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My LX200R OTA is impossible to collimate

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#1 laixiaolue

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Posted 07 November 2019 - 09:31 PM

I tried to collimate my new required LX200R 10" yesterday and found it was impossible. I almost unscrew the collimation screw and it seems I still have half way to go.

 

Today I remove the diagonal and looked through the rear hole. I found the primary baffle and the secondary mirror baffle won't line up at all. The primary baffle just points down too much. I opened the tube and moved the Optics out, mirrors and corrector are all in perfect condition. I measured the distance from the edge of the primary baffle to the tube. This distance is 4 7/16 inch if I measure from bottom, and it is 4 13/16 inch if I measure from top. The baffle is about 3/16 inch off center from the out-most side. It should be 3/8 inch off at the corrector plane. There has no way to correct this by moving the corrector or the secondary.

 

So what is the best way to fix this? It seems the baffle was weld to the rear cell, I don't find any mechanism to adjust it.

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Edited by laixiaolue, 07 November 2019 - 09:31 PM.


#2 markb

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Posted 07 November 2019 - 10:43 PM

Return it immediately, particularly if new.

 

It sounds like the rear cell was not properly assembled to the tube. Meade rear cells were, and I assume still are, epoxied on, so adjustment is not possible. It never should have left the factory I would think.

 

The secondary assembly and corrector might have been factory (mis)aligned to 'match' the baffle tilt, but I doubt this is possible given the extreme baffle tilt. My bought super cheap known-to-be-problematic M 8 was brought into near alignment this way. It started out uncollimatable and now collimates with out issues despite needing a bit of tuning still. I never measured the baffle clearance to the tube, I worked solely with a holographic concentric circle projecting collimator. Not sure why they have disappeared, best refractor/SCT tool, ever. The rear cell 'aiming' approach, a pinhole sight plug may help, should work if you try to realign things, if this is even possible with this extreme tilt. Aim to get the perforation in the bare corrector centered as far as possible first, then rotate the secondary holder to find the spot where the actual secondary mirror approximates being centered.

 

But that scope 'only' needed the secondary (factory glued) 3mm off center in a corrector 3 or 4mm off center, the corrector plate touching the cell on one side, and the secondary offset and holder both as far to the same side as possible.

 

This is, I believe, one of the major reasons for 'spin' aligning the 3 optical elements; to find the best match of the mechanical and optical axis centers. The corrector, secondary and usually primary are all then marked to allow maintaining the alignment on disassembly/reassembly. The 3 o'clock position on the tube corrector 'big end' is used on Celestrons and perhaps Meade secondaries on the edge. Some Meades have a white-out smear on a concealed (by the retaining ring) edge of the corrector and cell, with a sharpie mark across the corrector-cell joint, allowing easy alignment. Corrector plates are not always centered, and should be returned to the factory position, don't attempt to center them.

 

I doubt your extreme misalignment can be remedied but you can try if the scope cannot be returned.

 

I believe my M 8 made it out of the factory by the assemblers rotating an offset glued secondary in an offset corrector to reach an acceptable, but not perfect, optical - mechanical axes coincidence. One of the experts here has opined that the resulting effective corrector tilt would not have a noticeable effect on the images.



#3 Justin Fuller

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Posted 08 November 2019 - 12:24 AM

Return it to whoever you purchased from for a refund.

#4 laixiaolue

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Posted 08 November 2019 - 12:49 AM

The telescope was bought from craigsliat. Before it was sold to me, it was shipped from ID to CA. I believed it got hit on the rear side during the transportation. The hard case provides no enough protection for the rear cell. Meade use 1/16 inch aluminum for the rear cell of the ACF line, it is really weak. My LX50 OTA has 1/8 aluminum rear cell.

 

The rear cell is not flat, and the axis has 1.1 degree off. It needs some work.

 

 

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#5 Blackbelt76

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Posted 08 November 2019 - 08:23 AM

I'm surprised you didn't notice this earlier in the FOV while observing?

How old is the scope? Almost sounds like a issue for Meade.



#6 laixiaolue

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Posted 09 November 2019 - 02:35 AM

OK. I think now I have 80% of job done.

 

Today I setup the OTA on a cast iron table plate and applied the woodworking clamp. I left it there for a while and released it to measure the baffle angle. Now the digital angle meter show it was 0.1 degree off axis. I thought that was good enough, so I put the optics back. I looked through the rear hole, the secondary baffle was centered well. But when I peek through the edge, I found out the primary was still a little off, it points to the bottom-right direction. Maybe just 2-3mm off center.

 

I tried to do collimation, it works, but I have another problem: if I reach collimation on one side of focus, the other side will be out of collimation. I believe this is because the secondary is not centered. I think I can move the secondary to solve the problem, but I still prefer to solve the root problem. I will open the OTA tomorrow and redo the clamping job.

