Eta carina

[Prickly]

Hi all,

21mins (42x30secs), balanced mount, drizzle (to a slight degree), no autoguiding

Some unusual star halo (bright) not due to processing. Any ideas? Focus? Chromatic abberation? Filter reflection. Doesnt seem to be with blue stars.

Cheers
David

[pmrid (Peter)]

Originally Posted by Prickly

Hi all,

21mins (42x30secs), balanced mount, drizzle (to a slight degree), no autoguiding

Some unusual star halo (bright) not due to processing. Any ideas? Focus? Chromatic abberation? Filter reflection. Doesnt seem to be with blue stars.

Cheers
David

Hi David. Great subject isn't it. Seems to me you have 2 issues going here. Focus is the first. It's out. The brighter stars also show some halos that look like they might be chromatic abberation. But this would only be true if you were using a refractor. It may be that with sharper focus, that may go.
So taking one thing at a time, that's where I'd start.
cheers
Peter

[Prickly]

Hi Peter,

Thought I had the focus ok but I'll see what happens next time. The main star images seem reasonable. The double ring around the brighter stars is whats got me. Has anyone experienced that with poor focus?

A little bit of CA on very bright objects I can accept (yes it is a refractor) - there will definitely be a small amount but in DSLR images it hasnt been too noticeable.

I'll also be seeing if the flip mirror unit could be contributing in some way by removing it from the equation and as mentioned will be very careful with focus.

The double ring halo has me quite perplexed.

Cheers
David

[pmrid (Peter)]

Originally Posted by Prickly

Hi Peter,

Thought I had the focus ok but I'll see what happens next time. The main star images seem reasonable. The double ring around the brighter stars is whats got me. Has anyone experienced that with poor focus?

A little bit of CA on very bright objects I can accept (yes it is a refractor) - there will definitely be a small amount but in DSLR images it hasnt been too noticeable.

I'll also be seeing if the flip mirror unit could be contributing in some way by removing it from the equation and as mentioned will be very careful with focus.

The double ring halo has me quite perplexed.

Cheers
David

The 'halo' is similar to what I experienced with my own refractor made from Surplus Shed optics - an achromat only - so there was still some chromatic abberation on prime focus images through my DSLR - no matter how well they were focused. But the halo was more pronounced the more out of focus I was. I have partially eliminated this by using as Baader Fringe Killer filter. This blocks most (but not all) of the offending ends of the soectrum that were causing those rings.
You may like to ask around and see if you can borrow one for a bit of a test run and see what that does.
Peter

[Prickly]

Hi Peter,

I was using a UHCS with a little more cutoff in the blue.

The stars showing more halo didnt look blue to me but it could be CA I guess. As you say could be focus but I'm suspicious of internal reflections off the filter perhaps. Will try it next time with an 03 in the filter slider only and see what effect that has also with the flip mirror assembly removed.

Cheers
David

[Hagar (Doug)]

Great attempt. The halos look like a filter reflection or a reflection between the CCD and a filter. First call would have been a sharpening problem but when you say you used a filter I would bet on a reflection of some kind.

[Prickly]

Hi Doug,

You may be right. I had a UHCS at the front of the flip mirror. Im hoping bringing the filter closer may somehow help but we'll have to see. The many elements of the flip mirror might be causing a reflection somewhere so I might eliminate that too.

The original subs look quite sharp but when you stack them you get the halo. You can see the central star image fairly sharp and clearly in the image attached.

Hopefully further experimentation will lead to a solution.

Cheers
David