What's going on?

[Tyke 123]

Hi,

Is it normal when conducting an artifical star test to have different patterns either side of focus?

One side is ( to my eyes) spot on ,the other by the looks need to move towards 7-8 oclock position as the ring seems narrow at 1-2 o'clock 

I recently stripped the 127 mak down to flock it but never had star tested it before hand so can't say the problem (if it is a problem) was present before. 

[vlaiv]

This problem is related to moving mirror.

It is either due to direction in which you end your focusing or maybe due to bent rail - which makes primary slightly tilted and hence very slightly out of collimation in that focus position.

First try to see if it is down to focusing direction - try to end your focusing in same direction for both in and out focus images (in one case - defocus more and then reverse direction to get wanted defocus level - but in other direction use only one direction of defocus - does this make sense?).

 

[Tyke 123]

6 minutes ago, vlaiv said:

 

First try to see if it is down to focusing direction - try to end your focusing in same direction for both in and out focus images (in one case - defocus more and then reverse direction to get wanted defocus level - but in other direction use only one direction of defocus - does this make sense?).

 

Thanks for the reply but I'm struggling to get my head round this.

[vlaiv]

11 minutes ago, Tyke 123 said:

Thanks for the reply but I'm struggling to get my head round this.

I'll try to explain it better.

Sometimes mirror flop depends on last direction of travel with moving mirror focusing. If you've ever heard that there is image shift when changing direction of focus in Mak and SCT scopes - it is because of this same thing.

Mirror gets very slightly tilted when changing direction of focus. I'm going to exaggerate effect in diagram just to make it easy to understand:

When you "push" primary towards secondary (upper part of diagram), it can tilt slightly one way with respect to optical axis, and when you "pull" primary away from secondary - it will tilt the opposite direction.

If you want to see if this is what is causing slight collimation issue with in/out focus image, then you should do two things -

first normally "push" mirror in but when you "pull" mirror out - follow with slight reversal of direction to end with "push" to have same mirror tilt in both places - like this:

When defocusing in one direction - just defocus, but when defocusing in opposite direction - at the end - reverse direction of turning the focuser know for half a turn - to end up going the same way as you did the first time.

You can do the same with in focus - reverse there and do outward motion as is.

If one of these ends up in concentric rings on both sides of focus and other with collimation issue on both sides of focus - then it is mirror flop.  If you have same thing as you had before - regardless of any direction changes - then it is "bent focusing lead screw" - or following diagram:

Hope this clears things up.

[Tyke 123]

Thank you for that detailed and  clear explanation. 

I will investigate  and see if you're analysis is the culprit.

Would if lefr have any noticeable effecr on image quality?

[vlaiv]

1 minute ago, Tyke 123 said:

Thank you for that detailed and  clear explanation. 

I will investigate  and see if you're analysis is the culprit.

Would if lefr have any noticeable effecr on image quality?

I guess that casual user of Maksutov scope simply would not notice this under regular use.

Most would describe this as "a bit fiddly to find proper focus" - but in reality this is going back and forth with focusing mechanism until both happen at the same time - proper focus is reached and mirror is properly aligned.

There would also be noticeable image shift when changing focusing direction.

If there is something else going on and there is actual collimation issue one side of focus (bent mechanism) - then it really depends on use case. It might be that at proper focus everything is in collimation - and one would notice nothing, but with Maksutovs - it depends on accessories used - different diagonal or eyepiece or barlow or binoviewers - it all changes focus position and at some point - mirror might be in trouble area.

Image will be slightly softer there (but judging by level of miscollimation - just slightly).

Maybe there is nothing to worry about - just run a test again at less defocus both ways and rings might be concentric in that focus zone. Also - keep in mind to hold star on optical axis when doing these sorts of tests.