ATM Eq Platforms

[cshaw@ghgcorp.com (Chuck Shaw)]

Ray,

The decision to use a stepper or an AC motor (or even a DC Motor with a
simple DC to DC converter) hinges on how accurate you want to have the drive
system.  More to the point, how much trouble do you want to go to to achieve
accurate tracking.

If you can find an AC synchronus motor that turns at the right speed (or you
can find gears for it to get the final output the right speed) then this is
clearly the simplest way to go.  It also means you are slave to 110v power
(which may or may not be a problem for you)

A DC Motor, with its voltage stabilized by a simple DC regulator (VERY easy
to make from a Radio Shack variable voltage regulator that only costs a
couple of bucks), is the next simplest drive.  It will give surprising
accuracy too!!!  I drove my platforms with this for years and even did
photography with it to boot!  Mechanical "slop" in the drive linkage creates
more errors that do motor speed errors in general.  There are LOTs of small
DC motor/gearbox combos out there too......

I used a quick removeable AC synchro motor when I observed at home (or
anywhere near 110v), and a DC motor with a regulated DC voltage input when I
was away from 110v....Thats another way to have your cake and eat it too!

Steppers are a much more elegant solution.  You get very accurate speed, and
DC power to boot!  The circuits are really pretty simple with the advent of
driver chips like the Mototola SAA1042 and others that have everything but
the timing circuit in them.  And the timing circuit is dirt simple to make
too (a 50 cent 555 chip, and a resistor and capacitor and a pot and you have
the basic circuit......)

Mel's Alt/Az system and others like it are the ultimate answer for tracking,
and I am in the process of building a system based on Mel's design now.
Platforms are pretty simple to build, give surprisingly accurate tracking,
but suffer from that bane of all portable systems: the need for polar
alignment.....   Very casual alignment is more than sufficient for visual
work, with more precise alignment for better tracking, and very precise
alignment for photography (especially for ccd imaging).    The time I spend
getting a VERY accurate polar alignment prior to ccd imaging has started to
bother me (1 minute unguided ccd images are my polar alignment
standard)....so thats the motivation to try an Alt/Az/Field rotation
system....  to try and cut down the time spent polar aligning to get the
accuracy needed for 1-2 minute unguided ccd work.  If it were not for that,
the platform approach would be all I would ever need...

So, think about your accuracy requirements, and then your power
availability, and let those things lead you to the right system for you!!

Chuck


Chuck Shaw
cshaw@ghgcorp.com  (Home e-mail)
cshaw@gp801.jsc.nasa.gov (Work e-mail)
http://www.ghgcorp.com/cshaw (feeble 1st attempt at a homepage)