Re: Adjust CP4 Clock Frequency


Ray Gralak
 

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.
Or... the more likely reason, is your scope has unmodeled complex flexure. This can easily cause apparent tracking errors. Changes in temperature of just a few degrees C can change the Young modulus of materials in the scope enough to cause apparent tracking rate changes at the pixel level.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): https://www.astro-physics.com/apcc-pro
Author of PEMPro V3: https://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: https://www.siriusimaging.com/apdriver


-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Craig Young
Sent: Tuesday, May 5, 2020 2:24 PM
To: main@ap-gto.groups.io
Subject: [ap-gto] Adjust CP4 Clock Frequency

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.

Craig

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