I'm in the process of re-building a Spectroheliograph and a PST mod.
In the SHG I have, what I think is a good lens, a Carton 100/1300 objective.
I normal collimate the OTA - focuser to objective, using a objective target and a laser pointer, followed by the Cheshire side illuminated to show the "doughnut" reflections a la Suiter (p121-123)
I now have a very good variable brightness collimator available, and carried out the usual focuser to objective test.....BUT I noticed a reflected "dot" in the collimator target...this seems to be reflected from the optical surface(s) of the objective.
This reflection can then be used to collimate the objective lens to the OTA and focuser.
This is something new to me, having never used a collimator for this test. I haven't seen it mentioned as a way of collimating the objective, so I thought it would be of interest.
Ken, the reflection is normally used in Newts etc, but those lasers can be used for refractor collimation, in fact i used a very similiar one when i built my iStar 127mm f12 refractor. So yeah use an objective target which you have centre spotted, and your Cheshire to align the donuts so that any objective tilt is negated.
I guess you might be able to use the laser reflection but be very sure your laser is actually collimated correctly itself (ie it shoots straight). It is pretty easy to collimate a laser, and there are numerous threads about how to do that, so I won't go into that here.
Also, one other thing, make sure you rack your focuser in and out its range when your collimating to the centre spot on the objective, so you can find any focuser misalignment throughout its range of movement. Some focusers do not rack straight. I had one on a RC which wandered from one side of centre to the other.
Glen,
Yeah, I've had enough, and built enough, "laser pointer" collimators to double check the alignment before relying on their accuracy. The collimator I used is pretty good with no apparent movement over three metres when rotated in the V blocks.
I had obviously seen the collimator laser used on reflectors, but I had never seen any reports of it being used to collimate a refractor.
I'd like to think if this is a robust collimating method, that it will help many amateurs.
I had “issues” with my Tak TOA 130 a while back. The rear element of the triplet was loose, and I found the collimating screws all over the place. I was also ‘stranded’ in the wilds of Laos and didn’t have access to anything sophisticated.
I eventually got the scope collimated to my satisfaction after some weeks of experimentation and frustration.
What I noted at one stage, using a Howie Glatter laser colimator, was that I could see the individual reflections off each of the three elements back onto the colimator at the focuser. There was a difference in brightness in the reflected images but as part of the alignment process I could adjust the elements to each reflect back to the centre of the focuser.
I’m guessing that if you can adjust the elements separately, you might try for that alignment also.
In my case, the two front elements were/are locked together into a single cell. The rear element can be adjusted separately. While getting the reflections to align at the same point in the centre of the focuser is one thing, I also had to experiment basically trial and error with the separation between the front cell (two elements locked together) and the rear element. This was extremely finicky. With only a fraction of a turn on the collimating screws to move through the correct collimation setting.
For the Tak, the laser collimator was only a starting point. The ‘real’ fine alignment had to be done using an artificial Star.
