Refractor out of collimation - now what?

[Megawatt]

I had previously posted about poor star shapes in my William Optics GT81, and the consensus seemed to be something was being pinched somewhere. Which might still be the case, but last night I had the GT81 out on my HEQ5 for visual use (which I don't normally do with this setup). This allowed me to crank up the magnification using a 3mm Radian and 2x barlow, and I could immediately tell that collimation is off. My in-focus star images looked exactly like figure C below. 

Visually, the scope still performs very will in spite of this. I split both components of Epsilon Lyrae cleanly with an exit pupil of just 0.25mm. My question to the group is, how likely is this mis-collimation to be the source of my poor star shapes? I have attached three cropped images which are representative of what the stars look like at the centre of the frame. Two of the images were shot with a Canon T5i, and one with my ASI2600MC. Doesn't seem to matter which camera I use. Notice the stars are not just oval, but asymmetrical and somewhat flattened on one side. I've ruled out anything to do with polar alignment or tracking. Siril's tilt inspector indicates negligible tilt. This problem has been driving me crazy for a couple of years now. 

 

[Stuart1971]

It’s hard to tell with those images, as they have been compressed to much to be able to zoom in on them and have a good look at the stars, do you have some not compressed..?

[Stuart1971]

The last image, the stars look elongated as well as some other issues going on, but the compression is doing some of that..

[Megawatt]

44 minutes ago, Stuart1971 said:

The last image, the stars look elongated as well as some other issues going on, but the compression is doing some of that..

 

[Megawatt]

Markarian's Chain May 2024 siril ASI.tif

In this image, the stars turned out relatively okay. This is why I'm having such a tough time figuring things out; results are not consistent, and I can't see any reason for my star shapes to change night to night. 

The only other thing I can think of is that my focuser does not use a threaded connection to attach the camera or flattener, so I'm wondering if things end up a bit skewed after tightening down the compression ring. But I make sure the connection is solid and there doesn't seem to be any sag or play in either the focuser drawtube or the connection with the camera. 

Edited August 25 by Megawatt

[Albir phil]

18 minutes ago, Megawatt said:

Markarian's Chain May 2024 siril ASI.tif 149.33 MB · 1 download

In this image, the stars turned out relatively okay. This is why I'm having such a tough time figuring things out; results are not consistent, and I can't see any reason for my star shapes to change night to night. 

The only other thing I can think of is that my focuser does not use a threaded connection to attach the camera or flattener, so I'm wondering if things end up a bit skewed after tightening down the compression ring. But I make sure the connection is solid and there doesn't seem to be any sag or play in either the focuser drawtube or the connection with the camera. 

I have the same scope as yours my flattner/reducer screws directly onto the focuser then my camera screws onto the flattner reducer.Can you not do this with your GT81 or am I misinterpreing what you are saying🤔

 

 

[Stuart1971]

32 minutes ago, Megawatt said:

 

Well they don’t look to bad at all TBH, I don’t think there is a collimation issue there, a bit of pinching maybe as some of the stars have some extra spikes or flares, but I have seen worse on some scopes used with poor filters…

[Stuart1971]

30 minutes ago, Megawatt said:

Markarian's Chain May 2024 siril ASI.tif 149.33 MB · 1 download

In this image, the stars turned out relatively okay. This is why I'm having such a tough time figuring things out; results are not consistent, and I can't see any reason for my star shapes to change night to night. 

The only other thing I can think of is that my focuser does not use a threaded connection to attach the camera or flattener, so I'm wondering if things end up a bit skewed after tightening down the compression ring. But I make sure the connection is solid and there doesn't seem to be any sag or play in either the focuser drawtube or the connection with the camera. 

Ah, yes you deffo need to ditch the compression ring, that could very easily cause major issues and headaches…they should be banned 😂😂

[Megawatt]

51 minutes ago, Albir phil said:

I have the same scope as yours my flattner/reducer screws directly onto the focuser then my camera screws onto the flattner reducer.Can you not do this with your GT81 or am I misinterpreing what you are saying🤔

 

 

Mine is an older model with no threads on the focuser. Also non-adjustable flattener and no built-in focusing mask.

49 minutes ago, Stuart1971 said:

Ah, yes you deffo need to ditch the compression ring, that could very easily cause major issues and headaches…they should be banned 😂😂

Not sure how I can ditch it without ditching the whole scope. Which might be my best option, other than I can't afford to do it. 

This still leaves me with the question of mis-collimation, and how much of an impact it might have on my images. 

