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Any collimation experts able to help

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#1 dcaponeii

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Posted 18 September 2021 - 09:57 PM

I've posted a series of images taken with my 12" Meade ACF and it's got us stumped.  Trying to tune this beast and we think it's got a serious issue.  

 

https://www.cloudyni.../#entry11371151

 

Thanks in advance for whatever help you can provide as to sources of the issue, especially AT FOCUS.  Metaguide shows a centroid which does not appear in my images.


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#2 Borodog

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Posted 18 September 2021 - 10:24 PM

To summarize, Don has a Meade ACF SCT that seems to show such severe astigmatism that there is no or only a very small central spot in the Airy disk when at critical focus. The scope seems fairly well collimated but shows vicious astigmatism. We have been trying to help him diagnose what would be wrong with the scope (or the rest of his optical train) that could cause this.


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#3 C0rs4ir_

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Posted 20 September 2021 - 06:40 AM

send back to the vendor?



#4 Visit-the-Moon

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Posted 20 September 2021 - 07:10 AM

I have seen astigmatism like this in my 9.25" SCT when the primary mirror collar on the central baffle is tightened onto the primary mirror. This telescope has been dismantled several times. Easing the collar back to just light finger-tight and the astigmatism is gone. This telescope works like a champ. These mirrors are very sensitive. I have also seen this type of astigmatism in the Newtonians I've owned where any tab or bolt touching the mirror was overtightened (i.e. more than just touching the mirror). Has the telescope been opened up at any time? Also is the astigmatism more pronounced in some pointing positions?

 

A second possibility. The ACF design is a little different to the SCT design. In the SCT there are two spherical mirrors: adjusting the secondary simply roles a sphere to one side or the other. There is no optical axis inherent in either mirror and SCTs are quite easy to collimate. In my 8" and 9.25" SCTs the secondary mirrors are dead centre. In the ACF design the secondary is aspherical (hyperbolic) and consequently has an optical axis, so adjusting the tilt of the secondary moves this optical axis. However the ACF only has three adjustment screws and the mirror cannot be adjusted laterally. If you measure the mirror placement in the ACF you will probably find that the secondary appears to be off-centre by a few mm. Certainly the case in my 12" f/10 Meade. This factory placement achieves the lateral positioning to line the secondary optical axis up with the optical axis as defined by the eyepiece/camera. If the secondary optical axis is out of alignment, the telescope is likely to suffer from astigmatism. This is probably why many ACF Meade users complain about not being able to collimate their telescopes. Maybe they dismantled the secondary to install Bob's knobs or for cleaning but then centre everything. My 12" gives very sharp images but the collimation seems to be more sensitive than in my 9.25" SCT; the tiniest adjustments make a difference. When it is adjusted to be out of collimation there is some astigmatism. When collimated accurately there is no astigmatism. Has the centration of the secondary mirror been adjusted? I expect that an f/8 will be more sensitive than an f/10 system - I've only used my f/10 ACF.


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#5 luxo II

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Posted 20 September 2021 - 07:19 AM

That’s astigmatism and collimation can’t fix it.

As per above it’s possible one of the elements is under strain, from a clamping ring, retaining ring or lens cell that is too tight. This can happen to the corrector, the primary, the secondary.

If you have been tinkering with the scope think carefully what you touched and whatever it was, loosen it.

Retaining screws should not be tight, and if you are worried they’ll fall out, secure them with a dab of women’s nailpolish (use black, red looks hideous on a scope).

Secondly, the thermal expansion of glass and metal is different. In particular the corrector should be sitting on paper or cork shims that act as little spacers, for two reasons - to stop the glass making contact with metal, and to allow gaps so that the tube can expand and contract without squeezing the corrector laterally. If it does, the image will be degraded - I have seen this in a few SCTs where to owners did not understand the purpose of the shins and renived them - and screwed down the retaining ring tight to the point I was surprised the glass had not cracked.

