Collimation HELP !!!!!

[Astro_Baby]

Whats this Astro Baby needing collimation help ? Shome mishtake surely ?

Well guys as the queen of reflector collimation I have come unstuck (giggles). See I managed to go daft this month and bought a Unitron Refractor - I've only had it 5 minutes and I am asking for help already.

Question is HOW do you collimate a refractor ? The lens cell appears to have three small tabs behind the lens - I assume these are shims of some kind. The front of the cell also has three very small grub screws I am assuming this act as a tilt for the objective.

Am I right ?

Okies so how do you collimate a refractor - I am assuming star testing with a serious dose of patience. AT the moment I can even use the scope cos its cloudy and I am sure not messing with it until I see a star test - first rule of collimation - test it first

[F15Rules]

First off...YOU BOUGHT A UNITRON!!!..

Which model? Pictures please??

A refractor should not normally need collimation, if there are issues (unlikely) it is more likely to be the star diagonal which is out.

I would strongly advise you NOT to touch that lens...Unitrons were very well built and it is likely that it will be in good collimation already. Fracs are not like reflectors and don't need to be adjusted often.

Wait til you get first light, do some star tests, and ten to one it will be spot on..then you can concentrate on gloating about the fact that you have bought a classic scope!

[Astro_Baby]

Yup - I wasnt going to mess with it apart from fix the busted bits with the mount.

Its a baby 60mm Model 114 - I puts some details up in the classics group. The tube is immaculate but the mounts in worse shape than I had been led to believe.

All in all though she looks beautiful.

[steppenwolf]

I guess that mentioning laser collimators in this context would be a little too dangerous?

[Astro_Baby]

Steve -

Actually I did know not to mess with it but I like to know how. Anything that comes to pieces must by definition be taken to pieces if its around me. Its a kind of charcter defect I guess. I have to KNOW

[Merlin66]

AB, there's acouple of things you could "check" rather than pull it to bits (yet) to see how it works!

Put a paper target over the outside of the dew cap with a small central mark/ hole/ circle ( this will mark the optical axis) without the diagonal, put a collimated laser in the eyepiece holder; the beam should sit right on the mark .... if it doesn't then the focuser is not 100% aligned to the tube and the objective

2nd use a Cheshire eyepiece with the lens cap on and look for a series of reflected disks/ rings which should be concentric if the lenses are sitting square to each other.

The little shims are spacing the elements and should not be touched or moved.

I can scan the section from Suiter's book on refractor alignments etc if it would help you.

Ken

[Astro_Baby]

Thanks Merlin - - I have done a quick bit of reading. Enough to know that this should be left well alone. Unfortuanetly dear old Unitrons are .965 drawtubes so most modern stuff for collimation wouldnt work. Ah well - I shall leave well enough alone.

[steppenwolf]

AB, there's acouple of things you could "check" rather than pull it to bits (yet) to see how it works!

Put a paper target over the outside of the dew cap with a small central mark/ hole/ circle ( this will mark the optical axis) without the diagonal, put a collimated laser in the eyepiece holder; the beam should sit right on the mark .... if it doesn't then the focuser is not 100% aligned to the tube and the objective

This is exactly what I had in mind but I knew Mel would kill me - how come you are still alive, Merlin?

[m.tweedy]

found this link on collimating a refractor

http://www.spacealberta.com/equipment/refractor/collimate.htm

[John]

found this link on collimating a refractor

http://www.spacealberta.com/equipment/refractor/collimate.htm

Thats the method I've used with my 5" refractor which has a collimatable lens cell. The same test shows that my 6" non-collimatable refractor is off a bit but the star tests with it so far have been very satisfactory so I've concluded that refractors are less sensitive to a bit of mis-collimation.

[brianb]

The lens cell appears to have three small tabs behind the lens - I assume these are shims of some kind. The front of the cell also has three very small grub screws I am assuming this act as a tilt for the objective.

The shims are to keep the elements of an air spaced objective apart. Do not mess, you will regret it iif you do.

I don't know about the grub screws. They may simply be holding the cell together. It's unusual for refractors under 4" to have any provision for collimation.

[Astro_Baby]

Brian - your right. I did some fast reading and the lens on this scope is not collimatable. At least not in any practical sense.

Dont worry there was no way I was going to take this to bits in any hurry - I did think about it for about - ooohhh - 5 seconds Then I read some articles and decided a smart girl knows when she is well offf so leave it alone.

[ollypenrice]

You could do an artificial star test. I did this to collimate a refractor once, having spoken to the manufacturer at length.

Glue a small ballbearing onto a matt black card and find a long dark room. I used 'Captain Condom's' warehouse in Cromford. (I am not making this up.) You illuminate the ball with a decent beam like that of a good cycle lamp and take the scope as far away as possible. You then observe the lit ballbearing. I got a perfect artificial star that way, a real beauty. In fact with a few more ballbearings you could make your own Pleiades....

Olly