ATM A Cave Telescope

[Bill Abbay <b.abbay@humboldt1.com>]

The occasional reverence expressed for Cave telescopes, or at least
their mirrors, reminds me of an event that happened back around 1970. 
The local California State College physics department found itself with
an overage of funds one fiscal year and, faced with "using it or losing
it", they decided they needed a telescope.  What they wanted was a 12"
Tinsley cassegrain, german mount, drives both axes, bells and whistles
like that pictured in S&T ads at the time.  I remember lusting after one
of these truly fine instruments, myself.  Since the price exceeded the
amount they could just place an order for, the request had to go up for
bid.  With great care they wrote the specs for a Tinsley, confident that
they would actually get one.

Well, it seems Cave Optical got wind of the bid and convinced whomever
that they could meet the specs of a Tinsley and do it for less, some
$5000.  As it turned out, they didn't.  Nevertheless, after some months,
it arrived.

What was "equivalent" to a Tinsley was a 12" f15 Dall-Kirkham
(elliptical primary, convex spherical secondary).  Briefly, it did have
a beefy looking german mount on a 5' pier.  However, the RA and Dec
bearings were not pre loaded so they had a fair degree of play.  The
Declination drive was a tangent arm screw driven by a small synchronous
motor, all exposed.  The "paddle" was an off the shelf electronics box
with four Radio Shack caliber pushbuttons.  The RA speeds, we later
learned, were stop, sidereal, 2x sidereal.  I don't remember what speed
the Dec drive was.  It was fine for centering objects, but not
tracking.  Castings were adequate, though rough, and most fittings,
bolts, etc. appeared to be standard hardware store stuff.  No knurled
knobs here.

Since, at the time, the local astronomy club had an observatory but no
telescope, arrangements were made to mount the telescope in the club
facilities.  Power hadn't reached the site yet so the scope was driven
off a small utility generator.  As the frequency was somewhat too high
to drive the scope at sidereal rate, I rigged a throttle adjustment on
the governor to slow it down for tracking.  This worked pretty well, or
well enough to allow visual use.

A year or so later, we finally got power to the site.  With some
excitement, we got ready observe in total silence, without the incessant
racket of a running generator.  But the scope wouldn't track.  To our
astonishment, we found that the RA drive motor was 1/2 the speed it
should have been!  The generator with my throttle had simply corrected
the defect by delivering 120 Hz!  Though Cave sent us the correct motor,
the stock RA drive proved to be so poor that the college engineering
department replaced it with a Byers worm and gear.

To my knowledge, the optics were never tested outside the scope itself
so they may have been adequate, or even superior, as some on the group
would likely prefer.  Nevertheless, the overall scope design and
construction was decidedly inferior, certainly to the Tinsley,
effectively limiting the work that could be done.

The popularity of Cave telescopes suggests that this instrument was
simply outside their range.

Clear skies,
Bill Abbay