I’ve also seen one where someone decided it was a good idea to pack the space behind the secondary mirror with wadding (probably to stop it rattling). Mistake; the pressure (while seemingly tiny) was enough to ruin star images.

Edited by luxo II, 20 September 2021 - 07:32 AM.

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#6 dcaponeii

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Posted 22 September 2021 - 05:16 PM

As a follow up the other night I decided to believe that the ADC Align feature in Firecapture was actually trying to tell me something when the circles were still side by side after aligning my ADC in the vertical direction. Loosening the set screws on my focuser and adjusted the tilt on my optical train until the circles completely overlapped. Imaging was miraculously better mostly due to outstanding seeing conditions. BUT even as seeing degraded about three hours into the observing session the orthogonal flares on Jupiter’s moons did not appear as they have been for MONTHS prior to tweaking the alignment. Time will tell. I’ll get to be out again this weekend.
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#7 Tangent

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Posted 22 September 2021 - 05:42 PM

A second possibility. The ACF design is a little different to the SCT design. In the SCT there are two spherical mirrors: adjusting the secondary simply roles a sphere to one side or the other. There is no optical axis inherent in either mirror and SCTs are quite easy to collimate. In my 8" and 9.25" SCTs the secondary mirrors are dead centre. In the ACF design the secondary is aspherical (hyperbolic) and consequently has an optical axis, so adjusting the tilt of the secondary moves this optical axis. However the ACF only has three adjustment screws and the mirror cannot be adjusted laterally. If you measure the mirror placement in the ACF you will probably find that the secondary appears to be off-centre by a few mm. Certainly the case in my 12" f/10 Meade. This factory placement achieves the lateral positioning to line the secondary optical axis up with the optical axis as defined by the eyepiece/camera. If the secondary optical axis is out of alignment, the telescope is likely to suffer from astigmatism. This is probably why many ACF Meade users complain about not being able to collimate their telescopes. Maybe they dismantled the secondary to install Bob's knobs or for cleaning but then centre everything. My 12" gives very sharp images but the collimation seems to be more sensitive than in my 9.25" SCT; the tiniest adjustments make a difference. When it is adjusted to be out of collimation there is some astigmatism. When collimated accurately there is no astigmatism. Has the centration of the secondary mirror been adjusted? I expect that an f/8 will be more sensitive than an f/10 system - I've only used my f/10 ACF.

I wonder when Meade went to a spherical secondary in their non-ACF SCTs?  My Meade 10" 2120 (of MSCOG vintage) uses an aspheric secondary:

 

Meade2080.png

 

When I got the scope the original owner had sent in the original optics to get recoated due to tarnishing.  While he was careful  in putting the corrector back in the proper orientation, I had a lot of astigmatism, which was largely removed by changing the position of the secondary WRT the primary (I still have to do some work to finalize this process). 


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#8 Visit-the-Moon

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Posted 23 September 2021 - 03:42 AM

Tangent,

 

I had thought that Meade introduced the aspheric ACF design originally as the "RCX400 line" in 2005, where RC = advanced Ritchey-Chretien. This label was used to promote the design "Own your own Hubble, etc." and it created controversy. Two companies sued Meade, Star instruments and RC Optical Systems. They settled out of court in 2008. You can look all this up pretty easily. Anyway in 2009 the design was re-badged as the ACF - Advanced Coma-Free telescope. Pre-RCX Meades would be based on the original spherical secondary mirror design, though this manual excerpt is from 1983: https://deepskies.co...080-2120man.pdf - Interesting. I'm much more familiar with Celestron SCTs.

 

So this is the one with the silver coating? 

 

I only have one Meade (ACF), it was put into its box in 2016. I measured the distance between the external tube (inside face) to the outer edge of the secondary housing at various positions. The largest difference between opposite sides was 125.1 mm vs 122.8 mm. Perpendicular to this axis the distances were very close at 123.8 vs 124.0. So the secondary holder at least does appear to be slightly off-centre. I won't be adjusting that centration as the collimation is spot on.

