[APML] Re: Declination Drive problem

[Matt BenDaniel <matt@starmatt.com>]

The ST-4 doesn't have built functionality that allows you to easily tell
it to correct DEC in only one direction. It can help to use Average
Adjust, which reduces chasing of seeing (is that available on the STV
yet?). Sometimes I calibrate at 0.5x and guide at 0.25x (on a 600E QMD).
That is like reducing the aggressiveness on both RA/DEC with an STV. I
set the mount's electronic DEC backlash compensation pretty low. If it's
too high (even a smidge) with an autoguider it can thrash in a positive
feedback loop, which is true on just about any mount that has that
feature.

I think you can "fool" the ST-4 by changing the guide rate on the mount
during calibration at the right moments, but I haven't tried it.

Matt
--
> From Chris1011@aol.com on Fri, 18 Jan 2002 11:22:58 EST

Are you trying to guide manually or with a CCD guider? If you are
guiding manually, the backlash should be no problem because you would
not ever want to reverse the dec direction anyway. When guiding, the
idea is to give short pulses to the dec button to gently nudge the star
toward the crosshair, always in one direction, since dec drift will
always be in one direction. Don't hold your finger on the button and try
to remove it when the star has reached the crosshair. You will always
overshoot and have to reverse. People's hand-eye delay times can be on
the order of .2 to .5 seconds. You want to avoid reversing at all times.

If you are using an autoguider, there are ways to compensate for dec
backlash during calibration, but then during actual guiding there is
again no need for the dec axis to be able to reverse instantly because
dec drift is always in one direction. Just make sure that your
autoguider does not overcompensate for guide star error. Rather, set it
up to deliberately undercompensate so that the software will continually
try to nudge the guide star towards the center point and not constantly
oscillate back and forth.

The key to making your mount work is to understand fully how guiding is
done, and to set the guide loop up accordingly. In the case of
autoguiding, the loop consists of CCD chip, software and mount
electronics. In the case of manual guiding the loop consists of
eyeball/crosshair, finger/button and mount electronics. In either case,
dec backlash will have almost no effect on the outcome if you know how
to sidestep it. In fact, some people who monkey around with their mounts
to tighten up and compensate for backlash end up making the guiding
worse. There's nothing worse than having the backlash comp set just
right for instant reversal, and then finding this setting is too much
for a slightly different mount position, and the star jumps back and
forth. Best to undercompensate and leave some delay in.

Roland Christen


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