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Which way does the mirror flop ?

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#1 petert913

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Posted 22 April 2019 - 05:00 PM

In Mak and SCT telescopes we always hear about "mirror flop".  But I've never understood
mechanically which way it flops.

 

Is it :

1.  forward and back along the optical axis
2.  left and right, perpendicular to the optical axis
3.  Yawing - which would be disastrous to collimation
4.  other ?

 

c8hyOtom.png


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#2 Stardust Dave

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Posted 22 April 2019 - 05:12 PM

I think might be mostly 2. and a little bit of 3- is that why they tell us to finish focus CCW?



#3 freestar8n

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Posted 22 April 2019 - 05:29 PM

When you focus ccw the focusing rod pushes on the edge of the mirror - which tilts it a bit toward the baffle tube and locks it there.  So I think that is a bit of 3 or yawing as in your diagram.  Once it is locked there it will stay there unless the focus knob turns clockwise and allows the mirror to float a bit.

 

A slight tilt of the primary is ok and can be compensated for with a corresponding tilt of the secondary during collimation.

 

The only thing bad would be to collimate when the mirror isn't in its locked position - or to randomly move the focuser and allow the mirror to float.

 

So - under normal usage and with ccw focusing against gravity - the mirror shouldn't "flop" at all - even though it mechanically could "flop" if you turned the knob clockwise.

 

I think the play in the mirror has improved over the years, and for my EdgeHD11 the shift on focus with the primary is around 20".  And what matters is how repeatable it is when you pull back focus and then push forward with ccw motion.  In that case the object returns to the same location within a matter of arc-seconds.

 

This is a good case of a fairly simple and low cost focusing system that works remarkably well when combined with good technique.

 

Frank



#4 bobhen

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Posted 22 April 2019 - 06:42 PM

Mirror flop and mirror shift are 2 different things.

 

Mirror shift is cause because the focusing arm is offset and when you focus you are pushing or pulling the mirror from one side and this causes a slight tilt in the mirror that can shift the image.

 

Mirror flop is when the central retaining ring that holds the primary mirror in place is loose so that when the scope is pointed east and then is turned to the west the whole mirror flops slightly causing the scope to lose collimation.

 

Mirror shift can be mitigated/eliminated with an aftermarket focuser.

 

Mirror flop must be sent back to Celestron or Meade to be repaired. Or, if you are so inclined, you can take the scope apart and try and fix it.

 

Bob


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#5 Paradoxdb3

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Posted 22 April 2019 - 06:49 PM

I'm assuming mirror flop isn't an issue with a Mak, since the primary mirror can be tightened via the collimation screws?

#6 Stargazer3236

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Posted 22 April 2019 - 09:45 PM

Mirror flop is not prevalent in a Mak. Mostly, maks use an outside crayford focuser, so the mirror is not moved.



#7 freestar8n

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Posted 22 April 2019 - 10:00 PM

My Meade mak7 had very large flop as the focuser went back and forth. That’s what I mean by flop anyway. If you don’t load the mirror against gravity then it will tilt and flop as you cross the meridian. But if you load the mirror with final focus pushing up the effect is greatly reduced.

Frank

#8 choward94002

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Posted 22 April 2019 - 10:39 PM

In Mak and SCT telescopes we always hear about "mirror flop".  But I've never understood
mechanically which way it flops.

 

Is it :

1.  forward and back along the optical axis
2.  left and right, perpendicular to the optical axis
3.  Yawing - which would be disastrous to collimation
4.  other ?

 

c8hyOtom.png

Not exactly ... as Frank mentioned, there is mirror "shift" and mirror "flop" ... mirror "shift" is when the focuser arm loads and unloads against the mirror, and he described it pretty well ... that's item 1), movement along the optical axis ...

 

Mirror "flop" is item 3), a yawing of the mirror, and it's better understood if you think of what the mirror actually mechanically is; it's a tube with a mirror attached that rides inside of another tube of a slightly smaller diameter with two directions of yaw movement essentially constrained by a mechanical connection on one side of the inner tube anchored to the outer tube.  Note I said "two directions of travel", not "all directions of travel" ...

