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It Came Upon the Midnight Clear: New Focus Readout...(3/1/93)
on the 0.9-m Telescope
(from CTIO, NOAO Newsletter No. 33, 1 March 1993)

On the night of Dec 23 we installed a new focus encoder on the 0.9-m
telescope. The old encoder is still operating, so either readout can be
used. The new encoder (Futaba Pulscale) is a linear scale designed for
precision machine tool applications, has a variety of display modes,
and can read to 1 um resolution. An approximate conversion between the
scales is:

                     100 old units = 59 um.

Typical focus steps on the old system were 50 units, which therefore would
be approximately 30 um.

The old encoder is known to exhibit thermal drift, and has been
noticeably aging over the past year. As shown by brief testing, and
according to specifications, the Futaba scale should be very stable.
The secondary motor system itself (motor/gears, etc.) has backlash, and
when the directions are reversed, the motor can be heard running for
several seconds before the encoders indicate any position change.

There are instructions for the use of this readout and its impressive
display available in the 0.9-m console room.

The 0.9-m telescope does have some problems with focus stability, which
are probably related to one of the mirror supports. It also exhibits
astigmatic and spherical aberrations, and is a classical Cassegrain,
with coma over the field. We can now know with precision how poorly the
telescope focuses. We would appreciate any comments from observers on
focus stability, jumps in focus, or pointing glitches.

Optical and mechanical tests are continuing. On the following night
with the Tek 2048, the collimation appeared nominal (as noted in
previous Newsletters, the coma centroid is ~2 arcmin from CCD center).
The observer also noted on several exposures a rapidly moving luminous
object, with red and green coloration, and a leading, luminous
proboscis. A hearty "Ho Ho Ho" was entered on the logsheets.

R.A. Schommer, O.Saa, J. Briones,
R. Gonzalez, M. Fernandez

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