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WHAT IS MIRROR SHIFT

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#1 smiles233

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Posted 22 November 2020 - 11:34 AM

i have read about mirror shift, what is it?



#2 elwaine

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Posted 22 November 2020 - 11:57 AM

Perhaps a better term for mirror shift is image shift. The image, say the planet Jupiter, moves when the telescope is focused. The shift may be so minimal as not to be noticed... or if noticed, not to be bothersome. But sometimes the shift is so pronounced that your target moves out of the field of view as you attempt to improve focus. The reason the image shifts is that the primary mirror is pushed or pulled to one side. That tilts the mirror very slightly. The tilt throws the object you were viewing from the center of your field of view to the side, out of your field of view. If you look into a hand held mirror and tilt the mirror to one side, you will experience mirror, or image shift.

 

Reflecting telescopes that come to focus by moving the primary mirror back and forwards (like SCTs and Maks) can exhibit the phenomenon of mirror shift. It depends on the mechanical quality of the telescope. Not all telescopes that focus via moving the primary mirror exhibit the shift. 

 

Another term you will encounter is mirror flop. That can happen when the telescope crosses the meridian. A heavy primary mirror that is not stabilized in some manner, will, at some point in crossing the meridian, quickly tilt in the direction of the crossing... throwing your target out of the field of view. 


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#3 bobzeq25

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Posted 22 November 2020 - 11:58 AM

In some telescopes (like most simple Celestron SCTs, the scope focuses by moving the mirror up and back.  The mechanism is not perfect, and the mirror can shift a bit side to side.

 

This is no problem for visual astronomy, which is basically what those scopes were designed for.  Inexpensive powerful scopes for looking through, every aspect of the design was aimed at economy, like that focuser.

 

It's a real problem in DSO astophotography.  The camera is _much_ more sensitive to that shift than your eyes.

 

The thing is, such scopes are terrible to try to start out in DSO astrophotography anyway.  You want short focal length, light, and optically fast.  They're long focal length, heavy and slow.  Starting out with such scopes in DSO AP has been a source of unhappiness for many.  I collect quotes like these, to better advise beginners, have many like these.  Note that the Edge is a somewhat more expensive SCT, designed to reduce mirror shift.  It does that well, and it isn't enough to make the scope good as an initial DSO AP scope.

 

"I regret spending the first 6 months trying to learn imaging with an 8" Edge, with that scope it was a losing effort. Fortunately got a nice little refractor, and not only have the quality of my images improved but I'm actually enjoying the process of learning how to do it!"


Edited by bobzeq25, 22 November 2020 - 10:00 PM.

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#4 SeattleScott

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Posted 23 November 2020 - 12:02 AM

I thought it was focus shift/image shift or mirror flop. Both have been described well.

#5 Gastrol

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Posted 23 November 2020 - 01:13 AM

The image shift annoyed me each time I turned direction of the focus knob on my 11” SCT when focusing, I installed a Feathertouch crayford focuser.   Pricey, but well worth the addition.


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#6 Chuck2

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Posted 23 November 2020 - 02:10 AM

The image shift annoyed me each time I turned direction of the focus knob on my 11” SCT when focusing, I installed a Feathertouch crayford focuser.   Pricey, but well worth the addition.

 

Agree, my old Meade 10" LX 200 from the early '90s never had the image shift out of view, but it could not hold focus. It had so much mirror slop it would continually sag and drift out of focus.

 

Way back around 1991, I had the inventor of Feather Touch focusers custom machine one for the scope and never used the Meade factory focus knob again.



#7 Eddgie

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Posted 23 November 2020 - 09:12 AM

i have read about mirror shift, what is it?

The mirror on an SCT(and most MCTs) is a conical mirror that is only supported by the mirror carrier, which is a tube that runs through the center of the mirror.

 

The tube itself slides on the baffle tube at the center of the scope.  This tube has a coating of grease on it so that the mirror will slide smoothly. To allow room for the grease film and for proper movement and  in all temperatures, the baffle is very slightly undersize. 

