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Microtechnologies for Science Instrumentation Applications 139

A similar program proposed by JPL is the Advanced Segmented Silicon Space
Telescope (ASSiST), which utilizes thin silicon wafers as the building blocks of
highly segmented space telescope primary mirrors. 59–61 Using embedded MEMS
actuators operating with high bandwidth control, this technology can achieve
diffraction-limited image quality in the 3–300 mm wavelength range. The use of
silicon wafers as cryogenic mirror segments is carried forward considering a point
design of a future NASA ORIGINS mission. Individual segments of the ASSiST
consist of 1-mm thick, 300-mm diameter silicon wafers with 10-mm deep
frames, assembled into 3-m diameter rafts. This achieves considerable reductions
in primary mirror mass through the elimination of a heavy back plane support
structure. Rather, they exploit the micromachining capabilities of silicon
processing technology to achieve sophisticated control of a highly segmented
mirror using high-bandwidth, high-stroke MEMS actuators, which will ultimately
be built directly into the mirror segment, resulting in an integrated optics package.
Thus, a single segment can perform the traditional light-focusing function of a
telescope as well as the control functions, and quite possibly the space deployment
functions.

7.3.3 SPECTROMETER APPLICATIONS
The size of spectrometers, especially infrared spectrometers, has been rapidly
reduced in recent years due to uncooled IR detectors with ultrasmall pixel size
and modern micromachining techniques. 62 Infrared spectrometers are some of the
most important instruments since most molecules show a characteristic ‘‘finger-
print’’ spectrum within this range. A reduction in size for these instruments will
have a major impact on space-based observations, as well as for terrestrial sensors
for chemical and biological agent detection. One example is a Fabry–Perot (FP)-
63
based interferometer. A FP interferometer or etalon consists of two flat, parallel,
semitransparent plates coated with films of high reflectivity and low absorption.
The pass band of the etalon is determined by the separation between the plates,
which is generally varied using piezoelectric actuators. For any large aperture wide
field telescope, low-resolution FPs are an ideal option for narrow-band imaging as
opposed to linear or circular variable interference filters as they ease size require-
ments on filter wheels and offer flexibility in choice of spectral resolution. Tunable
filters on space telescopes will require operation at cryogenic temperatures, where
piezo actuators alone do not provide sufficient translation to tune the etalon over the
desired orders of interference without becoming large and cumbersome. In addition,
low-resolution infrared etalons require cavity spacings on the order of a micron.
Figure 7.10 and Figure 7.11 show a photograph and the schematic of a FP interfer-
ometer design developed at NASA GSFC. 29
In this design, the mechanism is fabricated in two sections that are assembled
into the final FP filter. The stationary mirror structure consists of a micromachined
350-mm thick silicon wafer coated with a multilayer dielectric (MLD) in the
aperture. The moving mirror structure is also machined from a 350-mm thick silicon
wafer and is identically coated with MLD over its aperture. Its reflector is attached

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