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LX200 rebuild

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#1 John Tucker

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Posted 03 March 2021 - 02:35 PM

A few years ago I bought an ancient LX200 as my first telescope.  Whoops!  Lousy pictures, won't focus, eventually took it apart and found the primary not perpendicular to the baffle tube and the collimation screw holes in the plate the secondary was glued to sufficiently mis-positioned that it was impossible to center the secondary over the primary.  

 

Bought the Piekiel book, shimmed the primary to get it it properly aligned.  Checked per the methods in the Piekiel book. 

 

Tossed the plate the secondary was glued to, bought an aluminum disk from EBay, drilled properly centered holes, glued the secondary to it with silicone aquarium cement.  Installed it and checked with a sight tube that it is decently centered over the primary. 

 

Whoops!  In the course of getting my grubby fingers all over the secondary doing this and cleaning the mirror, I accidently removed the alignment mark on the secondary.  The whiteout mark that shows the proper alignment of the corrector is still visible, but I don't know if I believe it given all the other problems with the construction of this unit.  

 

Anybody have thoughts on this?  Which is more important to align radially, the corrector or the secondary?  Any chance that one or the other can be mostly neglected?  Or that they are semi-independent of each other?  

 

Is it time to give up?



#2 markb

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Posted 03 March 2021 - 03:35 PM

I've been through this with a misconstructed meade 8 (crooked rear cell epoxied on, resulting in a crooked baffle tube), and a large Celestron 11 where all the original markings had been misaligned, in addition, the more recent factory addition of placement of the corrector plate over the baffle tube was gone.

 

The marking on the secondary mirror was, in part,  made after the factory rotated the corrector and secondary assembly, to center the secondary mirror over the baffle tube. If the baffle tube was properly centered and parallel to the tube walls, little adjustment or intentional off-centering of the secondary was needed. If the baffle tube is crooked, the factory selected, apparently, a secondary glued off center to the puck. This way, when the corrector was rotated the secondary moved eccentrically, allowing them to line it up with the baffle tube and, if the primary was glued properly (yours wasn't, yikes), lining it up with the optical axis of the primary.

 

My Meade had the baffle tube 8 mm or so off center at the corrector end, and the secondary mirror was glued two to three centimeters off center from the aluminum puck. A mess to be sure. By shifting the corrector plate all the way over to one side, getting a slight realignment of the baffle tube, and rotating the secondary over to the same side, the secondary is now aligned over the baffle tube. I started out with horrible coma even when collimated and now have a pretty good performer that will improve once the terribly blistered and discolored secondary is recoated. Silvered secondaries, another failed experiment by the manufacturer.

 

So the short answer is, don't despair, in reshimming the primary you did the hardest part of the job!

 

If you, as I would anticipate, carefully centered the secondary on the secondary puck, and equally carefully centered your threaded hole triangle, the secondary should not move eccentrically when the corrector is rotated, and you should be able to simply center the corrector perforation over the baffe tube to complete the opto-mechanical alignment.

 

When it comes to your particular situation, don't think about aligning things radially, think about aligning the baffle tube axis with the center of the secondary. You may get comments or see comments that the spherical secondary has no optical axis, but the fact that the colliimation pivot is very distant from the center of curvature means you can treat it as if it does have one.

 

However you do it, centering the secondary mirror itself over the baffle tube should be your goal.

 

If this means the corrector plate appears uncentered in the cell, that isn't a problem since the goal is not centering it in the tube, but rather, centering it over the baffle axis. The Celestrons Edge HDs actually have set screws to more easily align the corrector 'off center'. also allowing it to be cleaned and restored without losing alignment.

 

On mine, I completed this step with a holographic bullseye projection, but there are other ways to do it. The damaged secondary surface ironically helped see the pattern.

 

On the 11, the secondary seems to have been glued concentrically, so I simply centered the corrector perforation over the baffle tube axis with the holographic projection. Once I get a 3D printer I'll make a slip on mask to allow me to physically center the secondary itself, but performance at this point is extremely good. When I first got the 11 it was virtually unusable with horrible soft images. The mirror said itself gives an excellent Ronchi test. It's performance now is either optimal, or close to it.


Edited by markb, 03 March 2021 - 03:38 PM.


#3 markb

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Posted 03 March 2021 - 03:42 PM

Since you have re-glued and repositioned your secondary on the aluminum puck, centering the secondary should also end up centering the corrector. When the factory uses eccentrically glued secondaries this would presumably mean the secondary was centered after tuning, and at the corrector was slightly off axis, apparently not a significant performance issue.



#4 John Tucker

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Posted 03 March 2021 - 03:52 PM

Since you have re-glued and repositioned your secondary on the aluminum puck, centering the secondary should also end up centering the corrector. When the factory uses eccentrically glued secondaries this would presumably mean the secondary was centered after tuning, and at the corrector was slightly off axis, apparently not a significant performance issue.

