Peterson Engineering Corp. Sky Division
The EZ Focus
Kit, and how it affects 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.
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. Your SCTís 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 does not address this secondary backlash source directly.
SCT manufacturers provide a heavy grease on the outside of the baffle tube to reduce the amount of mirror "wiggle" or "shift" that occurs when focusing. This heavy grease squeezes into the gap between the inside of the mirror assembly and the outside of the baffle tube. To some extent, this grease prevents the mirror from shifting.
A lack of grease on the baffle tube is sometimes a source of major mirror shift. If this is your problem, redistributing the grease on the baffle tube and working the grease into the gap between the mirror assembly and the baffle tube will significantly reduce your mirror shift. However, if the gap is extreme or if there isn't enough grease, this "fix" will not work.
The EZ Focus Kit instructions do provide a simple and effective procedure for redistributing baffle tube grease. And the grease provided in the kit is the same grease used on the baffle tube. Removing the corrector plate and regreasing the baffle tube is beyond the scope of the kit procedures, but at least you've got the grease to do the job.
The third source of focus backlash if 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.
Someone in Yahoo's LX200GPS user group differentiated between mirror shift and mirror flop. It's a fine distinction but an important one for us 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. In the Meade LX200GPS, using the mirror lock prevents mirror flop.
Due to the basic design of the Meade and Celestron SCTs, there will always be some image shift during manual focusing. The question is, how much image shift is acceptable? I've heard of extreme cases where images moved completely out of the field of view in a 26mm Plossel eyepiece, or where it took 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.
My personal experience has with a Meade 10" purchased in 1999 and my current 12" GPS purchased this year has been barely noticeable image shift in the supplied 26mm Plossel, and no more than 1/10 of the field of view in a 9mm Plossel. Yet I've used an 8" LX200 where the image shifted 1/2 way out of the FOV in a 26mm Plossel.
Personally, I rarely use the motorized focus, as the EZ Focus kit permits my 12" 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.
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