Collimation errors (with pictures)

[stevejack (Steve)]

I created this thread to pictorially demonstrate and troubleshoot some problems I'm having with the collimation of my scope (10" Bintel dob).

Some of the pics are hard to make out so bear with me...

The pic called "cheshire1" shows the view through the cheshire - everything looks ok here (or at least it's as good as I can get it at this stage).

"vanilla laser" shows the non-barlowed laser hitting the mark (there is some slop in the focuser, but its pretty close).

"barlowed" shows the barlowed laser hitting the primary - it looks off... the beam hits only the edge of the center mark.

But then check the image "barlowed error" - is this a problem? The beam shown here pointed at the ground shows a faint circular outline, but the brightest part is only an elongated semi-circle (the middle of which hits the center mark on the primary).

Is it a problem that the center mark is only just being illuminated by the edge of the barlowed laser given that any adjustments to the secondary at this stage will throw the cheshire alignment off?

"barlowed return" shows the shadow of the center mark on the barlow itself and I assume this means its ok? Have I missed something?

Star tests show very obvious comet tails when out of focus, and the tail changes sides at either side of focus, but so far I've not been able to fix this in the field.

Is there a problem with the mechanics of my collimation process that's making it impossible to collimate or will it simply take time in the field when star-testing to tweak the errors out?

[monoxide]

and the tail changes sides at either side of focus

sounds like astigmatism?
either its in your eyes or your mirror is pinched, im not sure if the guys at bintel check if the 3 rubber lugs holding the mirror in its cell are too tight

i know they collimate the scopes before sending them out so surely there cant be anything majorly wrong

[rmcpb (Rob)]

Have a look at this article to check if any of your mirrors are pinched.

[stevejack (Steve)]

Curiosity got the better of me and I thought people might also like to see how things go together in the Bintel/GSO dobs.

Here's what I found when I took the secondary apart...

The pictures show some form of rubbery resin on the edges of the mirror which I will have to sand to achieve a looser fit - I'm concerned it will just peel off making matters worse though so I will look at this carefully before I decide to do anything.

There was some tape on the plastic holder which was keeping the mirror in place as well. I'll try slipping the mirror in and out a few times to check the true fit now that this tape isn't holding it in.

[erick (Eric)]

How easily did it come out of the holder? I had to slide a knife blade around the edges and jack mine off the adhesive tape, bending plastic along the way!

ps. fit an AstyroSystems DG-1 dew heater while you have it off
http://www.sdmtelescopes.com.au/shop...products_id=46

[stevejack (Steve)]

It took some persuading with a small screwdriver, maybe 5-6 minutes of work to get it free without damaging anything.

I am working on the primary too but it should be all back together by tonight (hopefully) for some testing.

The heater would be great but that's a purchase for another time when funds allow it (read: after christmas)

[stevejack (Steve)]

She's all back together with both primary and secondary mirrors cleaned (tip: don't leave your scope near a campfire all night...) and the clamps sufficiently loose.

Hopefully things look better tonight... if not, it was still a fun process and I'll know there's something else to tinker with down the line.

Cheers for the help guys, and a big thank you to Mike and Starkler for their great articles on mirror cleaning and optical issues - following these made the whole process very straightforward.

ONE more thing I will say to other beginners like me: I was scared to adjust ANYTHING when I bought the scope. I didn't want to touch any screws or fix any visual errors for fear of making things worse. And to even think about TOUCHING the secondary? Are you kidding? I didn't even want to breathe near it.
My advice; Rip the scope apart and start over. Buy the collimation tools, read the articles, and break the thing down. You soon figure out how it all works when you have a scope in pieces on the floor and you're in a state of panic
After all that, tweaking a few knobs on the primary will be nothing.

[wavelandscott (Scott)]

Originally Posted by stevejack

ONE more thing I will say to other beginners like me: I was scared to adjust ANYTHING when I bought the scope. I didn't want to touch any screws or fix any visual errors for fear of making things worse. And to even think about TOUCHING the secondary? Are you kidding? I didn't even want to breathe near it.
My advice; Rip the scope apart and start over. Buy the collimation tools, read the articles, and break the thing down. You soon figure out how it all works when you have a scope in pieces on the floor and you're in a state of panic
After all that, tweaking a few knobs on the primary will be nothing.

This is very good advice!

[Blue Skies (Jacquie)]

That's how I learnt to do it!

[stevejack (Steve)]

Ok so after all that it turns out the little comet tails change when I move my head... :\

damn

damn-damn!

[bmitchell82 (Brendan)]

ahhh but think of all the experience you just gained.