Re: Adjust CP4 Clock Frequency

Christopher Erickson

Craig, where are you observing that seeing allows you to keep a 1 arcsecond accuracy on a single pixel for several hours?

Here on Mauna Kea we frequently get sub-arcsecond seeing but we can't even keep the mighty Kecks to that standard of precision for hours at a time. The seeing, even at the summit of Mauna Kea, just simply doesn't allow it.

I frequently (with my personal AP mounts) do photometric and millisecond GPS-timing measurements of stellar occultations and using nearby stars for photometric calibration of each FITS frame. My AP mounts don't have abs encoders and I don't even bother to autoguide.

What kind of OTA are you using for your measurements? I see lots of gravitational sag in my cheaper APO refractors and compound OTA's. Especially in a 20" RCOS truss OTA that I use. At least the sag is predictable and is directly correlated to the OTA's orientation to gravity.

-Christopher Erickson
Observatory engineer
Waikoloa, HI 96738

On Wed, May 6, 2020, 11:32 AM Craig Young <craig.young.m8@...> wrote:
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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