Hi, I wanted to ask a question but more of a curiosity....
I noticed that Losmandy mounts up to about 2005/2008 had one of the two screws with two washers, which in current mounts for example are no longer present i.e. there is only one for each screw. What was the purpose of the two washers, just to identify it from the top adjusting one or as a special shim and if so, what were they used for over the years?
?I enclose a photo of a not recent Losmandy G11 taken from the Internet for better understanding.

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Thank you for your reply, Paul.? Also, I appreciate the link to Mark Crossley's webpage.

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For the "non-integer gear reduction" I referred to, I meant the non-integer reductions between the individual gears, as Mark Crossley mentions in his webpage. As I understand it, the MI-250 was built with Losmandy motors and gear boxes.? I know my MI-250 has a current Losmandy motor for RA, and the gear boxes look very Losmandy-like.

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I've read various accounts of misaligned worm blocks in Losmandy mounts built prior to the the OPW (my G11 is from 1999 and has separate worm blocks).? The worm bearing housing in my MI-250 is "in between" the original Losmandy design and the OPW design: the in-board and out-board beaning housings are bolted to a common piece of stock.? Variation in alignment of the worm bearings when everything is disassembled and reassembled should be minimal.? However, there is the possibility the three pieces aren't machined exactly square and parallel, thus introducing misalignment and the "pinch" Mark Crossley refers to.? Perhaps another similarity between the G11 and MI-250.? I understand your comment about PEC not being able to correct for the "76 second error".

I also remember all the discussions about the "76 second error", that was long before I was interested in photography and if my G11 has it, it didn't bother me at the eye piece.

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Thank you for confirming the "2x worm cycle" is a feature in Gemini II.??

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I understand that worm phase shifting between sessions is a separate issue from gear boxes, worm bearing block alignment and linear drift.

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Since you bring up linear drift: I've seen a few posts about that.? None of the posts I've read mentioned if the drift manifested itself over several sub-frames or during a single frame; and in either case, how long it took for the drift to become noticeable.? Does the linear drift manifest itself as oblong stars, or does it take long enough it manifests itself as the target object slowly drifting off-center?? Something else?

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The reason you have a PEC sensor and robust software that looks across multiple periods is so you don't have to have an exact integer (as opposed to rational) number in a gearbox reduction.

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Not to be too pedantic about it but I think you're both misusing the term "integer".

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You can easily get a non-integer ratio: Take a 10 tooth gear and mesh it with a 7 tooth one. That produces a 10/7=1+3/7 = 1.428571 and so on ratio.

10/7 is a rational number, not an integer.

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In this hypothetical example an algorithm that looks across 7 periods would pick up the subharmonic caused by the remainder of 3.

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A 10 and 5 combo would produce an integer ratio: 10/5 = 2

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And yes, proper alignment of the two-part worm blocks in older mounts removes a lot of the problem, which was not caused by gearing but by misalignment of the two separate worm blocks working against the balls in the bearings of the blocks. I, like others before me, clamp a piece of known flat angle iron to the blocks so they form a temporary one-block mounting while doing worm adjustments. After tightening the two blocks (but not overtightened - the metal is thin at the edges) in place remove the angle iron.

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Best regards,

Mark Christensen