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off center out of focus rings in Mac Cas

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#1 gfeulner

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Posted 08 December 2018 - 02:49 PM

A friend of mine just bought a new Skywatcher 127 Mak Cas. We were outside testing it out last night. With an 8mm eyepiece we noticed that when an out of focus star is in the middle of the field the rings are perfectly concentric on one side of focus but slightly off center (slightly below center) on the other side.There is no observable image shift and both sides of focus were centered. The rings are round on both sides. Not oval. To us it's odd that one side would be okay but the other is slightly off instead of both being off center. The in focus image of a double star is perfect. Iota Cass was split perfectly and the "E" star in the Trapezium was easily seen so the alignment is apparently okay. This is not a serious problem but we are wondering why it looks like this. Any Ideas? Thanks

Gerry


Edited by gfeulner, 08 December 2018 - 11:15 PM.


#2 Eddgie

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Posted 08 December 2018 - 05:00 PM

Tilt or centering (which in the end shows in the same way as tilt).

 

On a very bright star, defocus a bit to see if the Possion Point is exactly centered in the secondary shadow.  If it is not centered on both sides of focus, possible that there is some centering issue between the secondary and primary.

 

Poisson Point.jpg

 

 Of course you should check collimatoin.


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#3 Asbytec

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Posted 08 December 2018 - 06:29 PM

...in the middle of the field the rings are perfectly concentric on one side of focus but slightly off center (slightly below center) on the other side.


I've seen and fixed this. In my experience it's caused by a slight mechanical misalignment of the focuser/baffle movement with respect to the optical axis.

In my case, it was well collimated on the side of focus I used for collimation and off on the other side. As I recall, I finished collimation in focus, then checked it and saw it was slightly off inside and outside focus in opposite directions.

I ran across this when I pulled the mirror cell off for cleaning and reinstalled it incorrectly rotated 90 degrees from the factory setting. The phenomenon disappeared when I reinstalled the mirror cell back to its original position and the primary moved more true to the focuser axis.

My scope has very minimal focus shift. I do not know if this is caused by the primary baffle (which the primary rides during focus) not pointing directly at the secondary or not, or why this happened at all. But above has been my experience with the problem you describe.

#4 gfeulner

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Posted 08 December 2018 - 09:48 PM

Thanks for the replies. They were helpful.

 Gerry



#5 Bill Barlow

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Posted 08 December 2018 - 09:58 PM

It is not unusual for the defocus patterns to be slightly different in a Mak.  It is the in-focus image that matters most.  Sounds like your friends 127 Mak is collimated just fine.  My Intes MK66 also shows slightly different diffraction patterns, but they are very close to being identical.  Just a very small amount of de-focusing on a star will show you the "break-out" diffraction ring.  If it is evenly bright around the airy disc, your collimation is very good.

 

Bill




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