43 replies to this topic

[rfcooley]

Hello All,

 

I saved a Celestron C114GT from a crushing death. I am well aware of the difficulties with a Jone/Bird design. I understand the J/B is probably the most hated OTA on this forum and others.  But, I got to try. I have come across this design many times in outreach and have avoided these OTAs like the plague and told others to do so also.   I am sometimes my own worst enemy. So, I could not resist having one for myself to play with.

 

The issue is collimation and it goes like this:

• Collimate like Celestron recommends in the manual. Center everything up looking down the focuser.  Take it outside the field of view looks like a bunch of little comets. Can't get any sharp focus, tails get smaller but they don't ever really go away.
• Collimate with a Cheshire. Every thing is concentric and matching according to Vic Menard's illustrations in his books. Take in to the field. The field of view is a bunch of little comets across the field. Not as bad has previous method but still not good.
• Collimate with a Laser. J/B lens removed. Glatter laser. Everything aligns. Re install J/B lens making sure it is in correctly. Take it into the field and, you guessed it. Little comets through out the field of view. The comet tails are smaller than the previous collimation, but still there.
• Repeated all of the collimation methods several times. Still same results. (What is it they say about crazy people.) 
• Disassembled OTA to inspect for any kind of damage. Removed mirrors, cell, secondary and spider  Carefully visually inspecting all components and reassembling I found a small dimple in the tube assembly directly under the focuser assembly. I thought this could cause a misalignment in the focuser and possibly the culprit. I worked the dimple out. I do have some experience in auto/aircraft sheet metal work. I was able to pop it out. Re-assembled OTA. Installed new stainless steel primary springs and secondary spring, just in case. New secondary adjusting knobs, slightly longer than the Allen hex heads that came with the OTA. To allow for more adjustment if needed.
• Collimated assembly with a Cheshire. Took it out in the field and.... comets. !@#$%^.............. The results however were slightly better than previous attempts. I believe owing to the dimple under the focuser causing some focuser misalignment. However I would have thought that would have showed up in previous collimation attempts???

Since the last attempt was the best of the previous tests I did a star test. Popped a 9.0mm ep in which would give me max usable mag in average skies, went to Spica because it was about half way between horizon and zenith at my location and began adjusting the primary mirror.  Moving image in the direction of the tails to edge of field. Re centered Spica. Repeated this 3 times, each time tails got shorter image appeared to get sharper until I achieved a reasonably pin point star. Reasonable in the respect of what model of OTA I was working with. I was satisfied that the telescope was usable and used it for most of the evening on Globs, open clusters to see what kind of resolution I could achieve. Mindful that it is a 114GT. However generally the results were better than I expected. I was able to push mag to 180x as sky condition improved (20.3Mpsas) and still resolve some stars in M28; N6638; N6642 and A lot in M22. Some nebulosity in Lagoon and Trifid and open clusters in the area were resolvable. All and all pretty good I thought.

 

Now for the kicker and the question. I looked through the focuser to check collimation and everything was askew.  Stuck the Cheshire in and nothing was lined up. It all seemed offset. Cheshire crosshairs appeared to be over center of primary mirror, secondary significantly off to the side in the lower quadrant of the crosshairs  Put an ep back in and the views were still good..... What is going on?   T

he views were good I could just leave it the way it is and forget about it, since this project was just to fiddle with anyway. But the views are reasonable for what it is.  Reasonable enough that I would take it out once in awhile just to see what I can see. But, like the Enquirer, I just gotta to know why everything appears so off when collimating and still give a reasonable view?

 

I check collimation on all of my OTAs before each use.  I have taken OTAs completely apart and put them back together and collimated with all of the above methods and other than having to make some fine adjustment during a star test the collimation is generally pretty close.  What I don't understand is why this 114GT looks so far out when using a collimation device requiring large adjustments during a star test to get fairly sharp focus and reasonably pin point stars.

 

Is this a phenomenon of the spherical mirror? The secondary?  As I understand it, with a spherical mirror coma will worsen has you go out toward the field edge. This 114 does exhibits this aberration clearly and is to be expected. I estimate that the 20 to 30% of the field edge is aberrated. Or, could this be a consequence of the J/B lens?  I am inclined either way to believe there is something involved in the mirrors/lens themselves but I don't know the how and why?

