Coloured fringes on my stars

[almcl]

For a while now I have been concerned about multi-coloured stars turning up in my final, processed image.  Here's one such image (heavily downsampled) :

and here's one of the stars from the upper left part:

At first I thought this was just down to a lack of processing skills on my part, but then I noticed that individual raw subs also had some colour separation:

and wondered if this was a feature of the SW Coma Corrector which I use with my slightly modified SW200P.  So last night as it wasn't going to get fully dark, I removed the coma corrector and attempted to image with just the camera sensor and main and diagonal mirror in the light path.  The results (which were tricky to obtain - the OAG really picks up the coma stars big time) surprised me: there are still odd coloured fringes to the stars:

So what's causing these coloured fringes?  Is it chromatic aberration and if so, where's it coming from?  

Or is it a characteristic of the astro modded Canon 700 (with the Baader filter that Juan at Cheap Astro Photography supplies)?

Any thoughts on what the cause might be and how to mitigate it would be most welcome.

[Skipper Billy]

Looks like your collimation is a wee bit off ??

 

[philhilo]

Quote

Nearly the same rig, 200PDS and a modded misbehaving 650D (I do have a 700D but not modded). I am using the SW coma corrector and have the same fringes. The picture is a close up of one sub. I had wondered about collimation not being spot on.

 

[almcl]

Thanks for running that through CCD inspector, Skipper Billy.

I have often suspected that while I can visually collimate through the eyepiece (Cheshire, Concenter &c) hanging nearly a kilo of camera, home-made TEC cooler, coma corrector &c off of the slightly modified Skywatcher focuser probably undoes at least some of this at least as regards focuser tilt.  Although the CCDI tilt measurements don't look too bad?

Does anyone know of a way to collimate with the camera in situ?  I've seen a couple of ideas somewhere but they were a bit vague on what to adjust if the out-of-focus star in the center of the field of view doesn't have the secondary circle in the middle (and having read Suter's Star Testing Astronomical Telescopes, am not sure that merely racking the focuser out a random amount is a completely valid test.)

[Skipper Billy]

1 hour ago, almcl said:

Thanks for running that through CCD inspector, Skipper Billy.

No problem - I cant help you with the collimation question sorry - reflectors give me a strange feeling that men in white coats might not be too far away 😉 

[philhilo]

1 hour ago, almcl said:

Thanks for running that through CCD inspector, Skipper Billy.

I have often suspected that while I can visually collimate through the eyepiece (Cheshire, Concenter &c) hanging nearly a kilo of camera, home-made TEC cooler, coma corrector &c off of the slightly modified Skywatcher focuser probably undoes at least some of this at least as regards focuser tilt.  Although the CCDI tilt measurements don't look too bad?

Does anyone know of a way to collimate with the camera in situ?  I've seen a couple of ideas somewhere but they were a bit vague on what to adjust if the out-of-focus star in the center of the field of view doesn't have the secondary circle in the middle (and having read Suter's Star Testing Astronomical Telescopes, am not sure that merely racking the focuser out a random amount is a completely valid test.)

I am trying to get the hang of the collimation tool in APT - obviously this uses the camera in situ utilising the live view mode and an out of focus star. Had a play the other evening but done more research since. Similar view with PA and all the kit. This I think is why guiding around the zenith is poor with the 200PDS. Lots of flexure going on.

Out of interest I can hear the mirror (or something heavy) shift when I move the OTA, do you get anything like this? I do use a laser collimator but it doesn't sit flush on the focuser and rocks about so is pretty poor. 

[almcl]

2 hours ago, Skipper Billy said:

reflectors give me a strange feeling that men in white coats might not be too far away 😉 

I can identify with that one !

2 hours ago, philhilo said:

I am trying to get the hang of the collimation tool in APT 

Hadn't thought of that, thanks! Will give it a try during the light nights.

2 hours ago, philhilo said:

Out of interest I can hear the mirror (or something heavy) shift when I move the OTA, do you get anything like this? I do use a laser collimator but it doesn't sit flush on the focuser and rocks about so is pretty poor. 

I've changed the mirror cell mounting arrangements on my 200p (longer mounting bolts, stronger springs).  This helped with mirror flop/guider flexure but at the same time I also switched to an OAG so whether this stopped mirror movement or just concealed it isn't clear. I've not heard any movement - but am fairly deaf (and don't usually wear my hearing aids when out with the scope) .  I do find the scope needs both primary and secondary tweaking every time it gets mounted and put this down to the tube flexing.

[Adam J]

11 hours ago, almcl said:

For a while now I have been concerned about multi-coloured stars turning up in my final, processed image.  Here's one such image (heavily downsampled) :

and here's one of the stars from the upper left part:

At first I thought this was just down to a lack of processing skills on my part, but then I noticed that individual raw subs also had some colour separation:

and wondered if this was a feature of the SW Coma Corrector which I use with my slightly modified SW200P.  So last night as it wasn't going to get fully dark, I removed the coma corrector and attempted to image with just the camera sensor and main and diagonal mirror in the light path.  The results (which were tricky to obtain - the OAG really picks up the coma stars big time) surprised me: there are still odd coloured fringes to the stars:

So what's causing these coloured fringes?  Is it chromatic aberration and if so, where's it coming from?  

Or is it a characteristic of the astro modded Canon 700 (with the Baader filter that Juan at Cheap Astro Photography supplies)?

Any thoughts on what the cause might be and how to mitigate it would be most welcome.

Its latteral chromatic aberation, a problem with all two element coma correctors.

[almcl]

Thanks, Adam.

I feel an upgrade of some sort may be on the way!

[almcl]

13 hours ago, Adam J said:

Its latteral chromatic aberation, a problem with all two element coma correctors.

Adam, just thinking about that, the third image was taken without a Coma corrector yet it still has colour separation (admittedly in a different orientation to the other two) - so given that the only things in the image train are the two mirrors and the DSLR sensor, could it be something on the sensor, Bayer matrix or Baader filter, perhaps? 

[Adam J]

A badder replacement filter can cause red halos but they are not normally blue. It may be the Bayer matrix itself showing up. Do you do a per channel alignment?

Edited May 17, 2020 by Adam J

[almcl]

4 minutes ago, Adam J said:

 Do you do a per channel alignment?

Yes, in DSS the box is usually checked:

[Adam J]

13 hours ago, almcl said:

Yes, in DSS the box is usually checked:

There are better bebayer algorythems in some other stacking programs like APP. Might be work having a go with the free trial and see if it improves things.

[happy-kat]

I never use the align RGB channels in DSS and prefer to control alignment manually in processing where/if needed