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Meade 8in OTA Collimation

cassegrain collimation Meade mount NV SCT
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#1 normhughes

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Posted 14 December 2019 - 08:25 PM

Good Evening.  I am looking for some information regarding the secondary on my Meade 8in LX90 OTA.  I had the seconday basically come apart as i was not doing collimation correctly.  So i took it apart and set it back in place and back on the scope.  My question is, are there suppose to be springs unfer the secondary mirror or not.  This is a pre 2000 OTA, and basically it has a center point (or fulcrum), the mirror sits on and pivots according to adjustments from the 3 screws.  But there are no springs and I am wondering if they are suppose to be there.  Anyone with any information would be great.  Thanks In Advance for any response.  I appreciate it.



#2 ngc7319_20

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Posted 14 December 2019 - 08:48 PM

Don't think I've ever seen an SCT secondary mirror assembly with springs.  The fulcrum at the center point should always be in contact, so there is no need for springs to oppose forces on the three adjustment screws.  (But I've never taken apart an LX90.)


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#3 FLT-Astro

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Posted 14 December 2019 - 09:06 PM

I have a later Meade 8” F5 Newt.  The secondary is held by one large center screw (Phillips/cross Head) to adjust forward/aft and 3 small screws (Allen Wrench) to adjust angle/tilt/centering.  No springs.  If yours is the same setup, you probably loosened the larger center screw too much so it came out of its hole.


Edited by FLT-Astro, 14 December 2019 - 09:09 PM.


#4 bbqediguana

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Posted 14 December 2019 - 09:19 PM

I have a later Meade 8” F5 Newt.  The secondary is held by one large center screw (Phillips/cross Head) to adjust forward/aft and 3 small screws (Allen Wrench) to adjust angle/tilt/centering.  No springs.  If yours is the same setup, you probably loosened the larger center screw too much so it came out of its hole.

The scope that Norm has is an SCT which is different than a Newt.

Having said that - no, there should be no springs under the secondary. ngc7319_20 has it right - the 3 collimation bolts should be tight enough to keep the secondary in contact with the fulcrum point.

 

Here's a link to the LX90 Manual. Collimation instructions start on page 41 and I find the Meade instructions quite good:

 

https://deepskies.co...LX90_manual.pdf

 

Cheers!

Rick



#5 J A VOLK

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Posted 14 December 2019 - 09:21 PM

no springs, start with them evenly snug. When you loosen one, the others should be equivalently tightened to maintain contact with the pivot. If you loosen one a smidge you may not need to tighten the others.

Edited by J A VOLK, 14 December 2019 - 09:21 PM.

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#6 markb

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Posted 14 December 2019 - 09:28 PM

No springs, no center screws, just the little bump inside the plastic secondary housing, and the small matching (sort of) hole in the center of the aluminum puck.

 

The positioning is done just by tightening the screws during collimation, with the three tightened screws holding the puck and glued on secondary in place for optical collimation.  

 

There was a thread yesterday with a related problem (note the factory rotational alignment info is for Celestrons, most Meades use the method below)

 

Read through the thread carefully, and it has very useful tips for counting threads to get the secondary approximately level, and the Robin Casady link will get you close enough for a nighttime in-focus collimation. The shadow method is not really precise, I have found the daytime Casady reflection method a much better starting point, and you can do it in the daytime! Don't forget the in-focus tweak on a star at night.

 

https://www.cloudyni...tion-nightmare/

 

NOTE: You may have inadvertently lost the rotational alignment of your corrector and your secondary.

 

The secondary has a wide sharpie marker stripe on one edge. That should point to 3 o'clock viewed from the front/corrector end of the scope.

 

Meades correctors usually have a 1/4" or so stripe of white-out on the edge of the front of the corrector running onto the metal scope edge normally hidden by the retaining 'circle'. There is usually also a black sharpie line going from the corrector onto the metal scope rim on the white-out 'stripe'. Just line the sharpie mark to make one line. There may also be a chinese chop mark on the ground edge of the corrector, at the top, depending on the vintage.

 

The rotational alignment often makes the difference between a terrible soft image and a nice sharp one.

 

If there were cork spacers around the rim of the corrector under the retainer, just leave them of replace them where they were. if none, without extra testing equipment simply center the corrector and shim the edges with folder business cards.

 

If you removed the plastic retainer (8" ring) do NOT overtighten it, it WILL crack.


Edited by markb, 14 December 2019 - 09:31 PM.


#7 normhughes

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Posted 15 December 2019 - 09:11 PM

Thanks for all the replies folks.  I just wanted to make sure i wasnt missing them.  I did in fact put everything in its original place, as i made my own marks before removal.  Thanks Ranger 4 for the link.  i will review it carefully,  Again, thanks everyone, i appreciate the info


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