Collimation

[luka]

Hello, i recently got a 12" GSO dobsonian, i know how to collimate it but im not sure about why almost every time i rotate it, it needs collimation again? Am i missing some collimation tightening screws? Thank you for any advice!

[Mr Spock]

What I found when I bought my StellaLyra 12" is the springs were weak. I replaced them with stronger springs from Bob's Knobs - Springs for common Newt primary Item No CNspr

It now holds collimation all night. I do check it between each session. It needs minor adjustment sometimes due to being jostled onto the mount.

[luka]

Oh thats actually a reasonable fix, ill definitely try that out.

[John]

I agree with @Mr Spock that the primary cell collimation springs are weak and replacing them much improves the holding of collimation. I had the same issue with my Meade Lightbridge 12 inch which was also made by GSO.

It is frustrating that GSO have not addressed this issue - collimation is one of the main issues that folks are wary of with newtonian telescopes and the GSO weak springs problem has been discussed often for more than 15 years on astro forums such as SGL and Cloudynights 😒

Edited April 26, 2023 by John

[garryblueboy]

Yep I would change the springs makes a big difference to holding collimation 

[PeterStudz]

On my first telescope, a small skywatcher reflector, I had an issue collimating - every time I move the OTA it would go out a bit. As a beginner it took me several months to work out what was going on.

Eventually I found that a couple of primary mirror clips were not done up properly, so much so that they would physically rattle around, causing the primary to move in its cell. Of course it’s a fine line between too tight & too loose. 

[bosun21]

Another vote for upgrading the primary springs. I also found that fixing the collimation point with the collimation springs being well compressed gives even better results.

[luka]

1 hour ago, PeterStudz said:

On my first telescope, a small skywatcher reflector, I had an issue collimating - every time I move the OTA it would go out a bit. As a beginner it took me several months to work out what was going on.

Eventually I found that a couple of primary mirror clips were not done up properly, so much so that they would physically rattle around, causing the primary to move in its cell. Of course it’s a fine line between too tight & too loose. 

I actually tried thoose clips alredy, they were very tight when they came from the factory so i just untighten them a little bit cause i had a feeling it was too much

[Spile]

I assume there are no lock bolts on the primary mount of 12”?

Edited April 28, 2023 by Spile

[John]

1 hour ago, Spile said:

I assume there are no lock bolts on the primary mount of 12”?

The GSO primary cells do have locking bolts. They are a mixed blessing though because the cell components are made of quite thin metal and using the locking bolts can cause the cell containing the mirror to flex, which can itself alter the collimation.

[Mr Spock]

I don't use the locking bolts. I find the heavier springs work just fine. I checked the scope again last night before I started and it was still spot on 👍

[Spile]

I know a lot of folk replace the springs but with my Skywatcher I find the locking bolts keep the primary aligned ok.