Last night I measured the exact frequencies of the RA drive on my new-model
(3.34L) 8" LX200. The results were somewhat surprising.

My technique was to use an HP universal counter to measure the frequency of
the encoder pulses on the middle test point on the RA drive board. I used a
stopwatch and counted the pulses over a period of exactly 8 minutes (give or
take maybe 0.1 second).

First the good news: Assuming the drive covers 8 minutes of RA in 5400 (=
21600/4) pulses, the sidereal rate is *very* accurate. Calculated rate,
5415 pulses; measured rate, 5415 plus or minus one.

Also, pressing "E" stops the RA drive, and pressing "W" causes it to track
at 2x sidereal speed.

Now the surprise: The "frequency" numbers (58.0, 60.0, etc.) are not
proportional to drive rate! Apparently at some early stage the LX200's
designers wanted to make them equal to the frequencies that would be
supplied to a 60-Hz motor (e.g., on the early Meade LX3), but they abandoned
that idea or drifted away from it.

The actual relation is:

Drive rate relative to solar = 0.4952 + 0.008444 * Displayed Frequency

bearing in mind that I'm using more significant digits here than are
actually warranted.

Accordingly...

Solar rate is 59.8, not 60.0 as indicated in Meade's documents.

Topocentric lunar rate (correcting for parallax) is 57.2, not 57.9 as in
Meade's documents.
(Ignoring parallax it's even lower.)

60.1 Q and 60.1 M are the same rate, which is sidereal and very accurate.

It might be interesting to have someone replicate this with an earlier-model
LX200 and see if the design changed.



Michael A. Covington / AI Center / The University of Georgia
http://www.ai.uga.edu/~mc http://www.CovingtonInnovations.com <><

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