[Peter Drew]

If this is what you are seeing visually at image "C" then that definitely shows alignment error due either objective collimation, focuser alignment or image train sag.  My money would be on the objective.  No sign of pinched optics.    Try a laser in the eyepiece holder and see if the beam exits centrally through the objective, this will confirm or eliminate focuser problems.     🙂 

[Peter Drew]

Just had a look at the spec of the GT81 and noted that it is a triplet.  The centre component is very sensitive to alignment, if you have to attempt collimation I would suggest trying this component first.  Very small adjustments would be needed as the apparent collimation is not too far off.  Back off an adjustment screw a fraction to start, tightening in the first instance could chip the edge of the lens.    🙂

[Megawatt]

27 minutes ago, Peter Drew said:

If this is what you are seeing visually at image "C" then that definitely shows alignment error due either objective collimation, focuser alignment or image train sag.  My money would be on the objective.  No sign of pinched optics.    Try a laser in the eyepiece holder and see if the beam exits centrally through the objective, this will confirm or eliminate focuser problems.     🙂 

Hi Peter, 

My laser comes out dead centre all the way through the focuser's travel. Everything else seems solid once connected. 

I'm very hesitant to mess with collimation. When I remove the dew shield, there are several sets of 3 grub screws around the lens cell. Do you think it's safe to assume that each screw is for centring a different lens element?

[Albir phil]

51 minutes ago, Megawatt said:

Mine is an older model with no threads on the focuser. Also non-adjustable flattener and no built-in focusing mask.

Not sure how I can ditch it without ditching the whole scope. Which might be my best option, other than I can't afford to do it. 

This still leaves me with the question of mis-collimation, and how much of an impact it might have on my images. 

Well t b h  it's already been said, I personally don't think there is a mager issue with your image. If it was my scope I would not mess with it unless I new what I was doing,as Peter Drew dose🫡

[Peter Drew]

3 hours ago, Megawatt said:

Hi Peter, 

My laser comes out dead centre all the way through the focuser's travel. Everything else seems solid once connected. 

I'm very hesitant to mess with collimation. When I remove the dew shield, there are several sets of 3 grub screws around the lens cell. Do you think it's safe to assume that each screw is for centring a different lens element?

Yes, there is a set of three collimation screws for each lens element.  The fact that your laser image is centred further points towards collimation of the objective being slightly out.  as commented by Phil, if you are happy enough with your images perhaps best leave well alone.   🙂

Edited August 25 by Peter Drew

[Megawatt]

2 hours ago, Peter Drew said:

Yes, there is a set of three collimation screws for each lens element.  The fact that your laser image is centred further points towards collimation of the objective being slightly out.  as commented by Phil, if you are happy enough with your images perhaps best leave well alone.   🙂

On the contrary, I feel that these distorted stars are ruining my images. 

In your opinion, would such mis-collimation result in the distorted stars from my sample images? I can't seem to find examples online of how collimation impacts images from a refractor. 

 

[Stuart1971]

14 hours ago, Peter Drew said:

Just had a look at the spec of the GT81 and noted that it is a triplet.  The centre component is very sensitive to alignment, if you have to attempt collimation I would suggest trying this component first.  Very small adjustments would be needed as the apparent collimation is not too far off.  Back off an adjustment screw a fraction to start, tightening in the first instance could chip the edge of the lens.    🙂

I would certainly not recommend this at all, he would be much better off paying a small fee and sending the scope to RVO who have all the proper Zygo test and adjustment kit for doing this job, they can get it spot on for the OP for minimal cost, they do all there scopes including WO scopes before they sell if required by the buyer, I had my Esprit 100 checked and was good..before I purchased from RVO..

[Peter Drew]

9 hours ago, Megawatt said:

On the contrary, I feel that these distorted stars are ruining my images. 

In your opinion, would such mis-collimation result in the distorted stars from my sample images? I can't seem to find examples online of how collimation impacts images from a refractor. 

 

The collimation of a triplet does not necessarily mean tipping of the lens stack as with a doublet.  Radial displacement of a component of a triplet, particularly the central lens can produce the problem mentioned, I have corrected a few triplets based on the procedure I described and offered the advice if there was no alternative other than DIY.

[Peter Drew]

49 minutes ago, Stuart1971 said:

I would certainly not recommend this at all, he would be much better off paying a small fee and sending the scope to RVO who have all the proper Zygo test and adjustment kit for doing this job, they can get it spot on for the OP for minimal cost, they do all there scopes including WO scopes before they sell if required by the buyer, I had my Esprit 100 checked and was good..before I purchased from RVO..

I agree.  My advice was on the grounds of "if all else fails".  The OP is in Canada and may not wish to send his telescope abroad.    

[Stuart1971]

1 hour ago, Peter Drew said:

I agree.  My advice was on the grounds of "if all else fails".  The OP is in Canada and may not wish to send his telescope abroad.    

Good point, and missed by me, I stupidly assumed they were in the UK, apologies…👍🏻

[CraigT82]

You could check the lens collimation with a Cheshire. Looking through the Cheshire you’ll see multiple circular reflections of the angled face of Cheshire. All these circles should be concentric with the dark spot in the centre (the reflection of the Cheshire peephole). Misscolimation of a lens should present as non-concentric circular reflections. Although it doesn’t really tell you which lens is out, it might confirm your suspicion of misscollimation and not pinching or something else.