 

If a laser/beam of light, visually observed, is straight then it stands to my reason that would indicate mirrors are in line and and collimation should be close.  Not requiring large amount of adjustment such as the 114 does. In all of my OTAs that require periodic collimation  it only takes slight adjustment during a star test to achieve good results regardless of which method I use. 

 

I apparently have gone down a rabbit hole, which usually has several exits. I believe that one of those exits has the answer to why this 114 is so weeeeird. Any suggestion will be considered and tested if possible.

 

Clear Skies, Never Lose the Wonder.

 

RF

 

 

 

[Starman1]

The lens in the focuser could be:

--of poor quality (high likelihood of this)

--tilted relative to the optical light path as collimated without the lens in place (high likelihood of this)

OR

--the center marker on the primary could be seriously off-center on the mirror. (remove mirror and measure)

OR

--your laser is badly miscollimated (test by rotating 90°, tighten screw, repeat 3 times and see if laser beam makes a circle on the mirror)

 

So, if the scope is well-collimated without the J-B lens, and the secondary is centered under the focuser, and the secondary tilt has been adjusted well using the laser,

and the Barlowed laser or Cheshire shows the primary well-collimated, then the addition of the lens miscollimates the scope.  The fact you can adjust the primary mirror to eliminate the comet-like star images

says this is the case.  One easy way to verify it is the lens is to remove the lens and substitute a 2x or 3X Barlow in its place.  If the star images on axis are point-like, then you know it's the lens.

If it is just as far off as before, look to an off center center marker on the primary.

 

Many of the ones I've seen did not have a center marker on the primary mirror.  I hope you're not attempting to collimate one without a center marker...

[ram812]

...Or the barlow lens in the focuser is "Flipped". I collimate mine by removing the lens first, then re-install and check again. Made all the difference. The real crux of the setup is the focuser/rack. Nice and tight is the rule. If it's been apart just remove and flip. My OTA shows beautiful star clusters and the planets so well it's my "Grab'n'Go" scope now over my ST80. Beats lugging the XT10/CGEM out for a quick view😁!

Good luck!
Ralph



Edit: These do have "Sweet spots" on the primary. Secondaries are pretty good size but the thing can be fickle. Center spotting the primary, though, will help lots...especially troubleshooting!😃

Edited by ram812, 19 June 2025 - 05:57 PM.

[rfcooley]

The lens in the focuser could be:

--of poor quality (high likelihood of this)

I thought of that unfortunately I don't have another j/b lens to test this. But I will try removing the j/b lens and try it with a Barlow.

• Do I place the Barlow in the ep holder or disassemble Barlow and drop it in the focuser tube where the J/B resides.

--tilted relative to the optical light path as collimated without the lens in place (high likelihood of this)

 

OR

--the center marker on the primary could be seriously off-center on the mirror. (remove mirror and measure)

• I center marked the primary with a doughnut . First thing I did when I got the scope. So I know it is right. I suspected the previous owner wasn't collimating it right because there was no center mark. He was using the method Celestron writes in the manual. Look down the focuser a make everything centered according to illustration.

OR

--your laser is badly miscollimated (test by rotating 90°, tighten screw, repeat 3 times and see if laser beam makes a circle on the mirror)

• My Glatter, and the other lasers have been collimated and adjusted on a laser bench in the college physics department by me and the physics instructor who is also the Director of the Observatory.  They work fine on all the Newtons we have. The SCT are collimated with a HoTech we have at the observatory.

 

So, if the scope is well-collimated without the J-B lens,check and the secondary is centered under the focuser,check and the secondary tilt has been adjusted well using the laser, check

and the Barlowed laser or Cheshire shows the primary well-collimated,check then the addition of the lens miscollimates the scope.  The fact you can adjust the primary mirror to eliminate the comet-like star images

says this is the case. I concur  One easy way to verify it is the lens is to remove the lens and substitute a 2x or 3X Barlow in its place.  If the star images on axis are point-like, then you know it's the lens.

If it is just as far off as before, look to an off center center marker on the primary.

 

Many of the ones I've seen did not have a center marker on the primary mirror.  I hope you're not attempting to collimate one without a center marker...I am not.

