laser collimation and Mak/Cass scopes

[rider]

this is copied from another thread where I didnt get a reply....
might do better as its own thread.

Hi, all

I have an 8 inch mak, and believe that I can't use a laser collimator on it.

My reasoning is that the laser beam would hit the center of the (fixed) secondary mirror and return directly (hopefully) back to the laser collimator - ie: it would never touch the adjustable primary mirror.

is my reasoning correct? - I hope that I'm wrong, because I hate wasting prime viewing conditions to do star based collimation.

rider

[bojan]

Yes, your reasoning is correct. Laser collimator that goes into eyepiece barrel is not useable.
However, cheshire collimator can be used with catadioptrics.

[Max Vondel]

You may not be able to collimate a cat/sct but I think rider that the laser beam must have some kind of width. The curved secondary (mirror or miniscus) is only tangentally perpendicular to the beam at one (hopefully the central) point. Any photons outside this line, should be reflected away from the source (ie away from the incoming beam) and be spread. This spread maybe wide enough to reach the primary. All of this depends on the level of curvature and the width of the beam. Might be an interesting test. I will get some batteries for my collimator and check it on the sct. I suspect that the red laser light should be visible on some part of the primary mirror. Could anyone check this and save me buying batteries ? :whistle:

[Merlin66]

Laser's no good on these types of scopes; save your money!
A cheshire eyepiece will help and a webcam image of the diffraction pattern ( and initial donut) will get you there.

[OneOfOne]

Yes, you do get scattered light from the beam. Kendricks make a laser collimator for SCTs (I think it can be used on Maks) that uses the scattered beam to project an image on a paper target placed at the near focus point (10 to 30 m away). The theory is that you can use this beam for collimation....however, I bought one for this reason and you can forget it. Any slop in the fitting of the laser in the eyepiece tube MORE than exceeds the tolerance required. If you search the forum you will see that Rumples Riot got one and found he had to recollimate with a star test (I found this after I bought mine).

I don't know much about Mak collimation so this may not be applicable, however HoTech have a new laser out that they say allows you to check that the secondary hasn't moved once a good star collimation has been done. Their locking mechanism uses two rubber o'rings that expand inside the tube to remove any slop. The theory is that once the secondary is set, the laser can be used to check that the return beam comes to the same point each time on the face of the laser. If you google HoTech SCA collimator, you might get some info and if it is suitable for a Mak. I bought one a few months ago but it doesn't work in mine, the o'rings don't expand enough to grip. Maybe I should sell it...

[rider]

Thanks for all replys,

It seems I was correct in assuming that a good Mak collimation is only available through a star test.


Has anyone tried the "make your own star" in the projects section?

it seems to suggest that you can collimate without wasting veiwing time.

I was wondering if it would work better using a very low power laser and a VERY DARK FILTER (children, dont try this at home) rather than the LED/optic fiber approach...
have to be careful of the eyes dont we???

Rider

[HOTECH]

Hi Trevor,

"the o'rings don't expand enough to grip.."

Regarding the HOTECH SCA Laser Collimator fitting, we have replacement o-rings to fit focusers with wider tolerances. Please contact us and we will sent you the new o-ring. Thanks.:)