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Equipment Discussions >> Reflectors

Pages: 1
acochran
professor emeritus


Reged: 06/19/08

Loc: So. CA
Collimation screws near their tips
      #5521413 - 11/15/12 03:18 PM

I recently had my primary mirror on my "16" Newtonian Lightbridge refigured. When I reinstalled it, I had to move the primary mirror closer to the secondary so I could use my 20mm eyepiece. I moved the primary by unscrewing the 3 collimation screws. The screws are now almost out of the mirror cell.
Last time I used the scope, about 3 hours into the viewing session, I rechecked my collimation with my Howie Glatter Laser Blug and the primary was quite a bit off--the centermark was about half an inch to the right on the Blug.
So to try to figure out why it moved so much, I just took apart the lower end of my Lightbridge. All I can see wrong is that the 3 collimation screws are near their ends (but still in the mirror cell). I'm not even sure this is why my collimation moved so much. The mirror itself may have shifted.
I can think of 2 things that might help, get longer screws or maybe get a low profile focuser. I got my screws+springs from Farpoint. Maybe I will e-mail them.

Edited by acochran (11/15/12 03:33 PM)


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catboat
professor emeritus
*****

Reged: 12/01/09

Loc: Maine
Re: Collimation screws near their tips new [Re: acochran]
      #5521582 - 11/15/12 05:06 PM

Not good to have the collimation screws extended so far. And longer screws may encourage movement/flex.

Can you remount your tailgate/cell assembly farther forward -- drilling fresh holes for the attachment screws? Better yet might be to shorten the truss poles.


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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Collimation screws near their tips new [Re: catboat]
      #5521922 - 11/15/12 09:12 PM

The further forward the mirror, the lower the force the springs exert. Longer bolts with washers under the springs to shim them might solve the problem.

Jon


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panhard
It's All Good
*****

Reged: 01/20/08

Loc: Markham Ontario Canada
Re: Collimation screws near their tips new [Re: Jon Isaacs]
      #5522007 - 11/15/12 09:59 PM

Jon is on the right track.

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acochran
professor emeritus


Reged: 06/19/08

Loc: So. CA
Re: Collimation screws near their tips new [Re: Jon Isaacs]
      #5523012 - 11/16/12 02:43 PM

Quote:

The further forward the mirror, the lower the force the springs exert. Longer bolts with washers under the springs to shim them might solve the problem.

Jon



Thanks Jon, that's a good idea! I went ahead and put 5 washers under each spring to compress them more. We'll see if that helps.
I checked with Bob's Knobs and he did not carry knobs with threaded rods longer than 2 inches. He referred me to a company called McMaster-Carr (www.mcmaster.com). They have knobs with 3 inch long threaded rods, so I ordered 3. I will shorten them a little with a hacksaw when they get here.


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okieav8rAdministrator
I'd rather be flying!
*****

Reged: 03/01/09

Loc: Oklahoma!
Re: Collimation screws near their tips new [Re: acochran]
      #5523165 - 11/16/12 04:37 PM

Quote:

Quote:

The further forward the mirror, the lower the force the springs exert. Longer bolts with washers under the springs to shim them might solve the problem.

Jon



Thanks Jon, that's a good idea! I went ahead and put 5 washers under each spring to compress them more. We'll see if that helps.
I checked with Bob's Knobs and he did not carry knobs with threaded rods longer than 2 inches. He referred me to a company called McMaster-Carr (www.mcmaster.com). They have knobs with 3 inch long threaded rods, so I ordered 3. I will shorten them a little with a hacksaw when they get here.




Before you cut those rods with a hacksaw, be sure to run a nut down the threads inside of where you will cut first. Then after you cut the rods, running the nut down to where you cut the rod will condition the threads on the end of the rod where you cut it. Sometimes, cutting a threaded rod will damage the threads just enough that it's hard to thread a nut back onto it. Not always, but it's caused problems for me a few times.


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acochran
professor emeritus


Reged: 06/19/08

Loc: So. CA
Re: Collimation screws near their tips new [Re: okieav8r]
      #5549180 - 12/01/12 06:17 PM

FYI: Lightbridge 16" collimation screws are metric. Longer than 2 inches, I had to make my own screws with metric threaded rods, hacksawed to proper length and metric handles that were bought online (All Metric Small Parts). I epoxy glued handles onto rods.
When the clouds go away, we'll test them.

Edited by acochran (12/02/12 01:44 PM)


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saxmaneagle
sage


Reged: 08/21/07

Loc: Saint Francis, MN
Re: Collimation screws near their tips new [Re: acochran]
      #5556101 - 12/05/12 06:07 PM

I used a small file, and rounded the tips of my screws, (where they touch the secondary). I found that the thread ends on them caused little 'jumps' when the last bit of adjustment was done. This seemed to make the adjustments much smoother, and more accurate.

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