RE: Major Mark IV Problem

[Tom Droege <droege@wwa.com>]

Mike and all,

>Or is the mount flexible enough to allow this kind of vibration away from
>the motor?

That is the problem.  We are looking at a 3/4" steel axel with about 40# of
weight on the end of it.  Possibly an 8" moment of inertia.  It has a very
high Q.  Once you start it vibrating, it wants to keep going.  I can whack
it and see it moving 10s of seconds later.  I will have to see what I can
build in the way of a "dash pot" to damp it.  The problem is that it is
only moving a few mills, and it is hard to attach a dash pot in a way that
does not have some backlash.  The design is really much more complicated.
I will try to describe it tomorrow when I don't have the distraction of
trying to take data.  

The other problem is that the natural period of vibration is within an
order of magnitude of the step rate.  You may recall that I changed the
step rate by getting a motor with a different gear ratio, and it helped the
problem.  

I found a way to make it somewhat better.  I will put up another star
tomorrow.  

Tom Droege

At 04:54 PM 3/16/99 -0500, you wrote:
>I too see the double peak but the dip is close to the noise level (I get 125
>ADU's sigma noise.)
>
>What bothers me is that the separation of the peaks is about 5 pixels which,
>if I remember right, is about 20 steps of the RA motor.  Can the vibration
>be this large and not affect the repeatability or stability of the motor?
>Or is the mount flexible enough to allow this kind of vibration away from
>the motor?
>
>Mike G.
>
>
>		-----Original Message-----
>		From:	Chris Albertson
>[mailto:chris@topdog.pas1.logicon.com]
>		Sent:	Tuesday, March 16, 1999 4:40 PM
>		To:	mgutzwiller@lanvision.com
>		Cc:	tass@wwa.com
>		Subject:	Re: Major Mark IV Problem
>
>		mgutzwiller@lanvision.com wrote:
>		> 
>		> Is it possible that the problem is not vibration but a
>miscalibrated drive
>		> rate?  It would be easy to tell the difference.  If the
>star streaks get
>		> longer with longer exposure it's a drive rate problem.  If
>they are constant
>		> for differing exposure lengths then its most likely a
>vibration problem.
>		>
>
>		Mike,
>
>		Look at Tom's sample star image.  I see a double peak.  It
>could be
>		caused by sinusoidal oscillation where the dwell times would
>be longer
>		at the ends then at the center.  My "good" software is at
>home so I'll
>		look in more detail later but by eye I see two peaks that
>each look
>		like streaks them selves.  The streaks are diagonal so maybe
>there is
>		some error in the tracking rate and some polar misalignment
>so they
>		_would_ get longer over time but if the double peak is real
>I think
>		it proves there is vibration also.
>
>		 
>		--Chris Albertson
>
>		  chris@topdog.logicon.com                Voice:
>626-351-0089  X127
>		  Logicon RDA, Pasadena California          Fax:
>626-351-0699
>
>
>