IR star fringing through Astronomik CLS-CCD filter?

[Carl M]

So I've been having problems with my stars not being perfectly round, most of that in the past I put down to collimation. However, now I believe that something else is at play.

On most stars it seems like there is a chromatic abberation type effect, in particular it seems to be the orange/red stars that appear to show it worse. My camera is a 1000D full spectrum modified (clear glass replacement) and Baader MPCC with the Astronomik CLS CCD clip filter installed, which I thought included IR cut. The red channel seems to be the channel that shows the fringing, blue and green channels the stars are round and not bloated. Here are a set of images from a raw 5min sub to try and show the effect. It almost looks like the fringing has not reached it's focus point, which gives the appearance of the star bloating. Since it is only apparent in the red channel, would that point to IR?

In a single image it doesn't look so bad, but after stacking the fringe becomes worse. I'm aware there are some techniques that could reduce it in post processing like this one which I am going to try http://budgetastro.com/micro/articals/red_halos/red_halos.html

I would much prefer to combat the issue at it's source though, if I can find out what it is! I'm going to borrow my old unmodified 1000D from my sister to see if the issue still remains.

Any suggestions on what it might be?

Cheers,

Carl

[LightBucket]

First of all are you sure its the CCD version of the CLS filter you have, and not just the standard CLS without IR cut...secondly are you re focusing between filters as the focus point may well be slightly different...even if the filters are meant to be parfocal...

[Carl M]

I am sure it is the CCD version of the CLS filter, that's what it says on the filter at least. I always focus with the filter already in place since it's the clip in one.

I went back and looked at some images I took with my C9.25 which was with the same camera and filter, these also has the same red fringes around stars. This should rule out the coma corrector and the scope being a problem I guess?

[LightBucket]

Sorry my mistake, as you showed the three colour channels i was thinking you were mono imaging with RGB filters, so the focus issue, will not be the problem...

the only other thing is internal reflections from one of the glass elements in your focal train..

[Carl M]

Could be internal reflection, but then I'd have thought the bright stars would probably exhibit a bit more of this effect.

This is quite a close crop mind, but it's certainly visible in full frame. Other thing to note is that the coma like tail moves throughout the image. Presumably the result of the coma corrector trying to correct it maybe?  This is the full frame 5min sub.

[Adam J]

It probably is the filter to be honest as I get a slight effect like this on my CLS and thats with a 130PDS so nothing that should be causing chromatic aberration. My general rule is that I dont let it bother me unless its visible when viewing the image as full scale, pixel peaking is pointless who looks at a single star in an image?

[Carl M]

1 hour ago, Adam J said:

It probably is the filter to be honest as I get a slight effect like this on my CLS and thats with a 130PDS so nothing that should be causing chromatic aberration. My general rule is that I dont let it bother me unless its visible when viewing the image as full scale, pixel peaking is pointless who looks at a single star in an image?

Yeah you are right Adam, I try not to pixel peak but collimating this f/4 has turned me OCD about star shapes - that's the only reason I noticed it

Quick way to reduce the effect was to run one or two star reductions on the red channel.

[Adam J]

1 minute ago, Carl M said:

Yeah you are right Adam, I try not to pixel peak but collimating this f/4 has turned me OCD about star shapes - that's the only reason I noticed it

Quick way to reduce the effect was to run one or two star reductions on the red channel.

I seem to recall that the filter is only spec'd down to F3 by astronomik and from shots I have seen of people using it at F3 I would say that is optimistic. You do get weird color effects as the F-number is reduced with this filter F4 will strain it.