Don,

 

Thank you for your thoughts and prompt response.  I wondered if the J/B was a problem. I just could not figure out how to find out. Some articles I read said J/B is a Barlow other said it isn't quite.  I did remove and replace flipping the J/B earlier to see if possibly it was in wrong. It is in correctly. I am going to try the replacing the J/B with a Barlow as you mentioned. I have several of those about.

 

If I disassemble a Barlow I am not sure if it will fit into the cavity where the J/B resides.

 

I will try this today and test it tonight.

 

Clear Skies, Never Lose the Wonder

 

RF
 

[Starman1]

Don,

 

Thank you for your thoughts and prompt response.  I wondered if the J/B was a problem. I just could not figure out how to find out. Some articles I read said J/B is a Barlow other said it isn't quite.  I did remove and replace flipping the J/B earlier to see if possibly it was in wrong. It is in correctly. I am going to try the replacing the J/B with a Barlow as you mentioned. I have several of those about.

 

If I disassemble a Barlow I am not sure if it will fit into the cavity where the J/B resides.

 

I will try this today and test it tonight.

 

Clear Skies, Never Lose the Wonder

 

RF
 

Don't disassemble the Barlow, just try it in front of the eyepiece as you normally would.

Since it will double or triple the f/ratio of the scope for the eyepiece, you might get to the point where the difference between spheroidal and paraboloidal primaries are <1/4 wave.

I think that occurs around f/8-f/9 for a 4.5" mirror.

[rfcooley]

Don't disassemble the Barlow, just try it in front of the eyepiece as you normally would.

Since it will double or triple the f/ratio of the scope for the eyepiece, you might get to the point where the difference between spheroidal and paraboloidal primaries are <1/4 wave.

I think that occurs around f/8-f/9 for a 4.5" mirror.

Thank you Don. I will try that.

 

RF

[Starman1]

Thank you Don. I will try that.

 

RF

The Barlow will not control the spherical aberration from the spheroidal mirror, but it will tell you whether the lens in the focuser is simply a bad, or tilted, lens.

[rfcooley]

The Barlow will not control the spherical aberration from the spheroidal mirror, but it will tell you whether the lens in the focuser is simply a bad, or tilted, lens.

The wind is gusting to 20mph and averaging around 10 to 12. So I did not set up and test your suggestions. I will try Saturday or Sunday night.

 

A couple of question I had was about the J/B lens being tilted. Would this be tilted in the housing or tilted in the focuser.  There appeared to be no shims in the housing. Did they sometimes have shimming to correct a tilt?  Also, I have recently come across some conflicting information about the proper position of the J/B lens. It is a doublet, there is a thinner lens cemented to a longer lens. It originally came out with the thinner lens going toward the focuser.(The person that gave me this scope said he did not take anything apart because he didn't know anything about it. He doesn't like tinkering with things)  Is the proper configuration thinner lens toward focuser or toward the secondary, or does it really matter.

 

RF

[Starman1]

The wind is gusting to 20mph and averaging around 10 to 12. So I did not set up and test your suggestions. I will try Saturday or Sunday night.

 

A couple of question I had was about the J/B lens being tilted. Would this be tilted in the housing or tilted in the focuser.  There appeared to be no shims in the housing. Did they sometimes have shimming to correct a tilt?  Also, I have recently come across some conflicting information about the proper position of the J/B lens. It is a doublet, there is a thinner lens cemented to a longer lens. It originally came out with the thinner lens going toward the focuser.(The person that gave me this scope said he did not take anything apart because he didn't know anything about it. He doesn't like tinkering with things)  Is the proper configuration thinner lens toward focuser or toward the secondary, or does it really matter.

 

RF

https://www.telescop...e_corrector.htm

Go down the page to Jones-Bird Corrector.

The concave surface faces the eyepiece.

If the lens is cemented in its housing, it could easily be tilted.

If the lens merely inserts in the focuser rather than thread in, it could be tilted in the focuser.

The source of the tilt could be and/or.

 

If it threads in, try unthreading it slightly to see if that improves the image.  You can always add rubber o-rings so it stops at the right place if its best image is farther from the eyepiece.

[rfcooley]

https://www.telescop...e_corrector.htm

Go down the page to Jones-Bird Corrector.

The concave surface faces the eyepiece.

If the lens is cemented in its housing, it could easily be tilted.

If the lens merely inserts in the focuser rather than thread in, it could be tilted in the focuser.

