Re: AP900GTO - DEC stiffness

Roland Christen

In a message dated 3/22/2010 10:36:28 AM Central Daylight Time,
scancelli@... writes:

Another imaging session last night and still the occasional ping pong
dithering guider move between exposures. Once it settles however, it
make dec correction happily for long exposures without oscillation.
Just one more thought about this oscillation. In almost all cases,
oscillation is the result of improper calibration, which results in loop gain larger
than 100%. During calibration procedure, the mount is asked to move in RA
and Dec for a certain number of seconds so that the guiding software can
figure out how many seconds it takes to move a certain number of pixels (Pixels
per second). Because of inevitable delay in reversing Dec (RA does not have
this delay), there is always an undercount of the time it takes to move a
certain distance. Normally that delay is on the order of 0.5 seconds, but if
the geartrain in Dec is not fully meshed, it could be as long as 2 seconds at
1x, 4 seconds at 0.5x and 8 seconds at 0.25x! So you can see that if you
try to calibrate at 0.5x or even at 0.25x, you will undercount the time.

Now on top of that, if you calibrate for only 5 seconds, you will never get
any movement in the reverse direction in Dec, and the resulting parameter
numbers in your guiding software will be way off (too low by an order of

So, to make matters really bad, if you do 3 things wrong: use less than 1x
guide rate to calibrate and guide, have some significant de-mesh in the worm
gears, and calibrate for less than 10 seconds - then you will indeed end up
with bad parameter numbers and will likely see oscillations during guiding.

Therefore, you must do the opposite: use 1x guide rate to calibrate, make
sure you have good worm mesh, calibrate for at least 10 seconds time period.

Some other himts: if you are using MaximDL, make sure you are always using
Dec value = 0 - do not use any other Dec value to modify the guiding. Do
your calibration once, near the celestial equator and use that value everywhere
in the sky - don't worry, it will work perfectly even near the poles. Also,
please check your parameter numbers after calibration to make sure they
make sense. What is a parameter number? That is the number of pixels per second
in both RA and Dec that the guide program has calculated from your
calibration routine. These numbers should be nearly equal or at least close in value
for both RA and Dec when you calibrate near the celestial equator. If they
are quite different (by more than 10%) then that is a very good indication
that you have a bad calibration result.

An example of parameter numbers: if your pixel scale is 1 arc sec per
pixel, then the proper parmater numbers that shoudl show up each time you
calibrate will be 15 in RA and 15 in Dec. 15 is the number of arc seconds per
second that the mount moves at 1x sidereal rate. If your resultant parameter
numbers are 15 in RA and 7 in Dec, then you have a bad calibration (excessive
backlas due to poor mesh, or not enough cal time, etc). Bad calibration will
result in loop gain larger than 100%, which will result in oscillation or
hunting, as the mount is consistently given too large a correction signal for
the star's position error, and the guide star will seem to bobble back and

Good luck and good hunting, er, guiding.


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