Major Mark IV Problem

[Tom Droege <droege@wwa.com>]

Well, the Mark IV vibrates in RA due to the pulsed drive of the stepping
motor.  I have done what I can with the magic gunk, and there is still too
much vibration.  I see two ways to proceed.

1)  Work on it now until I fix it.

2)  Get the Mark IVs out into the field taking data.  This will get a start
on all the software.  Count on being able to engineer a fix.

I lean towards 2).  The system is plenty good to take engineering data.
Possibly even some real measurements.  For a typical star see the attached
IStar.FTS.  In this image, North is to the left and West is up.  The N-S is
smeared out because I do not yet have a good north alignment.  E-W (Up-Down
in the image) is smeared out from the vibration.  I suspect in the summer
it will be better when the rubber is warm.  I am tempted to put a heater on
the rubber coupling to see what it does.  

My present plan is to switch to a synchronous motor for the RA drive.  This
would get away from the 8/1 rewind to forward speed problem as I would
expect to be able to drive a synchronous motor faster.  This will take an
additional electronics card and testing, and ... and would take a few
months.  

So should I continue the production line, get the Mark IVs into the field,
and then work on a fix, or should I stop now and work till it is fixed?

I would particularly like to hear from all who are due to get a system.

Meanwhile, I will spend a few days working on other schemes.  Giant dash
pots come to mind.  Something to take energy out of the system.  It will be
hard as there is a big mass on the RA axis and it is only moving of order
0.004" peak to peak.  Sigh!

Tom Droege