts are suspicious.. if the focus was not OK, and this is the shadow of diagonal, then the fainter stars should show the same feature, and they do not.
This is something else.
And, no, definitely not CA.

[bojan]

Did you use RAW or some other format when taking those pictures?
And, how did you process them?

[Alchemy]

its definately a focus issue, you are just the tiniest tiniest amount off,

done it myself. see image below just a small crop of a much larger image... done with an APO refractor , so its not the mirror.

however just check your method of stacking if you are converting from a bayer array ... ie raws or fits, try them all and see if one is better than another.

[Geoff45]

You could be right. The size of the structure in an out of focus star should not depend on the brightness. Strange.
Geoff

[pmrid]

Hmmm! Focus is certainly a bit off but I suspect only a bit. I've tried a reprocess with Adoe CS2 from the TIFF produced by DSS and it doesn't show the same artifact. I guess I pushed something a biut too far in the curves. It's odd though and I'll watch for it again - if this darn weather ever clears long enough. Thanks folks.
Peter

[bojan]

After DSS, I am adjusting curves (TIFF) with Canon DPP only, without any problems.
Here is the same object, stack 8x30sec ISO 1600 taken with MTO-1000A (4", F11) on EQ6 several nights ago..
That is why there is more noise in my image. Also, my LP (Melbourne VIC) was much worse than yours..

Yes, this effect can be reproduced if curves are pushed too much and if there is inflection at some point (curve goes up, then down and then up again).

[TrevorW]

Peter if yo uare using DSS to stack and use cosmetic SETTING INCORRECTLY ie: "detect and clean reamining hot pixels" can cause this type of artifact appearing in the stars in the second image