ATM Re: Measurement and elimination of PEC with sync motor drives

["Tom & Lou Krajci" <krajcit@3lefties.com>]

>From:    Donn Starkey <starkey@FWI.COM>

>...Now my problem is PEC....

>Tonight, I am taking a series of CCD images of a star over several 30
.minute periods [one exposure every minute] and will measure the movement
>of the star by counting the pixels that it displaces from frame to
>frame.  I can convert the displacement to arcseconds.  The next thing is
>to plot the PEC.

Depending on how smooth, or sudden your periodic error is, I'd recommend CCD
exposures every 5 or ten seconds instead of once a minute.

>I am driving the RA motor with an AccuTrack Model 4000 drive corrector
>that has a thumb wheel system for changing the speeds by changing the
>drive frequency.  I can manually change the frequency from 45 to 75 Hz
>in 10000 steps

It sounds like you can adjust the overall driving rate, but it would be
fancy to integrate PEC commands with the thumbwheel input.  You may have to
use a different drive corrector for good PEC training/use.

Too bad you can't offset your mount's polar axis to the east or west about
five degrees, and take CCD exposures of stars on the equator/meridian.  If
the drive rate was perfectly constant you'd see a steady declination drift.
As the imperfect drive slows and speeds, you'll get a periodic wave shape
for star trails instead.  The smaller the amplitude, the smoother the shape,
the more uniform the shape from worm revolution to revolution...the better.
Bad drives have sudden moves of arcseconds in a matter of seconds that give
PEC training and autoguiders hard troubles.  This method of drive analysis
is a simple way of "plotting many data points" for drive rate variations.

Good luck!

Tom Krajci