The source of the tilt could be and/or.

 

If it threads in, try unthreading it slightly to see if that improves the image.  You can always add rubber o-rings so it stops at the right place if its best image is farther from the eyepiece.

Gotcha, 

 

The J/B lens is not glued into the hosing. Once you remove the retaining ring it slips right out. However, it appears that the housing the lens sits it is glued to focuser tube. I see inside the the housing what appears to be a collar that appears to be threaded inside the housing.( possibly some kind of adjustment that you mentioned that would raise and lower the J/B lens in the housing)   Any suggestions as to how I could remove the housing from the focuser? I hesitate to use a little heat due to the plastics.

 

RF

[Asbytec]

So, the issue with J/B is the lens is it has an optical axis that must be collimated with the primary optical axis, as well. There is no real way I am aware of to ensure the lens is collimated other than trusting it is not tilted in, and is assumed to be aligned with, the focuser axis.

 

As I understand your progress so far, you collimated with your quality and collimated tools but experienced what seems to be coma on axis. Then, you tilted the spherical primary and found a sweet spot. So, there is a sweet spot despite the corrector lens in the light path.

 

The kicker problem is, as you describe, everything seems out of whack. The cross hair was still on the primary center suggesting focuser (and lens) alignment, but the diagonal was tilted out of position. The good star images indicate, then, also indicate the primary axis is aligned despite the diagonal displacement. 

 

I do not understand how tilting the primary caused the position of the diagonal to no longer appear concentric with the focuser while retaining focuser axial alignment. The diagonal may well have needed to be where it is to align both axes suggesting it was not well centered under the focuser to begin with. 

 

It is possible to align both axes and produce good star images in the center of the field with almost any diagonal placement. Do you have a picture of the out of whack alignment taken through a pupil of the Cheshire or site tube? It's possible your scope needed primary star alignment to correct the primary axis. 

 

I'm curious, too, if a Barlow lens can adequately correct the spherical aberration of the spherical primary mirror. 

Edited by Asbytec, 21 June 2025 - 11:25 PM.

[Starman1]

So, the issue with J/B is the lens is it has an optical axis that must be collimated with the primary optical axis, as well. There is no real way I am aware of to ensure the lens is collimated other than trusting it is not tilted in, and is assumed to be aligned with, the focuser axis.

 

As I understand your progress so far, you collimated with your quality and collimated tools but experienced what seems to be coma on axis. Then, you tilted the spherical primary and found a sweet spot. So, there is a sweet spot despite the corrector lens in the light path.

 

The kicker problem is, as you describe, everything seems out of whack. The cross hair was still on the primary center suggesting focuser (and lens) alignment, but the diagonal was tilted out of position. The good star images indicate, then, also indicate the primary axis is aligned despite the diagonal displacement. 

 

I do not understand how tilting the primary caused the position of the diagonal to no longer appear concentric with the focuser while retaining focuser axial alignment. The diagonal may well have needed to be where it is to align both axes suggesting it was not well centered under the focuser to begin with. 

 

It is possible to align both axes and produce good star images in the center of the field with almost any diagonal placement. Do you have a picture of the out of whack alignment taken through a pupil of the Cheshire or site tube? It's possible your scope needed primary star alignment to correct the primary axis. 

 

I'm curious, too, if a Barlow lens can adequately correct the spherical aberration of the spherical primary mirror. 

The Barlow wouldn't correct the SA, but it could easily tell if the problem with the comet-like star images are due to the lens or the other elements in the optical train.

I suspect the lens housing is glued into the focuser with a tilt so that when the lens is put in the housing in the focuser, it is tilted relative to the optical axis, creating astigmatism.

This might be verifiable by measuring the distance to the lens on two sides from the top of the focuser where the eyepiece goes.

[Asbytec]

The Barlow wouldn't correct the SA, but it could easily tell if the problem with the comet-like star images are due to the lens or the other elements in the optical train.

I suspect the lens housing is glued into the focuser with a tilt so that when the lens is put in the housing in the focuser, it is tilted relative to the optical axis, creating astigmatism.

This might be verifiable by measuring the distance to the lens on two sides from the top of the focuser where the eyepiece goes.

