Re: Adjust CP4 Clock Frequency

Roland Christen

Measured tracking rate over 600 seconds time period. Each 100 second segment used a precision lab encoder pulse count to determine the exact rate of axis rotation with respect to time. The result is an arc second error graph spanning 6 x 100 second pulse count segments. In the following 700 seconds I added a 0.1% offset error to the servo's sidereal rate so you can see what the effect is.

When the sidereal rate is chosen to drive the mount, the rate of axis rotation is quite accurate over long time periods, but that does not mean that the telescope optical axis will follow that rotation rate. I can easily get a differential rotation rate between two telescopes on the same mount so that if i guide with one scope, the other scope will not keep a star on center for any period of time. So which scope is following the axis rotation rate? The answer is unknown, because both can be flexing in opposite directions from the mount's axis of rotation. Add to that slight polar misalignment and atmospheric refraction effects and I can get easily errors of 5 arc seconds in a 5 minute tracking run.

If you had done the measurements that I asked for, i could continue with the direction needed to solve this for you. But you choose to keep asking for something that is not possible. If you count the encoder pulses, are you able to use a lab standard for the time interval? Computer timers are not accurate enough for meaningful results. We discovered that long ago when we first started to develop our encoder drives.

Your posts are misleading to other people who watch this user group. They are not accurate to what we actually produce and what our mounts actually do.

Roland Christen

-----Original Message-----
From: Craig Young <craig.young.m8@...>
Sent: Wed, May 6, 2020 4:32 pm
Subject: Re: [ap-gto] Adjust CP4 Clock Frequency

The ATrack design is very similar to DONUTS, introduced in 2013:

For high precision photometry it is very important that the image remain centered on the same pixel over the cadence run of several hours.  Dithering is not used.  Given that there can be random drift corrections during the run, the accumulated error can move the target off a pixel, which is why careful re-centering is required in addition to drift correction.

I will find a more precise method for measuring the actual sidereal tracking rate of the mount instead of using a piece of tape, which is not precise enough to yield the metric needed to correct it.  I still don't see why reporting the pulse count from the encoder is proprietary.  I can see "what you do with the pulses in an algorithm" would be proprietary but I don't see why reporting the pulse count is proprietary.  I will find another way then.


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