Gday Freddy
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Hang on............ in reality, its an end shaft encoder, what pro observatories use, nothing magic about it at all. It elimates PE because its after the gearbox/worm gear etc..
Put aside the advertising and take it for what it is, an end shaft encoder, and its a very good price indeed for the reported res. Almost too good in fact, ive seen shaft encoders at this res for over $7K, just for the encoder.
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This device is also being discussed on the LX200GPS Yahoo forum
Someone found the specs for the device as
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"Heidenhain ERN-480" encoder with a 1/8th arcsecond resolution
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The specs for the ERN-480 models show only up to 6000 lines
( which in quadrature mode gives about 54arcsec granularity )
As such, something else is going on, probably a precision geartrain between the RA axle and the encoder.
This may have PE in it, but that could be accounted for easily in the "black box" ( dunno if it is )
Also, unless the device takeoff is perfectly centred, you would still get slight tracking rate errors due to eccentricity
( assuming we are talking of 1/8 arcsec precision )
Soooo, it looks like a good device to remove the bulk of the geartrain errors, but guiding is probably still required.
Andrew