Collimation screws

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KathyNS Canada
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Collimation screws

#1

Post by KathyNS »


I had my first good look at the collimation screws when I had the mirror cell out for cleaning yesterday. I always thought that the screws had a very limited range of motion, and I have now confirmed this.

The first photo shows the cell (orientation is mirror-up; mirror is removed) with the collimation screw set so the cell is at its maximum aft position, spring fully compressed. As you can see, the screw stands proud of the cell surface. If the mirror were in place, the screw would press on the back of the mirror, so this position is not attainable in practice. So the cell cannot go fully aft.
cell is max aft.JPG

The threaded hole in the cell is quite deep, so the screw can be backed out quite a long way, moving the mirror forward. I would need longer springs to do that.

However, when the collimation screw is backed out just enough to not contact the mirror, the locking screw can just barely reach the mirror cell, as shown in the second photo (orientation is mirror-down). This is the farthest aft that the cell can safely be. So the range of motion between mirror aft (collimation screw contacts the glass) and mirror forward (locking screw bottoms out) is about two turns of the screws.
locking screw when screw flush.JPG

So, questions...(1.) Is this normal? (2) Would there be any advantage to using longer locking screws or shorter collimation screws? I have enough range to collimate successfully, but none to spare.
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DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
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Re: Collimation screws

#2

Post by notFritzArgelander »


I don't think this is something to be concerned about. I had the 200mm f4 and the travel of the screws was adequate to achieve collimation. At f4 you really don't need much travel on the collimation/locking screws. The fast primary means that small changes make big differences.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Gordon United States of America
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Re: Collimation screws

#3

Post by Gordon »


If you haven't had any problems with it, I would just leave it as is.

"If it ain't broke don't fix it."
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Scopes: Explore Scientific ED80CF, Skywatcher 200 Quattro Imaging Newt, SeeStar S50 for EAA.
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KathyNS Canada
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Re: Collimation screws

#4

Post by KathyNS »


Gordon wrote: Tue Nov 01, 2022 7:05 pm If you haven't had any problems with it, I would just leave it as is.

"If it ain't broke don't fix it."
Well, I will certainly be a lot more careful when adjusting the screws,knowing that they can press against the back of the mirror! Generally, I would agree with "If it ain't broke don't fix it."

OTOH, it looks like there is a simple fix that will remove the risk to the mirror and at the same time give me some adjustment room. I can replace the adjustment screws with threaded rod, with wingnuts on the outside for the adjustment. Unlike the existing screws, the threaded rod won't turn (I'll ensure that it doesn't), so there will be no contact with the mirror.

I just have to figure out the thread specs. It looks like it might be M4 or something like that.
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DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
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Re: Collimation screws

#5

Post by JayTee »


I don't know if this will help, but here is a picture of my mirror cell looking at it from the back of the telescope.
The straight screws lock the cell in place after the wing nuts have been correctly adjusted. You just snug those screws until they're just touching the mirror cell.

Mirror Cell.jpg
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
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KathyNS Canada
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Re: Collimation screws

#6

Post by KathyNS »


JayTee wrote: Wed Nov 02, 2022 8:07 pm I don't know if this will help, but here is a picture of my mirror cell looking at it from the back of the telescope.
The straight screws lock the cell in place after the wing nuts have been correctly adjusted. You just snug those screws until they're just touching the mirror cell.
That is pretty much what I am planning to do with mine.

Right now, the "pulling" screws have molded plastic knobs, and they turn, threading into or out of the mirror holder. I will replace them with fixed screws, like yours, that won't turn. Instead, the wingnut will pull the screw through the back frame.

I measured the thread: it is M6. I couldn't find any M6 threaded rod, but I did find some long M6 bolts. I can cut the heads off them. I got locking nuts and wingnuts. I have a photo session cued up for tonight, but tomorrow, I am planning to do the surgery.
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DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
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