 

The mirror is only supported by the baffle tube and the focuser rod, which extends from the mirror carrier out through a hole in the rear of the telescope.  Since the carrier tube is larger than the baffle tube by a very small amount, when the scope is pointed to the sky, the weight of the mirror is supported by the focuser screw on one side with no support on the opposite side, so if there is a tiny gap between the baffle and the carrier, the carrier will settle at a slight angle to the baffle.  The angle depends on the gap, the grease film, and the temperature. In most cases the mirror shift is rather small with Jupiter less than its diameter, but in severe cases, the mirror can shift more than this.

 

Please excuse the crude artwork.  I drew this for you so you could see more clearly what happens when the scope is pointed to the sky. 

 

Mirror movement.jpg


Edited by Eddgie, 23 November 2020 - 09:14 AM.

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#8 Peterson Engineering

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Posted 23 November 2020 - 11:22 AM

I answered this same question in another section of the CN forum last week.  Learn about shift, flop and backlash:

https://petersonengi...-ez-focus-work/


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#9 freestar8n

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Posted 23 November 2020 - 05:00 PM

Any focusing system will have some amount of play and shift at some level - but with the right technique it should not limit performance.

 

The diagram Eddgie provides above is a good depiction of the situation, but the key thing to keep in mind is that as long as that geometry holds - there will be no problem during collimation or focusing.  And they way to make sure it is held is always to finish focus with the primary knob while the scope is pointing up and the final turns are counter-clockwise.  The pushing up against gravity will help guarantee that the geometry of that canted angle is constant - and can be compensated for in collimation as long as that final ccw is also used for focusing prior to collimating.

 

When people complain about focus drift and flop and so forth - they usually provide no indication of this important step, yet it makes all the difference.

 

I image with EdgeHD11 and I focus just with a stepper on the primary knob - and I refocus about every 40 minutes with final focus involving ccw turns of the focuser.  It is very reliable, repeatable and accurate - and I have no issues with image shift or flop while imaging.

 

Frank


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#10 smiles233

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Posted 25 November 2020 - 08:23 PM

thanks for all the replies



#11 Daniel Mounsey

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Posted 26 November 2020 - 06:06 AM

Best mechanical  internal focuser design  is the zen mak and its not costly.



#12 Chuck2

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Posted 27 November 2020 - 12:17 AM

When people complain about focus drift and flop and so forth - they usually provide no indication of this important step, yet it makes all the difference.

 

Frank,

 

I have owned 3 Schmidt Cassegrains over the past 40 years and have opened and repaired many more from both Meade and Celestron. I'm very aware of how the Schmidt focusing mechanism functions and it's limitations. Over the years, many user reviews have noted design improvements as well as design limitations. One such reoccurring limitation has been the optical focusing mechanism described above. When precise operating tolerances become too expensive or difficult to achieve in actual production, the focusing techniques you described above become futile. 

 

I worked on a Celestron C14 using artificial stars, lasers and focusing masks to achieve extreme collimation accuracy, but when panning to each horizon, all collimation was lost as the mirror flopped. I owned a factory new 10" Meade that not only had mirror flop, but whether you achieved focus by turning in or out (pushing up or pulling down on the mirror) take your hands away and within a minute the excessive mirror play would cause sag and focus drift. And of course, many Schmidt owners have experienced image shift where you see the object dance to each side of the field of view as you focus in or out. Call it what you want... shift, flop, wiggle, sag, play, backlash or drift, many Schmidt's scopes have it in some form.

 

Instead of fighting inherent design limitations and sloppy build tolerances, many Schmidt users I know have taken a logical next step installing a Crayford style focuser to make observing fun, not tiresome.

 

I've looked through some great Schmidt scopes, as well as some real turkeys. Sounds like your Edge 11 works as well as my excellent 28 year old C5+. As much as Schmidt's are problematic, I still find them to be the  "swiss army knife" of telescopes. waytogo.gif

 


Edited by Chuck2, 27 November 2020 - 12:35 AM.

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