 

decaponii,

 

So you changed the lateral position and the tilt of an external focuser to improve alignment of the optical axis? That makes sense.


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#9 dcaponeii

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Posted 23 September 2021 - 01:02 PM

Tangent,

 

I had thought that Meade introduced the aspheric ACF design originally as the "RCX400 line" in 2005, where RC = advanced Ritchey-Chretien. This label was used to promote the design "Own your own Hubble, etc." and it created controversy. Two companies sued Meade, Star instruments and RC Optical Systems. They settled out of court in 2008. You can look all this up pretty easily. Anyway in 2009 the design was re-badged as the ACF - Advanced Coma-Free telescope. Pre-RCX Meades would be based on the original spherical secondary mirror design, though this manual excerpt is from 1983: https://deepskies.co...080-2120man.pdf - Interesting. I'm much more familiar with Celestron SCTs.

 

So this is the one with the silver coating? 

 

I only have one Meade (ACF), it was put into its box in 2016. I measured the distance between the external tube (inside face) to the outer edge of the secondary housing at various positions. The largest difference between opposite sides was 125.1 mm vs 122.8 mm. Perpendicular to this axis the distances were very close at 123.8 vs 124.0. So the secondary holder at least does appear to be slightly off-centre. I won't be adjusting that centration as the collimation is spot on.

 

decaponii,

 

So you changed the lateral position and the tilt of an external focuser to improve alignment of the optical axis? That makes sense.

Jury is still out but it looks like the astigmatism went away when I fixed the optical alignment of the optical train at the rear of the scope.  My secondary doesn't have any play in it's lateral position.  The mounting holes only line up in one orientation and there is no play in that position.  The secondary looked to be glued to the holder so I'm not sure I could adjust the lateral position of the secondary.  I'll have to check again to be sure.


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#10 Tangent

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Posted 23 September 2021 - 02:58 PM

Tangent,

 

I had thought that Meade introduced the aspheric ACF design originally as the "RCX400 line" in 2005, where RC = advanced Ritchey-Chretien. This label was used to promote the design "Own your own Hubble, etc." and it created controversy. Two companies sued Meade, Star instruments and RC Optical Systems. They settled out of court in 2008. You can look all this up pretty easily. Anyway in 2009 the design was re-badged as the ACF - Advanced Coma-Free telescope. Pre-RCX Meades would be based on the original spherical secondary mirror design, though this manual excerpt is from 1983: https://deepskies.co...080-2120man.pdf - Interesting. I'm much more familiar with Celestron SCTs.

 

So this is the one with the silver coating? 

It is, which puts it somewhere between 1983 and 1986 if I am correct.  Just for fun I looked at the manual for the LX200 (released in 1992), and it also refers to the secondary as being aspheric.  Puzzling for sure!

 

https://www.meade.co...ssic_Manual.pdf



#11 Borodog

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Posted 23 September 2021 - 05:09 PM

Don,

I am not at all sure that fixing the tilt in your optical train really fixed your astigmatism. I don’t see how it can. It can’t help to have sensor tilt, but I don’t think it can cause astigmatism. It will just cause misalignment between the focal plane and the sensor; you will have a stripe in focus and regions to either side that are inside of focus on one side and outside of focus on the other.
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#12 Visit-the-Moon

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Posted 23 September 2021 - 07:22 PM

"Jury is still out but it looks like the astigmatism went away when I fixed the optical alignment of the optical train at the rear of the scope.  My secondary doesn't have any play in it's lateral position.  The mounting holes only line up in one orientation and there is no play in that position.  The secondary looked to be glued to the holder so I'm not sure I could adjust the lateral position of the secondary.  I'll have to check again to be sure."

 

decaponeii.

