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Collimation issue:

Started by The Mad One, 02/09/2012 03:01PM
Posted 02/09/2012 03:01PM Opening Post

I have a quirk in my 8" that has me somewhat stumped. I took the primary out to give it the first cleaning in the nearly 4 years since putting it in the OTA. Side note, used the Photonics Cleaning solution, worked fantastic & the basic kit has enough in it to do 4 such 8" primary mirrors.... plenty in the basic started kit! Well I finally decided to place a center marker on the primary While I had it out, I've been using a Cheshire Eye for collimation for years, works even without a center marker. As you likely know it's all about the visual picture you see not a laser dot to the marker thing.

Well, here's the problem! When the OTA is properly collimated by the use of the Cheshire Eye the laser collimator dot always hits the upper left of the marker ring. The laser shows the Primary as true from that spot and this of coarse is confirmed by use of the Cheshire. I have triple checked the marker on the primary & it is dead center of the primary. I checked the focuser again, it is dead on the mark in the tube across from it. I have triple checked the secondary & spider and this too shows as being properly centered. There does not seem to be any shift of the primary in the cell, seems to be seated properly. Any input on possible causes of a fix? So far I am just ignoring the "new" center mark & checking the collimation the old fashion way with the cheshire. Star testing shows the OTA to be perfectly collimated with this method.

Used 2 different laser collimators with the same quirk. An Orion lasermate & a Hotech collimator. I may try rotating the primary cell in the OTA tube to see if maybe it's the tube causing the laser misalignment. The collimation with the Cheshire confirms the same findings, when visually collimated with Cheshire the marker ring appears to be in the same slightly off situation. Any ideals? Other than the possibility the mirror is shifted slightly out of center of the tube by the mirror cell position in the tube I'm stumped. :S


Posted 02/09/2012 05:13PM #1

A couple of comments:

- Have you checked the collimation of the lasers?

- To set the primary tilt you should not use the return beam of the laser, that is very sensitive to other errors like focuser tilt, slop in the focuser and miscollimation of the laser itself. You really need to be using Nils Olof Carlin's Barlowed Laser technique which is insensitive to other collimation errors.

The Barlowed Laser arrived on the scene about 10 years ago and is quite simple. You make a target for the focuser end of your barlow with a small hole in the very center. Put the Barlow in the focuser, put the laser in the Barlow.

On the primary mirror you will see a diffuse pattern and if you look at the target on the Barlow you will see the diffuse pattern with a shadow of the primary ring or triangle. You should center that shadow around the out going beam to collimate the primary.

Howie Glatter builds a number of items that make use of this principle, the Blug, the Tublug and a standard barlowed laser attachment for the laser. They are nice and that is what I use but a regular Barlow is just as effective.

Posted 02/09/2012 08:23PM #2
A red dot laser collimator can give a false reading if the telescope is not set up properly in the first place or if the laser itself is not properly used.

If, for instance, your laser is not pointed in the middle of the secondary (actually to a point near, but not quite, the middle), you can get a perfetly good return bounce, but improper collimation.

Posted 03/30/2012 04:01PM #3
Two suggestions:
1) If your mirror is f/8 or faster, the secondary needs to be offset away from the focuser and toward the primary, since the portion of the secondary away from the eyepiece intercepts a larger portion of the light cone. If the secondary seems centered in the eyepiece AND centered in the tube, the laser will give you a small skew. A good check is to see if, when you are "on target" with the laser, does the "return" spot from the laser it the same place as the "input" dot from the laser?
2) Simply rotate the laser in your focuser to see if it gives you an accurate center. If a rotating laser describes a small circle, then you need to compensate for a centering error in the laser itself.
Posted 03/31/2012 01:30AM #4
Thanks all for the suggestions. Problem was solved with a complete recheck of the moolite focuser in relation to collimation to the secondary. The focuser was slightly off center to the side of the secondary. I caught this after rechecking the focuser against the mark on the opposite side of the tube after re-marking the spot directly across from the center of the focuser tube. the original mark was placed based on the old original focuer which must have mounted into the hole slightly off ctnr. At any rate after re-marking the tube, collimating the focuser to the mark, putting the secondary back in place with a small ctr. mark as well the problem was solved. I used the Cheshire eye for all of the collimation procedures then after making sure the Orion lasermate was true found evrything to be as it should be. Red dot center of the donut, reflecter laser center back through the lasermate laser opening.

Now with the busy season of my PT summer business coming into full swing all I have to do is find some time to use everything!