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#7 markb

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Posted 09 November 2019 - 07:14 AM

Please reread my earlier post, #2.

 

You should be able to reach the goal of getting the optical and mechanical axes coincident, now.

 

Start by restoring the original rotational alignment marks, although I believe this is unnecessary on your scope.

 

Remove the secondary housing if possible, it should unscrew on a Meade. 

 

Then position the corrector to center the perforation with the baffle centerline using your sighting method. Not the secondary housing, that is next.

 

Shim or mark the corrector to cell position, it will not be centered in the cell. Reinstall the secondary and housing, but you may need to remove it to shim it within the perforation hole, as there is a bit of play within the hole.

 

Shim ( within the perforation), and then rotate the secondary housing to find the spot where the secondary mirror is centered, without changing the corrector perforation centering. You should be able to align the optical and mechanical centers this way. The primary is assumed to share centers with the baffle tube. The goal is to separately get the corrector centered, using the perforation hole edge, and the secondary centered, using the secondary mirror edge.

 

The secondaries are often not totally concentric to the plate they glued to, making this rotational method of centering possible.

 

Meades do not get any hand figuring AFAIK so the usual 'rotation alignment' is done to shift the secondary position slightly, to get it in the mechanical centerline, it is spherical so the secondary doesn't have a single optical center as I understand it.

 

I was going to suggest using a hydraulic press to 'fix' the cell, but thought it would crack. Well done using your method!

 

This general method got my problem M8 working and collimatable, despite a 6-8mm baffle offset measured at the corrector (measurements as I can best recall).



#8 laixiaolue

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Posted 09 November 2019 - 12:18 PM

Thank you for the advice! If the orientation of the secondary and corrector is not important, the adjustment is much easier. I just need to make the secondary off-center and rotate the corrector to find the best orientation. I will try to make the primary baffle as centered as it could, and then adjust the secondary.

 

 

Please reread my earlier post, #2.

 

You should be able to reach the goal of getting the optical and mechanical axes coincident, now.

 

Start by restoring the original rotational alignment marks, although I believe this is unnecessary on your scope.

 

Remove the secondary housing if possible, it should unscrew on a Meade. 

 

Then position the corrector to center the perforation with the baffle centerline using your sighting method. Not the secondary housing, that is next.

 

Shim or mark the corrector to cell position, it will not be centered in the cell. Reinstall the secondary and housing, but you may need to remove it to shim it within the perforation hole, as there is a bit of play within the hole.

 

Shim ( within the perforation), and then rotate the secondary housing to find the spot where the secondary mirror is centered, without changing the corrector perforation centering. You should be able to align the optical and mechanical centers this way. The primary is assumed to share centers with the baffle tube. The goal is to separately get the corrector centered, using the perforation hole edge, and the secondary centered, using the secondary mirror edge.

 

The secondaries are often not totally concentric to the plate they glued to, making this rotational method of centering possible.

 

Meades do not get any hand figuring AFAIK so the usual 'rotation alignment' is done to shift the secondary position slightly, to get it in the mechanical centerline, it is spherical so the secondary doesn't have a single optical center as I understand it.

 

I was going to suggest using a hydraulic press to 'fix' the cell, but thought it would crack. Well done using your method!

 

This general method got my problem M8 working and collimatable, despite a 6-8mm baffle offset measured at the corrector (measurements as I can best recall).



#9 laixiaolue

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Posted 09 November 2019 - 12:49 PM

It seems my secondary has already off-centered by default. I can rotate the corrector to make the secondary centered to my primary baffle. It is good enough so my eyes cannot tell the difference. I think I can use this as a starting point, put back primary and reset collimation screws. I can do the collimation by rotate the corrector. Once I get the best collimation, I will mark the new orientation and use the collimation screws to do fine adjustment.


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#10 markb

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Posted 09 November 2019 - 01:40 PM

Sounds good, and should work well, but, once you center the secondary to the baffle, skip further rotation of the corrector/secondary assembly.

I would check the centering of the bare corrector perforation first, then insert the secondary housing, align it by eye and then ONLY change the collimation screws.

That aligns 1) the primary and baffle opto-mechanical axes to 2) the optical and physical center of the corrector and, finally, 3) centers the corrector physically with the rest, at the same time centering it optically as well.

Your combined corrector-secondary method that you mentioned really should be close.

Once you have that, or if you just stick with the combined corrector and secondary centering you just tried (which should be fine I would think), DON'T further rotate anything. Rotation will shift the secondary out of the centerline. You will be ready to just collimate.

Just center everything, tighten it down, and do the REGULAR screw collimation (which should work without issues).