I believe you're right and I don't believe a Barlow corrects for spherical aberration, specifically, but extending the focal length/ratio might reduce it to about 1/4 wave at around f/9 or so. The corrector lens, which is likely designed to reduce spherical aberration resulting in 2x the primary focal ratio, may be tilted in the focuser.

 

But there was a solution of primary tilt - after collimation with collimated quality tools - that rendered nicer star images regardless of the lens. This configuration, for some reason, resulted in the "Cheshire" cross hair aligned with the primary center, but the diagonal position away from the center. That's confusing. 

 

So, no apparent focuser misalignment, but how was the primary alignment in the Cheshire after star collimation? Why was the diagonal "off to the side" after primary tilt?

 

It all seemed offset. Cheshire crosshairs appeared to be over center of primary mirror, secondary significantly off to the side in the lower quadrant of the crosshairs  Put an ep back in and the views were still good..... What is going on?  

Edited by Asbytec, 22 June 2025 - 01:42 AM.

[Starman1]

If it looked like this, it was collimated:

The secondary shadow is offset--it is not concentric with the other circles.

Attached Thumbnails

• 

Edited by Starman1, 22 June 2025 - 01:55 AM.

[Asbytec]

If it looked like this, it was collimated:

The secondary shadow is offset--it is not concentric with the other circles.

Agreed. If that is what he meant, or even the silhouette rotated toward the "lower quadrant of the cross hairs" is fine. Prior to tilting the primary mirror, this must have been his diagonal tilt and rotation holding the focuser axial alignment. Though, he was shocked "nothing(?) was lined up" after star alignment. 

 

After tool collimation, though, he was able to tilt the primary and achieve a "reasonably pin point star," corrected for coma on axis and spherical, and achieve high magnification despite the corrector lens. I wonder if the primary alignment was visually "askew." It had to be different from his tool alignment. 

 

But, there is a sweet spot in the field of view, and it may not correspond to his tool collimation of the primary mirror. I trust his tool collimation was adequate with reasonably concentric signatures, and he placed the primary center marker. Rare, but the sphere optical center could be off the primary center? 

Edited by Asbytec, 22 June 2025 - 04:54 AM.

[rfcooley]

So, the issue with J/B is the lens is it has an optical axis that must be collimated with the primary optical axis, as well. There is no real way I am aware of to ensure the lens is collimated other than trusting it is not tilted in, and is assumed to be aligned with, the focuser axis.

 

As I understand your progress so far, you collimated with your quality and collimated tools but experienced what seems to be coma on axis. Then, you tilted the spherical primary and found a sweet spot. So, there is a sweet spot despite the corrector lens in the light path.This would be correct.

 

The kicker problem is, as you describe, everything seems out of whack. The cross hair was still on the primary center suggesting focuser (and lens) alignment, but the diagonal was tilted out of position. The good star images indicate, then, also indicate the primary axis is aligned despite the diagonal displacement. This is also what I am experiencing

 

I do not understand how tilting the primary caused the position of the diagonal to no longer appear concentric with the focuser while retaining focuser axial alignment. The diagonal may well have needed to be where it is to align both axes suggesting it was not well centered under the focuser to begin with. 

 

It is possible to align both axes and produce good star images in the center of the field with almost any diagonal placement. Do you have a picture of the out of whack alignment taken through a pupil of the Cheshire or site tube? It's possible your scope needed primary star alignment to correct the primary axis. 

 

I'm curious, too, if a Barlow lens can adequately correct the spherical aberration of the spherical primary mirror. 

 

 

The Barlow wouldn't correct the SA, but it could easily tell if the problem with the comet-like star images are due to the lens or the other elements in the optical train.

I suspect the lens housing is glued into the focuser with a tilt so that when the lens is put in the housing in the focuser, it is tilted relative to the optical axis, creating astigmatism.

This might be verifiable by measuring the distance to the lens on two sides from the top of the focuser where the eyepiece goes.

 

 

I believe you're right and I don't believe a Barlow corrects for spherical aberration, specifically, but extending the focal length/ratio might reduce it to about 1/4 wave at around f/9 or so. The corrector lens, which is likely designed to reduce spherical aberration resulting in 2x the primary focal ratio, may be tilted in the focuser.

 

But there was a solution of primary tilt - after collimation with collimated quality tools - that rendered nicer star images regardless of the lens. This configuration, for some reason, resulted in the "Cheshire" cross hair aligned with the primary center, but the diagonal position away from the center. That's confusing. 

