Maksutov lens problem?

[matd]

Hi, today I picked up a Maksutov 100mm tele objective from a chap on Astrobuysell, the same as this one here:

http://www.365astronomy.com/Maksutov-100mm-Tele-Objective-1000mm-Focal-Length-F-10-Flat-Field.html

But when I focus on a star it appears as a ring - I think this means something is misaligned. Does anyone know how I might go about improving the alignment? I've looked a few guides on the web, but I can only see one tiny brass screw on the outside that looks like it might be for alignment. Here's a sample: Any tips would be welcome!

Thanks

-Mat

 

 

[John]

How long did you let it cool for before taking that shot ?

It's not quite in focus (hence the ring with the dark spot in it) but the rest might be because the lens has not cooled ?

[matd]

Thanks John, I think it was outside for half an hour before I took the shot. I thought I had the focus as close as possible - but I shall try again.

Many thanks,

-Mat

[RT65CB-SWL]

It looks that you may have not enough in/out focus. I do not know whether it is misaligned or a collimation issue and whether it can be collimated, as the stars are 'egg-shaped'/oval and the bright one has diffraction spikes.

Then again...

[Peter Drew]

Looks similar to the Russian MTO telephoto Maksutov telephoto system. If so, there is usually no collimation facility provided as this is determined by the accurate machining of the main body. The focusing is very critical on this design which could account for the apparent lack of focus. The field flattener which is buit in at the camera end might well be adjustable, the position of this will affect the star image size regardless of the focusing, it is essential to have this position correct to achieve good star images. The good news is that well set up they give excellent images. 

[matd]

Thanks for the replies. For your info, the sample is from the full height of the sensor, with just the sides cropped - so the rings are actually quite large!

With regards to the focusing - moving the focus slightly further out made the circles taller and thinner, moving it slightly further in made them shorter but wider. So does that mean I *do* have enough in-out focus, and the lens has a problem?

Cheers

-Mat

 

[Peter Drew]

Your description of the inside and outside of focus star images is a classic result of astigmatism. The optics in this type of "mirror lens" are usually butted up against the mechanics of the main tube to hold them in collimation, it's possible that one of the optical components is being held too tight.

[matd]

Thanks Peter. Is that something that a local telescope specialist might be able to help with, or would it have to go back to the manufacturer?

 

[Peter Drew]

Is focus achieved by rotating the front section of the tube ?, if so it would be worth checking which component is responsible for the astigmatism. You can do this by defocusing a star and noting whether the distortion rotates as you focus. If it does then the corrector is at fault, if not then it's the mirror. I'm probably your nearest telescope specialist ?  

[matd]

Yes, the front section rotates to focus - if I remember rightly the distortion doesn't rotate - it tends towards a vertical / horizontal line when you extend focus out/in.

Peter, might you be able to assist with repair? Should I PM you?

Kind regards

-Mat

 

[Peter Drew]

Yes Mat, send me a PM.