Since everything was taken apart, I highly recommend starting with the Robin Casady method in daylight, working from the secondary end, adjusting the screws until the reflections are concentric. See http://www.robincasa...ro/collimation/ .

It is a breeze if you sit in a chair at a distance that lets you match edges of reflections, with the scope in a cradle (I use a mitre box) or on a mount. Amazing how much easier it is when you are FACING the screws!

Only then do the out-of-focus shadow at night or on an artificial star. A small ball bearing in the sun at 100 feet is okay, but some 'artificial' SA might show up at that short distance, but can be ignored.

The Casady method should have you VERY close to perfect.

You are not done until you finish with collimation tweeks on a star in focus, preferably at night. The shadow method only gets close.

In my experience with mangled SCTs, the Casady method should MATCH the star method. If you check the Casady reflections after star aligning and one, or more, is decentered, you likely still need to tune up the optical and mechanical center alignments. After I move I will chart which misalignment moves which reflection edge, using my M8.

If the two methods are very close, you should be getting great images, assuming the usual good SCT optics. A ronchi or DPAC test will flag issues like zones, roughness, and SA. Astigmatism should show in the defocused collimation process.

The star collimation is the 'more correct' of the two if they don't match and you want to use the scope to observe.

My M8 is very close to matching the two methods, decently sharp (even with a bubbled secondary silvering), and was once uncollimatable and useless. It still needs a bit of tuning, but the Casady method almost matches the star collimation. It has a bad secondary coating, so it will await that fix.

My C11 was very soft but is now decently sharp, but the rotational alignment is off due to bad info online, and a secondary that was rotated during screw removal (the Fastar secondary holder has an alignment notch too), it should be extremely sharp once finished. The Casady method is still noticably off from the star collimation. I am across the country from it, so it may not be finished before I move.

If you are moving up from an 8", you should be amazed at the 10! I was.

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I have wondered if Meade makes intentionally off-centered secondaries to allow them to match slightly tilted baffles by rotation of the slightly eccentrically glued secondaries.

The Celestron EdgeHD whitepaper appears to confirm these opto-mechanical axis alignment thoughts, and much was gleaned from the C14 efforts on wilmslowastro.com. The Faster/Hyperstar needs to start with a coincident axis scope before the extra FS/HS optics are added.

Edited by markb, 09 November 2019 - 01:45 PM.


#11 laixiaolue

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Posted 09 November 2019 - 02:35 PM

I just put everything back and use the corrector orientation I got from eyeballing. I tighten all the collimation screws so there has no collimation. I point the telescope to a car about 300 feet away, the view is pretty sharp in a 40mm eyepiece. I de-focus on a sunlight reflection, the collimation looks pretty close, the in and out focus patterns look pretty consistent. I think I am pretty close. I will set up the artificial star tonight and do fine collimation.

 

I think Meade cannot guarantee the center baffle perfectly centered in mass production. The only option to fix the problem is making the secondary off-centered to compensate the axis error.


Edited by laixiaolue, 09 November 2019 - 02:38 PM.


#12 markb

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Posted 09 November 2019 - 03:25 PM

Excellent!

 

I hope my info helped. It took a bunch of reading, thinking, research and experimentation to figure out the details (probably obvious to pros in optics, of course). I think many 'soft' SCTs may suffer from opto-mechanical alignment issues rather than bad optical elements. Except those pesky Dynamax and B&L rough corrector travesties. Not that I am still grouchy that my first 'real', bought-new scope can never be made useful. But I do have a Mak meniscus to experiment with . . .

 

Try the Casady method with the scope as it is now, it is probably very close to fully concentric.

 

If the extra focal patterns match well, you may have a real winner with few if any optical issues. Errors usually show up there, the Suiter method.

 

I do daytime testing as well as night testing, in part because my location suffers from poor atmospheric turbulence.

 

I noticed long ago, and confirmed by observations, that snap to focus and 'snappy' daytime images usually are a fair predictor of night sky performance, at least for me. Not super critical, but a good indicator. Chromatic abberations are easy to see on distant branches and powerlines, too. If I can resolve head writing and the threads on a bolt 1/2-1/4 mile away, the optics can't be too bad. My close to perfect etx90 easy snapped to crisp focus on daytime targets noticably better than my other excellent etx90s that were a small step down but really, really, good.

 

Best of luck when you try it on a clear sky, let us know.

 

Your rear cell straightening experience is sure to help others, too. I would not have expected such positive changes. I may give it a shot on my M8 to reduce the error to 1-2mm.

 

I agree on the off-center secondary to puck gluing, they may use this during the optics matching process, particularly since they epoxy the cell to the tube, eliminating that alignment method.




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