 

So, no apparent focuser misalignment, but how was the primary alignment in the Cheshire after star collimation? Why was the diagonal "off to the side" after primary tilt?

 

 

If it looked like this, it was collimated:

The secondary shadow is offset--it is not concentric with the other circles.

Sending pics. Here are some pics taken yesterday and last night. Has you can see everything is out of whack once a star test is completed.

 

View through focuser only.

 

View through Cheshire after collimation.

 

View through collimation cap

 

After star test adjustment  Stars sharp in roughly 70% of field. Stars get better the closer you get to axis. But you can see everything out of whack but the views through the ep are decent. It does a nice job on open clusters. Some of the small globular are difficult to resolve more that a 1/2 dozen or so stars. Doesn't pick up the spirals on M13 but it does resolve quite a few stars. M5 resolves some stars but is mostly just a large cloud. Messing with the primary adjustments while looking at the globular doesn't really change anything. I think it is a limitation of aperture and optics of an inexpensive scope. But I have no explanation why collimation appears out of whack yet the views are IMO pretty good for all the apparent out of whackness I see.

 

In any case, these were the best pics I could get. I do not understand why the spiders appear to look offset. I have checked them for center in the tube and in the other pictures they look as they should. The crescent shadow that shows up in the the focuser image is due to the clamp on the cell phone holder partially covering the 45 degree reflection port on the Cheshire.The focuser itself is not of the highest quality and does shift around a bit with Cheshire, camera mount and cell phone all attached.

 

Can I improve on what I have or have I probably reached the limitations of this scope? What do you guys think?  Suggestion to try?

 

I do appreciate the consideration and attention you have provided.

 

RF

[Starman1]

I'm not sure what I'm looking at here.

is:

a=inside front edge of focuser or collimation tool?

b=outer edge of secondary mirror's reflective surface?

c=reflection of front opening of the scope?

d=reflection of the edge of the primary mirror?

 

Or do I have b and c reversed?

Attached Thumbnails

• 

[Asbytec]

I agree with Don. The image he reposted is a little confusing. The edge of circle "c" is confusing. It's all confusing. Is "b" the edge of the correcting lens? But what is "c"? Maybe it's misalignment may have something to do with tool collimation?

Still, the three tool collimation pics appear, at a glance, to be in the ballpark. The reflection of the primary is nearly centered under the focuser suggesting a nearly aligned focuser axis, and the primary center mark is in the pupil suggesting primary axial alignment. You can see all three clips suggesting the diagonal is pretty close to its proper location. Not perfect collimation, but ballpark. So, it looks like your tools are doing some work to collimate the scope.

Your last image is what we'd expect to see after tilting the primary. The cross hair remains close to the primary center mark, but the reflection of the collimation cap has drifted off the primary center. Spherical aberration is the same across the entire field, and the field lens corrects for spherical, to some degree, but it does not correct coma. Coma is an off axis aberration, so we expect to see the best star images near the center of the field and coma in the outer field. That is collimation.

I dunno, and I have to go to work this morning.


Edit: The diagonal holder appears to have an odd "fat" shape. Does it? That must be the same feature noted in Don's post. It also appears to be seen through the focuser but not reflected in the diagonal. Can you put a white paper background below the diagonal opposite the focuser to see if it goes away?

Edited by Asbytec, 22 June 2025 - 08:13 PM.

[rfcooley]

I'm not sure what I'm looking at here.

is:

a=inside front edge of focuser or collimation tool?

b=outer edge of secondary mirror's reflective surface?

c=reflection of front opening of the scope?

d=reflection of the edge of the primary mirror?

 

Or do I have b and c reversed?

I not sure what the reflection, triangle bright spot is in C. It is in all the pics I took. But I think it is the edge of the secondary. In the pic of after the star test that half moon shape is complete.

 

Blue is the edge of secondary. At least that is what I think? Pretty sure it is. Flexing the spider vanes makes that edge move up and down.

 

 

 

 I went out to take another pic Triangle is a light leak caused by rubber eye cup.

 

 

RF

Edited by rfcooley, 22 June 2025 - 08:37 PM.

[rfcooley]

I agree with Don. The image he reposted is a little confusing. The edge of circle "c" is confusing. It's all confusing. Is "b" the edge of the correcting lens? But what is "c"? Maybe it's misalignment may have something to do with tool collimation?

