Temperature Gradient Sensitivity
of the FAME Basic Angle
Marc A. Murison
Astronomical Applications Department
December 4, 1995
If the compound mirror assembly (CMA) in FAME were to be fabricated of a single block of Zero-
dur (instead of silicon carbide with glued-on Zerodur wedges as currently proposed), then, assum-
ing the thermal environment indicated in the Step 1 proposal, changes in the basic angle due to
temperature gradient fluctuations during the course of one spacecraft spin period are confined to
as. This gives rise to the interesting possibility that the proposed laser metrology system,
whose sole functional purpose is to measure short-term changes in the basic angle, may not be
needed if further, possibly inexpensive, attention is given to the thermal environment of the CMA.
A tolerance of 25
as on changes in the basic angle can be met passively if short-term gradient
changes are on the order ~5 mK/m or less.
Additionally, there is a potential problem that is independent of the metrology question: warping of
the CMA mirrors alone due to static gradients (as illustrated in the Step 1 proposal) will introduce
wavefront errors of order ~18 nm (~
= 550 nm). Similar warping of the primary mirror
will also occur.
1. Introduction and Summary of
This memo addresses the temperature gradient
sensitivity of the FAME basic angle. I adopt a
simple analytical model and assume that the so-
called "compound mirror assembly" (CMA) is
formed from a contiguous block of glass,
haps two bonded glass slabs (Figure 1). I con
sider for analytical simplicity two cases:
U.S. Naval Observatory
Current design calls for the CMA to consist of silicon carbide with thin Zerodur mirrors attached
with glue. However, the CTE of SiC is ~200 times that of Zerodur.