Collimation of Newtonion Telescopes

Space Dog (Ric) · #1 21-03-2007, 11:30 PM

I have a 200mm Newtonion / Dobsonion telescope Telescope that was supplied with a LaserMate collimator that is easy to use, however I'm not entirely happy with the result that I'm achieving.
The beam emitted from the collimator is not pinpoint and is very bright which I believe is leading to less than perfect collimation.
I have heard that the Cheshire eyepiece is a more accurate way of collimating a telescope.
Comments / advice on this subject would be most welcome.

Ric.

iceman (Mike) · #2 22-03-2007, 05:37 AM

Hi Ric.

Welcome to IIS!

I don't use a laser, but have heard that to use them effectively, you have to ensure your laser is collimated! The easiest way to do that is to put it in a v-block of wood or similar, and project the beam against a wall. Turn the laser and see whether the beam stays put or moves with it.

I use a sight-tube/cheshire combo, and I find it great. It's all i've ever used in 2.5 years. I got the longer-tube one (not the short one) - they're about $60 or so but will last a lifetime.

I have however, just ordered a set of Catseye collimation tools. I'm at the point now where I want to know my collimation is PERFECT, not "close enough", and so these catseye tools will help me achieve that. Not exactly cheap but i'm hoping they'll be worth it.

rmcpb (Rob) · #3 22-03-2007, 08:53 AM

Welcome to IIS Ric

I have both a Cheshire eyepiece and a laser collimator. I hardly use the Cheshire now as I mainly use it to centre the secondary and once that is done it hardly ever needs checking, maybe once a month at the most (haven't had to change it in over 18 months). I use the laser to ensure the secondary is hitting the centre of the primary, always OK as it is locked, and the beam is returned back to the secondary accurately, usually extremely close and only takes a tweak of the primary screws to adjust.

My answer is that both have their jobs and, considering their cost, they have a place in every kit.

Cheers

h0ughy (David) · #4 22-03-2007, 10:52 AM

thanks GUYS, enjoyed the info. Mike did you get a order like Strongman Mike's?

DougAdams · #5 22-03-2007, 11:16 AM

Quote:

Originally Posted by iceman

Hi Ric.
I have however, just ordered a set of Catseye collimation tools. I'm at the point now where I want to know my collimation is PERFECT, not "close enough", and so these catseye tools will help me achieve that. Not exactly cheap but i'm hoping they'll be worth it.

What makes the Catseye better than a Cheshire?

Starkler (Geoff) · #6 22-03-2007, 12:40 PM

Quote:

Originally Posted by DougAdams

What makes the Catseye better than a Cheshire?

Catseye tools include cheshire and autocollimator. These are the main differences as i see them.

The triangular centre spots precisely fit the illuminated ring of the cheshire. This greatly enhances accuracy.
Being two inch format allows a much easier to use autocollimater as images can be seen much further off axis.

Nils Olof Carlin wrote a piece about autocollimaters and I read what he wrote to mean that the order of accuracy given by the autocollimator to be somewhat unneccessary for normal use. ie not worth the hassle (unless you are trying to get a perfect image onto a tiny chip like Mike

)

astro_nutt · #7 22-03-2007, 03:50 PM

I use the older style of laser colimator and in order to check the alignment. With the scope horizontal, I use a small pocket mirror held under the secondary mirror to view the beam going back to it's source. The image I look for is an illuminated red "donut" around the exit point. This would suggest that the beam has returned to it's source and the beam flare has illuminated the exit point in a perfect circle. Some beam patterns are shaped like a flying saucer..the trick for this would be to rotate the collimator in the focuser until you see the flying saucer shape image in the pocket mirror..then gently nudge the spider vane holding the secondary..the return beam should sit over it's source of origin exactly.
Cheers!!

DougAdams · #8 22-03-2007, 04:05 PM

Quote:

Originally Posted by Space Dog

I have a 200mm Newtonion / Dobsonion telescope Telescope that was supplied with a LaserMate collimator that is easy to use, however I'm not entirely happy with the result that I'm achieving.
The beam emitted from the collimator is not pinpoint and is very bright which I believe is leading to less than perfect collimation.
I have heard that the Cheshire eyepiece is a more accurate way of collimating a telescope.
Comments / advice on this subject would be most welcome.

Ric.

The other alternative is to pop the laser in a barlow and look for the doughnut reflected back up the drawtube. I believe this skirts the problems of having your laser out of collimation. I've been using this method and it seems to work fine - very easy. I had to fashion a cap from an old eyepiece cover to fit over the drawtube inside the OTA (bit like the BLUG).

xelasnave · #9 22-03-2007, 07:15 PM

You may find this interesting
http://legault.club.fr/collim.html
alex

Space Dog (Ric) · #10 22-03-2007, 10:41 PM

Thank you everyone for your helpful advice.
You have certainly given me a few different options to improve my collimating skills.
Cheers, Ric.

Geoff45 (Geoff) · #11 23-03-2007, 11:11 AM

Quote:

Originally Posted by DougAdams

The other alternative is to pop the laser in a barlow and look for the doughnut reflected back up the drawtube. I believe this skirts the problems of having your laser out of collimation. I've been using this method and it seems to work fine - very easy. I had to fashion a cap from an old eyepiece cover to fit over the drawtube inside the OTA (bit like the BLUG).

This is by far the best method. Try rotating your eyepiece tube using the laser without the barlow, and I'll bet your "perfect" collimation changes. With the barlow, things stay stable. Google "barlowed laser" for more info, or look here http://gmpexpress.net/~tomhole/blaser.pdf
Geoff

Space Dog (Ric) · #12 29-03-2007, 06:29 PM

Well I have mastered the art of collimating my scope.
Made a sight tube to check the position of the Secondary and Primary mirror and used a Barlowed Laser and a small hand held mechanics mirror to view the Laser beam returning to its source for the fine adjustment.
Works a treat! Also did a star check and am very happy with the results.
Once again thank you all for your advice.

Cheers.
Ric.

astro_nutt · #13 29-03-2007, 11:08 PM

Well done Ric!..enjoy the views with your fine tuned scope!!
Cheers!