Still, the three tool collimation pics appear, at a glance, to be in the ballpark. The reflection of the primary is nearly centered under the focuser suggesting a nearly aligned focuser axis, and the primary center mark is in the pupil suggesting primary axial alignment. You can see all three clips suggesting the diagonal is pretty close to its proper location. Not perfect collimation, but ballpark. So, it looks like your tools are doing some work to collimate the scope.

Your last image is what we'd expect to see after tilting the primary. The cross hair remains close to the primary center mark, but the reflection of the collimation cap has drifted off the primary center. Spherical aberration is the same across the entire field, and the field lens corrects for spherical, to some degree, but it does not correct coma. Coma is an off axis aberration, so we expect to see the best star images near the center of the field and coma in the outer field. That is collimation.

I dunno, and I have to go to work this morning.


Edit: The diagonal holder appears to have an odd "fat" shape. Does it? That must be the same feature noted in Don's post. It also appears to be seen through the focuser but not reflected in the diagonal. Can you put a white paper background below the diagonal opposite the focuser to see if it goes away?

post-202154-0-38739400-1750617515.jpg

I just went to take another pic. The white triangle is from light leak caused by the rubber eye cup.

[Asbytec]

I just went to take another pic. The white triangle is from light leak caused by the rubber eye cup.

Okay so the blue line is the edge of the much larger (than it appears) diagonal mirror. The smaller illuminated circle surrounding the reflection of the primary is the mirror cell, but we're also seeing more of the upper bottom of the tube near the primary. If so and if possible, then the diagonal needs to move much closer to the primary mirror. That may explain why three clips are not visible in your skewed image.

But that still doesn't explain why you have to realign the primary axis using star collimation. Primary alignment looks good in your latest image, so why do you have to adjust it? You mentioned the center marker being very close to the center of the mirror. I'm puzzled by the fact you can find the coma sweet spot despite the corrector lens, but you have to tilt the primary to find it. Could a tilted corrector lens refract the primary axis optically but not visually?

Edit: Also noticable is the visible connection of the vane to the spider hub at left, not so much at top, and not visible on the right. I don't know is that means anything other than maybe the spider could be better centered in the tube, if possible. You should still be able to align both axes.

Wait to see what Don says.

Edited by Asbytec, 22 June 2025 - 10:58 PM.

[Starman1]

I not sure what the reflection, triangle bright spot is in C. It is in all the pics I took. But I think it is the edge of the secondary. In the pic of after the star test that half moon shape is complete.

 

Blue is the edge of secondary. At least that is what I think? Pretty sure it is. Flexing the spider vanes makes that edge move up and down.

 

20250527_162851 edge if secondary.jpg

 

 

 I went out to take another pic Triangle is a light leak caused by rubber eye cup.

 

 

RF

Well if blue is the edge of the secondary, and the stalk that holds the secondary is off the image at the top, then the secondary needs to be moved significantly down tube toward the primary mirror.

You do that by loosening the center bolt and tightening the collimation screws.  Do it with the tube horizontal so if the secondary fall off the bolt it doesn't hit the primary mirror.

 

Each bit you move it down, adjust the tilt with the sight tube (crosshairs) to make sure the tilt is adjusted to place the crosshairs on the center marker.

Eventually, the outline of the secondary will be concentric with the focuser and the reflected image of the primary will be centered on the secondary as well.

See the image in post 14.

[rfcooley]

Okay so the blue line is the edge of the much larger (than it appears) diagonal mirror. The smaller illuminated circle surrounding the reflection of the primary is the mirror cell, but we're also seeing more of the upper bottom of the tube near the primary. If so and if possible, then the diagonal needs to move much closer to the primary mirror. That may explain why three clips are not visible in your skewed image.

