Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

billyharris72

Problem with collimation - suggestions needed

Recommended Posts

Hi all:

I've run into a problem  collimating my scope (a Bresser Messier 200mm Dobsonian). I start by following the procedure below.

  1. Check the secondary mirror is centred in the focuser.
  2. Make sure the reflection of the primary is centred in the secondary.
  3. Adjust the primary mirror to get the primary mirror centre spot aligned with the eyehole in the collimation cap or Cheshire.

What I get ends up looking something like this (excuse my awful drawing).

horizontal.jpg.86a4a41f3d143d3315806f7e3

I then raise the tube in altitude and look through the collimation cap to see:

diagonal.jpg.7b2d46e44fee874aa15e68ef147

Everything looks the same except the centre spot is no longer where it should be. If I move the scope back down in altitude everything goes back to where it is supposed to be. And this happens in a smooth way - it does not feel as if anything is moving or slipping (it's a solid tube, and pressing on the primary mirror housing does not make anything move).

Has anyone experienced anything similar? I'm assuming it has something to do with my point of view rotating as the scope is raised (though bracing my head in the same position and trying to rotate in a fixed position to the focuser makes no difference) and that something is therefore not aligned properly with the optical axis. But for the life of me I can't see what it is that is out of place.

Suggestions gratefully received. :(

Billy.

Share this post


Link to post
Share on other sites

May be the focuser tube flexing if it's not tight (crayford??).  The mirror could be solid but if the cheshire is moving somehow you'd get the same thing I think.

My 200p has the same issue to a lesser extent, as I fitted collimation springs that are a touch too weak so f I collimate at low altitude then raise it up it goes off a little. (not sure if the Bresser has collimation springs or push/pull screws?)

 Work around is that I always try to collimate at the altitude I'm going to be observing (only takes 20 secs to stick the laser in to check if move to different altitude).  I should get some stiffer springs but just haven't got around to it yet.

This also happens if your mirror clips are loose (as they should be really!).  

Share this post


Link to post
Share on other sites

Actually I've just realised that in your second sketch the mirror clips are in the same correct position, so the cheshire & secondary must still be in alignment and it must be the primary mirror that is tilting slightly... I think!

Share this post


Link to post
Share on other sites

I think you might be right on that. I had had the scope on an equatorial mount. To test my idea that it was something to with rotation I put it back on the Dob mount with the focuser tube facing straight up. Collimated the scope horizontally and then raised it while keeping my head straight over it, so nothing was rotating. The same problem was there, albeit to a lesser extent. So I doubt it has anything to do with the mirror itself and is more to do with the primary moving around.

The Bresser does indeed have springs, so maybe this is the issue. Are these easy to replace? I've never tried removing the primary cell or anything similar before and am a touch nervous in case I can't get it all back together or end up causing some damage.

Share this post


Link to post
Share on other sites

Are the springs between the primary and the back plate tensioned, Collimating by turning the screws Anti-clockwise should bring the backplate and the mirror closer together and the spring tight to stop any mirror flop....??

Edit = Anti-Clockwise not Clockwise (now i have thought about it)

Edited by Tinker1947

Share this post


Link to post
Share on other sites

I would be inclined to check the secondary given the amount the centre ring has moved in your picture. Did you loosen the centre screw on the secondary to move it down the tube? The three secondary collimation screws are quite short and it would be easy to have reached the end of travel on one so that as you raise the scope from horizontal the secondary starts to swing free.

Share this post


Link to post
Share on other sites

I've got the same scope and have only checked horizontally. This has spurred me to go check it again vertically! Cheers! I always make sure the lock screws are tight and assume its all good

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.