Help please- what does this indicate?

[markse68]

I noticed on processing previous images that I was getting red fringing on stars sometimes but I put it down to focus changes between filters (LRGB on mono camera) as I haven't sorted out a focuser yet. But I'm just having a look at my M13 from the other night and something really strange is going on. What I'm seeing is the stars are moving around the frame and changing shape between filters! In L and R the stars are fairly round but they get elongated in G and really like lines in B. There's no way these will stack satisfactorily. What could cause this though? Does it indicate a tilted focal plane and the blue being focused to a smaller dot is showing greater elongation? Do I need better filters? The lines aren't radiating but fairly linear across the frame L to R. I attached a gif to show this- a crop from near the centre of frame. The subs were all taken in sequence so no physical reason for the image to shift between filters. I thought my collimation was pretty good as checked with several devices.

TIA Mark

 

PS the scope is an 8" f4.5, Baader RCC-1 cc and Astronomik 2c filters, QSI583 camera

[Clarkey]

How low in the sky was the target? I am wondering whether it was atmospheric distortions which does affect the blue channel most. Only speculation - I am not sure.

[markse68]

32 minutes ago, Clarkey said:

How low in the sky was the target? I am wondering whether it was atmospheric distortions which does affect the blue channel most. Only speculation - I am not sure.

Hi Clarkey,

it was at about 58deg according to Skysafari so quite high really.

Mark

[markse68]

I think I know what the problem is- and it's my own making  

I checked older sets of subs and they only suffer focus issues with stars growing larger in R particularly (I'm focusing in L) but they don't move around like this nor change shape like this.

So what changed? Well in trying to fix the vignetting I was getting from the 2c filters, I removed the glass from the cells and made up some 3D printed frames with bigger apertures. My guess is that while it helped with the vignetting, the filters are not accurately enough held flat and parallel to the sensor and this is causing the image shift etc. Doh.

So it looks like I will need new filters after all.

So next question, Baader or Astronomik? I bought an Astronomik clear filter the other day and they have changed the cell design from the ones I have- larger outside diameter and larger aperture- almost as large as the Baaders. And I read that they improved something to make them halo free.

Mark

[wimvb]

Good you have it sorted. I was about to suggest astigmatism from the cc. In that case you should see a difference in focus between R and B, with the help of a Bahtinov mask, for example.

Edited July 22, 2021 by wimvb

[markse68]

Hi Wim, I do see a difference in focus with Bahtinov mask but just assumed it was because of the glass in the CC. Can't do much about that other than re-focussing though as I need a CC. Is astigmatism a common issue with CCs? 

I did notice that the astronomik filters although spec'd at 1mm thick do actually vary in thickness a little

Mark

[wimvb]

Chromatic aberration is a focus position shift between filters (colours), but stars stay round. Astigmatism will give elongated stars.

https://www.britannica.com/technology/aberration#ref61613

[markse68]

Hmm- wouldn't that affect the least in focus colour most though which seems to be red (has the largest star shapes anyway)? Maybe I do also have a collimation issue with the light cone from the secondary off-axial which I think would also cause astigmatism- I did tweak things to try and centre the light cone on the sensor but maybe that wasn't such a good idea either

Mark

[markse68]

They didn't stack at all well lol