But that still doesn't explain why you have to realign the primary axis using star collimation. Primary alignment looks good in your latest image, so why do you have to adjust it? You mentioned the center marker being very close to the center of the mirror. I'm puzzled by the fact you can find the coma sweet spot despite the corrector lens, but you have to tilt the primary to find it. Could a tilted corrector lens refract the primary axis optically but not visually? This is a question I wondered about. When I aligned it with the laser (JB removed) everything lined up. Put the JB back in and checked with a Cheshire everything still looked lined up. Star test all stars looked like comets clear across the field. Adjust the primary and the tails disappeared

Edit: Also noticable is the visible connection of the vane to the spider hub at left, not so much at top, and not visible on the right. I don't know is that means anything other than maybe the spider could be better centered in the tube, if possible. You should still be able to align both axes. I have measured the centering of the spider with a regular ruler, with machinist ruler and with a pair of calipers equipped with long extension needles for measuring down inside a cylinder. All measurements are within ~.060" or less than 1/16 of an inch according to the Digital Calipers.  I would think this would be close enough. 

Wait to see what Don says.

The quest continues.

 

Well if blue is the edge of the secondary, and the stalk that holds the secondary is off the image at the top, then the secondary needs to be moved significantly down tube toward the primary mirror.

You do that by loosening the center bolt and tightening the collimation screws.  Do it with the tube horizontal so if the secondary fall off the bolt it doesn't hit the primary mirror. This may very well be the problem. When I disassembled secondary mirror there was no spring behind the secondary mirror and the mirror was flat against the support of the spider. Since I was unfamiliar with this particular model. I wasn't sure if it was right or not and was unable to find any info. This spring may have been missing from the get go. I put a spring behind the secondary mirror only because I have seen springs behind the secondary mirror assembly before in other Newtonian OTAs 

 

Each bit you move it down, adjust the tilt with the sight tube (crosshairs) to make sure the tilt is adjusted to place the crosshairs on the center marker.

Eventually, the outline of the secondary will be concentric with the focuser and the reflected image of the primary will be centered on the secondary as well.

See the image in post 14.

I will try moving the secondary down the tube in the morning and post some more pics as to what results I come up with. 

 

Maybe we have solved this mystery. I'll keep you and Asbytec updated.

 

I appreciate both of you for your time and attention. 

 

RF
 

[Asbytec]

I have measured the centering of the spider with a regular ruler, with machinist ruler and with a pair of calipers equipped with long extension needles for measuring down inside a cylinder. All measurements are within ~.060" or less than 1/16 of an inch according to the Digital Calipers.  I would think this would be close enough.

 

That's too close to center! Ease back, a little. (Kidding). 

 

The displaced diagonal should explain the "skewed" appearance you describe. Part of it, anyway. The diagonal looks pretty large, so the apparently smaller primary reflection should easily fit within the diagonal over a range of focuser travel. Provided the diagonal is reasonably well centered and rotated under the focuser. It doesn't have to be "caliper" close.

 

Once you adjust the diagonal longitudinal position as Don described, then center and rotate the diagonal under the focuser best you can. Then, once you align the cross hair or laser to the primary center, the primary reflection is necessarily concentric with the focuser draw tube. That is two of the three collimation signatures. The diagonal position is the third signature.

 

You can use the fact the primary reflection is centered under the focuser to assess your diagonal placement. The axes can still be aligned, however, with almost any diagonal position under the focuser. So long as see the entire primary reflection in the diagonal the field is fully illuminated and there is a tilt and rotation solution that will align both axes. That is fine. 

 

As you know, the last step is to tilt the primary axis (defined by the center mark) to the focuser axis (defined by the laser, the cross hair, or collimation cap pupil). This is where you ran into trouble causing you to do a star alignment. If you need to star align, without touching the diagonal, the primary axial alignment will become "skewed" away from tool alignment.

 

My fingers are crossed the simple movement of the diagonal will fix the "kicker" problem. But I'm curious because you mentioned the visual tool alignment is not affected by the corrector lens, but the star images might be. The visual tool alignment and the star test do not seem to agree, but they should, and both pass through the corrector lens.

[Vic Menard]

Here's what I see:
The outermost light blue circle is the reference circle for the focuser axis (light blue cross hairs).
The yellow circle is the edge of the tilted/decentered J-B lens.
The green circle is the actual edge of the secondary mirror,
The red circle is the edge of the primary mirror reflection (red cross hairs show the center).
The violet circle is the reflection of the secondary mirror.

 

The primary mirror center marker appears to be off center.
And the secondary mirror appears to be offset incorrectly, which may be caused, in part, by the J-B lens and the J-B lens tilt error.

Attached Thumbnails

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Edited by Vic Menard, 23 June 2025 - 06:59 PM.