[mmaimcal] Query from Jaap Baars

[Simon Radford]

Since the ACA antennae will be smaller than the ALMA antennae (6-8 m vs.
12 m), presumably the subreflector will also be smaller (350 - 500 mm
vs. 750 mm). For equal specifications (beam throw, transition time,
etc.), the nutator force requirements (and size) scale with the mirror's
moment of inertia, which scales at r^4 (or even r^5 when the thickness
is included). As an illustration, the SMA and ALMA nutators have similar
specifications. The SMA nutator, for a 350 mm mirror, uses a pair of 4
lbf (17.5 N) motors. The ALMA nutator, for a 750 mm mirror, will use a
pair of 100 lbf (444 N) motors. This is a scaling of r^4.25.

Hence ACA nutators could be much less powerful (and smaller) than the
ALMA nutators (3-20%). Equivalently, ACA nutators could have higher
performance goals than the ALMA nutators.

The cost of prototype nutators, however, probably would not scale nearly
as steeply as the motor and physical size. With the quantities planned
for ALMA (4 or 5 nutators), we probably will not see much economy of
scale in production.