Adjust CP4 Clock Frequency

Craig Young

Rolando mentioned in a recent posting (#70144) the tracking rate is dependent on the controller clock frequency.  Over the years of using my AP1600 with encoders I could never get APCC to provide the same tracking accuracy as other users, even with a 200 point model, so it had to be a problem with this particular mount.  Since this has never be resolved after several hours working with AP I wrote my own tracking program ATRACK which does provide very accurate tracking over several hours.

With ATRACK I know exactly what the RA and DEC tracking rates are because the target drifts no more than 1 or 2 pixels over 4 hours, without the use of an autoguider.  So comparing the 200 point model tracking rate with ATRACK I noticed a consistent error, in both direction and magnitude, which led me to the conclusion there is some base tracking error in the mount that APCC does not correct for.  One possibility would be the crystal frequency used for timing.  This is a known problem in radios which is why they revert to TCXO devices (Temperature Compensated Crystal Oscillators).  Also, on one of my other mounts, there is a setting to adjust the master clock frequency.  I know the basic tracking of the mount is excellent because ATRACK reports no sudden jumps (e.g., cable twists or mirror shift, etc).  And, if there is a clock frequency error then the encoders will provide excellent pointing (which they do) but accurate tracking would not work.  A 200 sample pointing model can provide excellent pointing and tracking but the tracking makes an assumption the sidereal tracking rate of the mount is perfect.  If the sidereal rate is too fast, like on my mount, then the model will not know this and accurate tracking is impossible no matter how big my model sample count or polar alignment.

To test this I ask the guys at Astro-Physics if there is a way to adjust the base clock frequency of the CP4 using APCC.  I made some quick measurements comparing ATRACK rates with APCC rates and found a discrepancy of 0.1% (0.001).  Since APCC is ALWAYS running fast I would then reduce the clock rate by 0.1% and see if it solves the problem.  The adjustment should reduce the tracking rate to match ATRACK.


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