TS optics 80mm triplet - acceptable chromatic abberation?

[R1k]

Hi guys, recently picked up a used TS optics 80mm FPL53 triplet. I have come from a SW evostar 80ED doublet. 

 

I am slightly disappointed with the colour correction with the scope, I am using the dedicated 0.79x FF/FR and a canon 800d at 61mm back spacing (as per manufacturer recommendation). I expected stars totally free of CA but instead find that bright white stars are surrounded by a small rim of blue CA (see autostretched single 150s frame of M45 attached as an example.) 

 

Would you guys say that this is acceptable CA for this scope or should I be right in being disappointed? I am considering sending the scope to TS optics who have agreed to look at it. 

 

Thanks

Rik 

Light_0028_ISO200_150s_19C-St.tiff

Edited December 15, 2020 by R1k

[vlaiv]

I don't think this is chromatic aberration in the image.

I think it is some sort of dispersion. Either atmospheric dispersion, or what is more likely - something to do with FF/FR and tilt.

From what I'm seeing - blue is shifted downwards and red is shifted upwards in these star images (blue is not halo around whole star).

Also, note bottom corner stars:

Other corners are fine except for opposite corner that is the worst offender in dispersion effect (at least it seems so to my eye):

I don't think this is due to scope, but it could be due to collimation of the scope. Maybe best thing to do would be to take a series of test images:

- star cluster high in the sky so you avoid any atmospheric dispersion

- with FF/FR removed - and examine center of the field - to see what the stars look like.

- aiming near zenith will also minimize seeing. Atmospheric seeing impacts blue wavelengths more then red (as they are shorter), and often blue is slightly more blurred than red and green channel due to this.

If you still have this RGB separation without FF/FR - then it might be issue with collimation or something. If you don't have it without FF/FR - then look into FF/FR connection. Do you have threaded connection or you use 2" clamping. I had issue with 2" clamping mechanism and this scope FF/FR combination.

It would shift - and one day I would have good images and next day - astigmatic stars in corners.

[Ken82]

Check this out, looks similar to an issue I had -

Can you post separate R G B subs ?

Ken

[R1k]

1 hour ago, vlaiv said:

I don't think this is chromatic aberration in the image.

I think it is some sort of dispersion. Either atmospheric dispersion, or what is more likely - something to do with FF/FR and tilt.

From what I'm seeing - blue is shifted downwards and red is shifted upwards in these star images (blue is not halo around whole star).

Also, note bottom corner stars:

Other corners are fine except for opposite corner that is the worst offender in dispersion effect (at least it seems so to my eye):

I don't think this is due to scope, but it could be due to collimation of the scope. Maybe best thing to do would be to take a series of test images:

- star cluster high in the sky so you avoid any atmospheric dispersion

- with FF/FR removed - and examine center of the field - to see what the stars look like.

- aiming near zenith will also minimize seeing. Atmospheric seeing impacts blue wavelengths more then red (as they are shorter), and often blue is slightly more blurred than red and green channel due to this.

If you still have this RGB separation without FF/FR - then it might be issue with collimation or something. If you don't have it without FF/FR - then look into FF/FR connection. Do you have threaded connection or you use 2" clamping. I had issue with 2" clamping mechanism and this scope FF/FR combination.

It would shift - and one day I would have good images and next day - astigmatic stars in corners.

Massive thanks for the detailed reply

I have uploaded:

- a single raw image of the doulbe cluster in perseus (right up at the zenith) with the FF/FR installed

- a single JPG image of Vega (sorry no Raw for this) of Vega in the centre of the screen showing the same colour separation) without the FF/FR attached. 

 

TS optics seem to think this is a collimation issue. 

 

Thanks again

 

 

Light_0009_ISO200_150s__18C.CR2

[R1k]

1 hour ago, Ken82 said:

Check this out, looks similar to an issue I had -

Can you post separate R G B subs ?

Ken

Cheers Ken - read the whole thread and was sad to hear you returned the scope!

 

Will post the R G and B channels tomorrow - using an OSC DSLR. 

 

Thanks

Rik

[vlaiv]

7 minutes ago, R1k said:

TS optics seem to think this is a collimation issue. 

It looks like that is indeed the case - best course of action would be to return the scope to get it collimated.