Thank you Mark.  I've seen a lot of conflicting comments on the importance of rotating the secondary and or the corrector, but it sounds like you did not find this an issue?  That would certainly simplify things!

 

I didn't get the secondary quite perfectly centered in the corrector as I was operating at the edge of my skills with an electric drill.  But its not too bad....



#5 John Tucker

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Posted 03 March 2021 - 04:18 PM

What I didn't mention was that in addition to rubbing off the rotational marker on the secondary, I also damaged the coating.  Thus I put this task down for about a year.  In the meantime I got a good price on a 6Se, but have been unable to guide it, as there is too much flex for a guide scope and the stars on the edge of the FOV have too much coma for an OAG.  So back to the LX200. 

 

Majestic Coatings did a great job on my secondary for about $110 or so.  Fast turnaound, nice work, good reputation. 

 

Tonight I guess will be the final test of a  hundred hours or so of work.,



#6 markb

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Posted 03 March 2021 - 08:24 PM

That is a good bit of information. I've put off sending my meade 8 secondary in for re-coat. I planned to send it to Majestic but had no idea what a secondary only would cost.

 

The secondary centering over the baffle is the key.

 

I understand it can be done with a sight tube but I found it only possible to do accurately with the holo laser, not an easy buy these days.

 

So the sight tube might be your best option, or a mask that doesnt touch the mirror surface, but allows centering of the secondary over the baffle tube. EDIT: By using a regular laser collimator, just to 'hit' the center spot on the paper mask.

 

As long as the secondary is directly over the baffle tube you will be good. As I mentioned, this is the goal of the rotational alignment.

 

If the corrector is not centered in the cell it is not an issue.

 

You can rotate the corrector + secondary as a unit to aid centering the secondary mirror over the baffle, basically duplicating the factory method.

 

Mark it when you are done if possible. If the corrector ends up off center, shim to match the true postion, and take photos, they really help.

 

An alternative I will try later is the 3d printed centering mask over the secondary. You might be able to make one from posterboard with care, sort of a truncated cylinder.


Edited by markb, 04 March 2021 - 01:56 AM.

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#7 John Tucker

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Posted 04 March 2021 - 06:27 AM

That is a good bit of information. I've put off sending my meade 8 secondary in for re-coat. I planned to send it to Majestic but had no idea what a secondary only would cost.

 

The secondary centering over the baffle is the key.

 

I understand it can be done with a sight tube but I found it only possible to do accurately with the holo laser, not an easy buy these days.

 

So the sight tube might be your best option, or a mask that doesnt touch the mirror surface, but allows centering of the secondary over the baffle tube. EDIT: By using a regular laser collimator, just to 'hit' the center spot on the paper mask.

 

As long as the secondary is directly over the baffle tube you will be good. As I mentioned, this is the goal of the rotational alignment.

 

If the corrector is not centered in the cell it is not an issue.

 

You can rotate the corrector + secondary as a unit to aid centering the secondary mirror over the baffle, basically duplicating the factory method.

 

Mark it when you are done if possible. If the corrector ends up off center, shim to match the true postion, and take photos, they really help.

 

An alternative I will try later is the 3d printed centering mask over the secondary. You might be able to make one from posterboard with care, sort of a truncated cylinder.

Well, I got it pretty good last night. Not a EDGE HD by any stretch of the imagination, but miles ahead of where it was when I bought it.  I had some residual coma, of which I eliminated 90% by rotating the secondary, though its quite possible that what I was really doing was fine-tuning the centering. 


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#8 John Tucker

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Posted 04 March 2021 - 09:17 AM

Before and after

 

 

image_2021-03-04_081728.png


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#9 John Tucker

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Posted 04 March 2021 - 11:28 AM

Mark I think you were right about the centering of the secondary.  The best "rotation" of the secondary cell seems to also do the best job of centering the secondary over the primary. 

 

With the sight tube, if I put the cover over the corrector so no light comes in that way, and shine a light through the side port of the sight tube, one sees a shiny circle or oval in through the peep hole.  As I move the corrector plate side to side, it goes from oval and off-center relative to the baffle tube to centered and round. It has reflections of the crosshair of the sight tube in it, and as you center the round circle the reflections of the cross hairs superimpose as well.  Someone told me to do this when I worked on this scope a year ago but I forgot about that.  

 

Will see if there is further improvement tonight. 

 

When I first put a sight tube in this OTA 2 years ago, only a tiny edge of the bright circle/oval was visible.  After all the stuff I did before last night it was oval and off center, but fully visible.  Now it is round and centered.  Let's see what happens. 


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