Focus Backlash, Mirror Shift and Mirror Flop

Focus backlash is defined as the excess play in the focus mechanism. The EZ Focus Kit will eliminate most of your focus backlash.

Let’s say you are focusing your scope at high magnification, and it’s almost in focus. You need to go through focus at least once and defocus a bit on the other side to really know where the best focus position is. So on purpose, or by accident, you go past the position of best focus. Now you have to turn the knob just a little bit in the opposite direction to get back to the best focus position. You don’t have far to go, so you slowly turn the knob. And turn the knob. And nothing happens. THIS IS WHAT WE DEFINE AS FOCUSER BACKLASH. You turn some more, maybe a bit faster now because you want something to happen. And then BAM! The mirror starts to move and before you realize it you’re through focus again, and back where you started. THIS IS WHAT’S BAD ABOUT FOCUSER BACKLASH.

Meade has used the same focuser mechanism on their large SCTs for more than 30 years. The newer Meade LX600 and LX800 scopes do not use this traditional mechanism.

With traditional SCT designs there are 3 primary causes of focuser backlash:

Typically 90% of your backlash is associated with play in the focuser mechanism itself. And this is the problem that the EZ Focus Kit addresses.
Typically, another 8% of the focuser backlash is due to mirror shift. A traditional SCT primary mirror assembly rides up and down the scope baffle tube to achieve focus. The focus mechanism is attached to a point way out on one edge of the mirror assembly. If the mirror assembly doesn’t fit tightly around the outside of the baffle tube, the mirror will “wiggle” a bit from side to side if you push and pull on one edge (which is what the focuser does). THIS WIGGLING IS WHAT WE CALL MIRROR SHIFT. You will be turning the focuser knob while the mirror is shifting, but since the center of the mirror isn’t moving until the mirror has completely shifted, you will be experiencing focuser backlash during this period of time. The EZ Focus kit addresses this problem indirectly.
There is backlash associated with the fit of the focuser mechanism onto the pin that extends out from the mirror sled. Typically this is very minimal – responsible for 2% or less of the initial focuser backlash.

Now let’s talk about mirror flop, mirror shift, and what amount of image shift is acceptable.

It’s a fine distinction between mirror shift and mirror flop but an important one for SCT users. Let’s formally define this as follows:

Mirror shift is mirror “wiggle” from side to side caused by focuser movement.

Mirror flop is the same movement, only caused by gravitational effects on the mirror as the scope orientation changes. Both cause image shift, or a movement of the image from the center of the field of view out toward the edge. Some Meade SCTs have a mirror lock to prevent mirror flop.

Due to the basic design of traditional Meade and Celestron SCTs there will always be some image shift during manual focusing. The question is, how much image shift is acceptable? In extreme cases images moved completely out of the field of view in a 26mm Plossel eyepiece, or it takes more than 4 complete turns of the focus knob before the mirror finished shifting and started to focus. I’d certainly consider those scopes to be defective. The EZ Focus Kit is intended to reduce focus backlash. The mirror shift component is usually reduced, sometimes significantly. In rare cases where there is a huge spacing between the mirror sled and the baffle tube, not so much.

My personal experience has been barely noticeable image shift with a 26mm eyepiece, and no more than 1/10 of the field of view in a 9mm eyepiece. Yet I’ve used an 8″ LX200 where the image shifted 1/2 way out of the FOV in a 26mm Eyepiece.

Personally, I rarely use my motorized Crayford focus for visual use, as the EZ Focus kit permits scopes to snap into focus with even the highest magnifications. If I had significant image shift, I’d be forced into using it. As to how much mirror shift is acceptable…. I can only suggest that you get what you accept.

Pete Peterson