 

That makes perfect sense. By adjusting the position/tilt of the focuser you have now aligned the eyepiece/camera with the optical axis of the hyperbolic secondary mirror. If you are off axis with respect to the secondary you can expect astigmatism. With the limited adjustments available in the ACF (i.e. only secondary mirror tilt) adjusting the focuser position is the best option. For some reason the secondary is not installed in its optimal position or was moved from that factory-set position but that would be a real pain to adjust. There are many posts on this forum where ACF owners have dismantled the secondary and then had issues. I don't have a Ritchey Chretien, however I did consider buying one at one time so looked into their collimation, they often have adjustment of the hyperbolic primary, hyperbolic secondary and the focuser positioning to achieve excellent collimation.


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#13 luxo II

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Posted 23 September 2021 - 07:52 PM

" The secondary looked to be glued to the holder"

Personally, IMHO that’s not optimal. It may be OK, but if you see astigmatism again I’d suspect the glue is deforming the secondary particularly if this appears to be temperature-related (differential expansion of dissimilar materials).

Edited by luxo II, 23 September 2021 - 07:55 PM.

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#14 Borodog

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Posted 24 September 2021 - 01:44 PM

"Jury is still out but it looks like the astigmatism went away when I fixed the optical alignment of the optical train at the rear of the scope.  My secondary doesn't have any play in it's lateral position.  The mounting holes only line up in one orientation and there is no play in that position.  The secondary looked to be glued to the holder so I'm not sure I could adjust the lateral position of the secondary.  I'll have to check again to be sure."

 

decaponeii.

 

That makes perfect sense. By adjusting the position/tilt of the focuser you have now aligned the eyepiece/camera with the optical axis of the hyperbolic secondary mirror. If you are off axis with respect to the secondary you can expect astigmatism. With the limited adjustments available in the ACF (i.e. only secondary mirror tilt) adjusting the focuser position is the best option. For some reason the secondary is not installed in its optimal position or was moved from that factory-set position but that would be a real pain to adjust. There are many posts on this forum where ACF owners have dismantled the secondary and then had issues. I don't have a Ritchey Chretien, however I did consider buying one at one time so looked into their collimation, they often have adjustment of the hyperbolic primary, hyperbolic secondary and the focuser positioning to achieve excellent collimation.

I just don't see the logic here.

 

If this were correct, that simply by changing the tilt of the sensor plane to align with the hyperbolic secondary's axis would produce get a coma free, non-astigmatic field (neglect the inherent field curvature of the ACF design here; we're talking about astigmatism), that implies that there is a coma free, non-astigmatic focal plane in space already. If we just tilt the sensor relative to this plane, I don't think we get astigmatism (although I am very willing to be proven wrong); we just get a sensor that has the focal plane in front of it on one side and behind it on the other.



#15 WadeH237

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Posted 24 September 2021 - 02:08 PM

I don't think that tilt can cause on-axis astigmatism.

 

But if you already have off-axis astigmatism (RC's do, but I don't know about the ACF scopes), then tilt can make it more apparent in the areas where focus is softer due to the tilt.

 

In reading the entire thread, I have to wonder if the optics are pinched.  That could account for the lobes in the images on the other thread.


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#16 dcaponeii

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Posted 25 September 2021 - 06:28 PM

Last night the weather didn't clear until after 10pm and then my mount started acting up on me so I haven't had a chance to assess the situation.  If the mount cooperates I should have a better idea of what's going on this evening.  The bottom line from the comments above is: It could be but probably not.  I tend to agree with that.  I think my original thinking that when seeing is good then I stay in the zone of best focus for more of the frames in the video and that minimizes the effect of the on-axis astigmatism.  I can confirm that the secondary is not pinched by anything in the mounting (the glue comment is intriguing).  I'm not brave enough just yet to pull the corrector to see if the primary mirror is being pinched by the lock nut on the baffle tube or whatever else is supporting the primary in the ACF design.

 

I can confirm that I get a focus change when changing from the eastern sky to the western sky or the reverse.  Possible that's another symptom of what's causing the astigmatism.

 

Bottom line will likely be that this will be my DSO scope and I'll have to tune up my 10" f/10 for planetary as soon as my new camera arrives.




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