 

So, suppose that I have my two tubes sitting vertically, one nestled inside of another.  The outer tube, as it's slightly larger than the inner one, is free to yaw back and forth N, S, E and W and any direction in between.  Now I have my focusing arm, let's suppose on the N side of the tubes, that mechanically couples the two tubes at that point ... that is going to constrain the N and S direction of the tube yaw  movement, but does *not* constrain the E or W yaw movement ... that's "mirror flop".  Even though I have my focuser properly loaded against the mirror (preventing mirror "shift"), even though my N and S yaw movements are constrained by the focuser arm the outer tube is still free to yaw in the E and W directions ... which typically happens when the mirror starts loading to one side, goes vertical and then loads on the other side ... it "flops" ...

 

The effect is more visible based on the size of the mirror (sheer weight), the age of the OTA (the grease will migrate over time, giving dry spots), the time of construction (Celestron had some good years and some bad years) and the condition of the baffle locking collar (it loosens sometimes) and the effect can range from barely noticeable to knocking collimation completely off ... you'll find threads here describing both ...

 

Some people will use mirror locks to attempt to constrain the E and W yaw movements, which works reasonably well ... I will typically collimate with the OTA loaded in one cardinal direction (say "E") then mark the OTA with some blue tape for that direction and am careful to always keep the OTA in that orientation (so, no meridian flips) so I don't have a "flop" event ...


Edited by choward94002, 22 April 2019 - 10:43 PM.


#9 luxo II

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Posted 23 April 2019 - 03:40 AM

It's 3 - yawing - as this is th only movement that can cause the image to shift laterally.



#10 rmollise

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Posted 23 April 2019 - 09:14 AM

In Mak and SCT telescopes we always hear about "mirror flop".  But I've never understood
mechanically which way it flops.

 

Is it :

1.  forward and back along the optical axis
2.  left and right, perpendicular to the optical axis
3.  Yawing - which would be disastrous to collimation
4.  other ?

 

c8hyOtom.png

 

If you're talking about flop, it will depend on the attitude of the scope. If you're talking about focus shift, it will depend on whether the mirror is being pushed or pulled. ;)



#11 Cpk133

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Posted 23 April 2019 - 06:44 PM

No need to over complicate.  Find a bagel and cut a hole in it.  Insert finger and point at different parts of the sky.  Watch bagel relative to finger.  Eat bagel.


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#12 AxelB

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Posted 24 April 2019 - 06:43 AM

In order to avoid the bagel effect, one would need to manufacture the baggle (and your finger) to extremely tight tolerance. Normal variation around the target specifications will produce some almost perfect samples and some "floppy" ones.

#13 RAKing

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Posted 24 April 2019 - 09:09 AM

The mirror always flops down - toward the earth.

 

Since mirror flop occurs when the GEM flips over the meridian to continue tracking from east to west, the mirror will be affected by gravity when the RA axis rotates.

 

How much flop is determined by how tight the mirror is fixed on the baffle and the angle of the Dec axis.  It is usually worse when the scope is pointed low to the south (basically a side to side flip of the axis).

 

My .02,

 

Ron



#14 WadeH237

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Posted 24 April 2019 - 11:56 PM

The mirror always flops down - toward the earth.

Well, sort of.

 

To use the bagel analogy, this would be true of just the bagel on your finger.  But that's not quite the scenario that we have.  The focus screw fixes the distance of the mirror cell right at the spot where the focus knob is.  Assuming that the mirror is properly attached to the cell, that part of the mirror cannot move forward or backward (but it can potentially move laterally) on the baffle tube, so that restricts how the mirror can flop around.


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#15 MortonH

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Posted 29 April 2019 - 02:58 AM

Mirror flop is not prevalent in a Mak. Mostly, maks use an outside crayford focuser, so the mirror is not moved.

Most Maks do not have an external focuser, e.g. the Skywatcher line.




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