Re: ATM Re: Worm Fabrication

[andydtg@phoenix.net]

I've been having an exchange of messages with Chuck Shaw
on the general subject of how to measure and correct rate
and periodic errors in the various forms of telescope drives.
I'm working on a photo quality alt-az mount for Al Kelly's
17.5" and Chuck is starting a Mel Bartels type design 
alt-az mount for his 14". Getting the rates and errors 
right on these type of mounts is critical for imaging
without guiding.


The normal procedure & the one I'm using now is to build the 
drive, assemble the mount, install the telescope, align 
everything & see how well it tracks stars. I've been using
Al Kelly's CB camera to take sequences of short exposure
images at 30 sec intervals, Al tracks & stacks the images
& gets a file of 'move' pixel coordinates which can be
converted to arc seconds & plotted vs time to see whether
the drive drifts or has periodic errors. This works well
*except* that you have to assemble the scope, wait for 
weather, get the data, process it, maybe disassemble the
mount, fix it,reassemble it, etc, you probably get the 
idea that this is *not* a convenient method to debug
drive errors if you're going to build more than 1 or 2
of them.

We considered precision encoders to read the oupuut rate
at various points in the drive system & extract drive
errors this way. This would work but the encoders are
*very* expensive at the accuracies/resolutions needed.
You also need non-cheap test equipment like precision
high speed counters, frequency meters etc.

Here's what we came up with & invite comments/criticisms,
even flames :(

Build a 1 axis platform with a *long* known radius sector
on it & drive it with a *good* leadscrew/tangential thin metal belt with
an accurate crystal controlled stepper, with << 0.1 arc sec steps.
At most it would only have to move about 5-10 degrees. The rate would be
about 10x sidereal. This would produce an accurately known angular rate with 
virtually zero periodic or scale factor errors.

The drive under test would get mounted on this & be run at the same test
rate in the opposite direction, so the two rates cancel. A flat mirror
on the drive would provide optical access to the motion of the test
drive relative to the reference. A *long* efl scope would view the
mirror. A pinhole star or narrow slit would be at the focus of the scope,
get collimated, go out the scope, reflect off the mirror, and return
to the focus. A beamsplitter would allow the video camera to view
the returned image. The scale should be something like 1/8"=1 arc
sec on the video screen. Any deviations due to periodic error etc
would be seen in real time on the monitor, allowing tweaking & fine
tuning. 

If a periodic error is identified at an accessible point in the gear
train (eg a motor/worm coupling), the error could be tuned out mechanically
with a coupling that allowed for both radial and phase adjustment, at least
for smooth sine wave type periodic error. Other error sources could be 
identified & corrected as well.

Andy Saulietis / DTG Alt-Az-Fp Drive Systems
12617 Harriet Ln
Santa Fe, TX 77510 USA
409-925-8854 Voice/Fax