Collimation of an imaging setup

[Thommy]

I collimate my Maksutov-Newton without the imaging train (filter wheel, off-axis guider, guide camera and CCD camera). First, I collimate the secondary mirror with a laser collimator, then the primary mirror with a modified Kroupa. Visually this works like a treat, but when the imaging train is installed the focuser flexes a bit depending on target. How do you solve this? Do you somehow collimate with everything attached?

Thanks,

/Thommy

[wimvb]

Since the flexing depends on where the scope is pointing, and this changes during the night, there's really only one solution: throw money at it and upgrade the focuser.

[Thommy]

Of course, you're right, but right now that is not an option. I'm looking for a "second best" solution. Now, for the first time I tried to collimate with all the equipment (minus CCD) mounted. I placed the telescope in a "typical" direction for imaging. Actually, there was a significant miscollimation, but if this relies on the new setup is not clear.

I thought this was an issue experienced by many imagers - surely not all use the best focusers available. I'm a bit surprised that there are no more responses.

/Thommy

[wimvb]

Do you have the possibility to "tighten up" the focuser? SkyWatcher focusers can be adjusted to become stiffer. This could possibly improve the flex situation.

Another option is to use extension rings. If you normally need to have the focuser tube half way out, then adding a short extension, you can have the focus tube longer in, and it may be stiffer.

[Thommy]

Thanks Wim,

Yes I adjusted the focuser to be as stiff as possible. I have the Skywatcher MN190 where there is a retractable part of the focuser. This added a lot of flexure, so I drilled two holes in the foucuser to secure the retractable part with screws. Even so there must be some flexure remaining in the focuser.

/Thommy  

[wimvb]

3 hours ago, Thommy said:

I have the Skywatcher MN190

I'm very interested in this scope as my next imaging upgrade. Can you show what modifications you made to the focuser?

On 02/11/2018 at 13:07, Thommy said:

a modified Kroupa

Just curious, but what's a "Kroupa"? Google only came up with prof. Pavel Kroupa.

[Thommy]

Sure, I will post some pictures, but is travelling a lot these days, so it will have to wait. I will not forget, though. There are a lot of threads on replacing the MN190 focuser, but that's because it it a very cumbersome task where a lot of people have failed.

Sorry, it is a "Krupa" collimator - my fault. It works really good. Mine is made so I can use it as it is, looking into it, or I can put a webcam onto the collimator which has the advantage that I can see the changes in collimation live, while adjusting the primary.

/Thommy

[Thommy]

Yeahh - I made it in 2018! Happy new year!

The screws are 3 mm but could be smaller. I never discovered how to remove the inner part, otherwise I would just have removed it.

/Thommy