Laser Collimators

See my post here:

www.irishastronomy.org/boards/viewtopic.php?t=8066&start=15

Since then I have found out that one can't really rely on a cap to align the secondary no matter how slow or fast the scope. The cap is fine for primary collimation but even then only on slower F6 and above.

One really needs a sight tube/Cheshire combe to get the secondary centred and rotated correctly and tilted. Problem is that at night its a pain holding a torch to the cheshire window while looking through the whole while your other hand is around the front adjusting the secondary screws.

As a result of my bog standard lasers that constantly go out of collimation, my focuser slop, my 2-1.25'" adapter slop in the drawtube............but my desire to be able to do it at night which really does need a laser to be able to do it quickly rather than the cheshire......

Thus I invested in a premium laser collimator from Howie Glatter. Triple the price of my €50 antares but will be worth it. I got the 2"-1.25" version which can be used as both. I will use it in 2" mode so that there will be no 1.25" adapters with slop between it and the drawtube. It is precision machined barrel so there is some slop eliminated too. It is built like a tank and guaranteed to be in collimation when it arrives and stay that way unless you drop it on concrete from about 10ft high. Lots of guys have dropped theirs from head height and found the collimation still perfect. There is also a holographic attachent which projects a laser grid for correctly set secondary centering, and rotation. It also comes with a big barlow target with a 45 degree white face that is inserted in the bottom of the drawtube. Truss users can point this backwards and see the target from the primary collimation screws end. Solid tube users like me can angle it forwards and either go forwards to check adjustments or mount a little vanity mirror to the tube to see the target from the back, but in my case with a laptop usually with me, I will just point a webcam at it and view the target on my laptop screen down the back end of the scope while I collimate.

I felt it worth the outlay because with all the issues I had with the cheaper lasers I just found my self frustrated with constantly worrying if the laser was sitting right in the focusser, worrying that the laser was out of collimation, worrying that my home made barlow target was of centre, had a pain constantly haveing to go back forwards to check adjustments of the primary etc etc In short, I just could never trust my collimation with the standard lasers and in the dark using the other tools were a chore.

question

I received my laser collimator in the post on friday and worked up the courage to laser collimate my 10 inch dob. I thought I had the optics lined up just using the collimation cap last month - achieving concentric circles between the mirrors as seen looking thru the cap....I used the laser device and made the necessary adjustments to the primary and seconary mirrors. I now have a laser light path that to my eye is 95% accurate

but. :shock: when I was finished the process, I thought I would :evil: check with the collimation cap on its own and find that the view is no longer concentric as viewed thru the pin hole. ....Is it possible the the focuser is not 90 degrees or have i missed something
BTW have had no chane to field test the adjustments

any comments welcome

Declan

Wait and do a star test. You should get a nice Airy pattern the same on either side of focus if your collimation is correct. If it's not and the laser-colli is indicating good collimation, then yes it may be the focuser needs adjusting or shimming to square it up (though this in itself should result in failure to collimate with the laser).
I get pretty perfect collimation using the Laser-Colli, and this is confirmed by doing a real Airy test on stars, I trust it. (Maybe the collimation cap is the problem).

Thanks for that John....maybe I have a crooked eye. I am waiting for some decent weather to check on the changes made. I am not a perfectonist - but I would like the scope to operate to it's full potential. I was just curious after noting that the optics seem way out as viiewed thru cap

Your scope could have an offset secondary, which may be causing the confusion. When you collimate with the cheshire and then put in the laser, what part of the secondary mirror is it hitting? Bang in the centre or slightly above centre? If your secondary isn't already centre marked, it is worth doing.

Michael, I assumed the secondary was correct, but don't know for sure. after laser collimation the seconary is off to the left as viewed thru cap

I don't know. If I removed the secondary to mark it (if thats what you mean) it would be an operation with more complexity than the moon landing :shock:

If I do nothing will it seriously effect viewing? i.e. comprimise between the two?

declan