Re: ATM A Cage as Secondary Holder

[Mel Bartels <mbartels@efn.org>]

Microstepping which involves current control to the windings is quite
successful in combating vibration.  Really, the vibration comes from the
motor's rotor slap to the next halfstep or wholestep.  This sends a
pulse through the mount and tube assembly.  If the motor's repeated
slapping as the scope moves matches a resonant frequency of the scope,
then the pulse translated to mechanical motion is amplified, and it
shows up at the eyepiece.  Also, when the rotor slaps to position, it
also enters a phase of decaying oscillation about the rotor.  This
frequency is generally too high to be picked up, but conceivably could
be in certain very stiff and short telescopes.  Microstepping at the
subarcsecond level introduces a higher frequency of much smaller pulses
that the tube assembly easily absorbs.  If there is an average in
amateur scopes, is that they have a characteristic resonant frequency of
several hertz (cycles per second).  Rap the side of the tube while
watching at very high power.  You will typically see several short
vibrations that die out in a second or so...  Microstepping tracking
rates are typically much higher than this, thus attenuated. I can attest
to perfectly smooth tracking at 6,000x in a 20"...

--
Clear skies, Mel Bartels // Programmer/Analyst, amateur astronomer
Springfield, Oregon,     // USA homepage: http://www.efn.org/~mbartels
mailto:mbartels@efn.org  // atm, atm-digest list-owner
Motorize A Dob: http://zebu.uoregon.edu/~mbartels/altaz/altaz.html