Collimating refractors

[Merlin66]

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.

[johninderby]

Using a laser to colimate a refractor is pretty standard but I just print out a target that fits the objective.  Never thought to check the reflected dot.

[Merlin66]

Yeah,

The usual laser alignment test, just checks the alignment of the focuser to the centre of the objective.....

it’s that extra step of using the reflection from the objective to verify the collimation of the objective to the OTA/ focuser that’s the different and interesting point.

Edited June 22, 2020 by Merlin66

[kazihu]

If the reflection is off on the pointer plate and the lens cell is not collimatable which is the right setting? 

1. The spot centered on the objective and off on the pointer plate

2. The spot off on the objective and centered on the pointer plate

 

 

[Merlin66]

Start with the conventional focuser to objective alignment test with the laser, #1 in your list.

Any mis-collimation of the objective to the OTA and focuser will show in the pointer plate as a mis-collimated point. 

You need to collimated the focuser first, then look at the objective.

 

[kazihu]

The second  is only a "what if" case. When the objective is not collimatable.

Back on the first: The laser point is in the center of the objective doesn't  necessarily mean that the drawtube is paralell with the optical axis (because the drawtube has a play). If it doesn't that may cause the off center reflection. At least IMO. 

[Merlin66]

I agree.

The first test only shows that the focuser is aligned with the centre of the objective.

Once this has been established, I believe any off centre pointer plate laser dot, points to the fact that the objective is not collimated to the focuser.

If the objective is not collimatable, then there's not much you can do about it.

 

[John]

It is possible to shim an objective cell and a focuser to introduce some tilt, if it is needed to achieve good collimation. It's a bit trial and error but it can be done.

 

[kazihu]

I got a laser and test my refractor. The reflection was in the center even if the spot on the objective was a little bit off. I tried perfecly center the spot on the objective but not so easy to find out where is the issue.

The laser has a play in the 1,25 adapter, the adapter has a play in the drawtube and the rawtube may not well aligned...

 Even with compression ring design when i tighten a tumbscrew, it moves the spot.

And the laser itself is needed to be collimated. I tried to rotate, and saw it "circling" a little, but i can only rotate it with loose thumbscrew so when i touched the laser, the spot moved...

At last my method to collimate the laser:  I mark the spot position when everything is tight then loose a thumbscrew and rotate the laser 90 degerees, tight and mark again . I adjust the laser until the spot stays on the same  mark. I hope that after each "loose and tight" the laser was the same position...

 

 

[philj]

Thats the way I have always checked refractors and the reason I love collimateable cells, these new fangled cnc,d jobbies are ok but push pull screws are tge best.

I never collimate an ota with a diagonal in place, I do check after to see what effect the diagonal has tho. Most of the collimation issues I find are with the focusers unless there has been trauma or finger trouble in the cell.

I wrote about my experiences some time ago here

https://www.google.com/url?q=http://philjay2000.tripod.com/usefulstuff/adventures.pdf&sa=U&ved=2ahUKEwjw2-fF2ZzqAhW1Q0EAHTbIBqMQFjAAegQIBxAB&usg=AOvVaw2rzHdLJuEtui4RFpCFFgLD

[Merlin66]

Yes, the laser needs to be checked before you start.

I'm suggesting that the return spot from the objective replaces the Suiter "side illuminated Cheshire".