Jump to content


Photo

Collimation screws near their tips

  • Please log in to reply
7 replies to this topic

#1 acochran

acochran

    Viking 1

  • -----
  • Posts: 542
  • Joined: 19 Jun 2008
  • Loc: So. CA

Posted 15 November 2012 - 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.

#2 catboat

catboat

    Viking 1

  • *****
  • Posts: 680
  • Joined: 01 Dec 2009
  • Loc: Maine

Posted 15 November 2012 - 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.

#3 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44765
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 15 November 2012 - 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

#4 panhard

panhard

    It's All Good

  • *****
  • Moderators
  • Posts: 13669
  • Joined: 20 Jan 2008
  • Loc: Markham Ontario Canada

Posted 15 November 2012 - 09:59 PM

Jon is on the right track.

#5 acochran

acochran

    Viking 1

  • -----
  • Posts: 542
  • Joined: 19 Jun 2008
  • Loc: So. CA

Posted 16 November 2012 - 02:43 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

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.

#6 okieav8r

okieav8r

    I'd rather be flying!

  • *****
  • Administrators
  • Posts: 4692
  • Joined: 01 Mar 2009
  • Loc: Oklahoma!

Posted 16 November 2012 - 04:37 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

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.

#7 acochran

acochran

    Viking 1

  • -----
  • Posts: 542
  • Joined: 19 Jun 2008
  • Loc: So. CA

Posted 01 December 2012 - 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.

#8 saxmaneagle

saxmaneagle

    Ranger 4

  • -----
  • Posts: 380
  • Joined: 21 Aug 2007
  • Loc: Saint Francis, MN

Posted 05 December 2012 - 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.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics