Max Planck Institute for Astronomy - Annual Report 2007

Cryogenic Wheel Mechanisms for the Miri and NirSpec
Instruments aboard JWST
The MPIA is the only European institute which is
engaged in the development of two of the four scientific
instruments for the James Webb Space Telescope
(JWST). In close collaboration with ZeiSS (Oberkochen),
the Institute is developing filter and grating wheel mechanisms
for Miri and NirSpec – the cryomechanical
hearts that enable the full operational functionality of
these complex instruments. In addition to the hardware
contributions, MPIA is responsible for the electrical
system engineering of the Miri instrument. Members of
the Institute are deeply involved in the ground-testing of
Miri and also involved in the science teams of both Miri
and NirSpec.
The infrared space observatory JWST is jointly developed
by the US, European and Canadian Space Agencies.
Its 6.5 m mirror will allow acquisition of images that will
be as sharp as those taken by the smaller Hubble mirror
in the optical range. To avoid blinding of the sensitive
cameras by its own thermal radiation, the primary mirror
is cooled radiatively to –230 C. This “passive cooling”
is possible at the Lagrangian Point L 2 at 1.5 million km
anti-solar distance from Earth. Nasa will have overall responsibility
for the JWST mission to be launched aboard
an European ariaNe-5 rocket in the year 2013.
JWST is equipped with four scientific instruments,
two of which are built mainly in Europe: Miri, a camera
with coronograph and spectrometer for the mid-infrared
range (5 to 28 µm), is built by a consortium of 20
European institutes, with JPL providing detectors and
Fig. IV.5.3: Filter wheel mechanism for the Mid-Infrared
Instrument (Miri). The wheel carrying filters, coronographic
masks and a douple prism is moved by a central torque motor.
Axial Balancing
Weight
KIP between
bearing and Filter
Wheel
Filter Wheel with
Index bears
IV.5 Instruments for Space Observatories 115
the cryomechanical cooler. Nirspec, a near-infrared range
(1 to 5 µm) multi-object spectrograph capable of observing
more than 100 objects simultaneously, is developed
by esa and an industrial consortium led by EADS-
Astrium, Germany.
Common to all focal-plane instruments is that they have
to be operated in a cryo-vacuum at temperature between
– 267 C (Miri) and – 240 C (Nirspec) so that
their own thermal emission will not outshine the cosmic
infrared radiation. Another common property of the instruments
is that all of them have large optical exchange
wheels with numerous gratings, filters beam splitters,
mirror, prisms and coronagraphic masks mounted on
them. Although every space technician is anxious to
avoid mechanisms like these wheels (...may fail...), powerful
scientific instruments without moving parts are
not feasible. Because of previous successful developments
of such mechanisms for the European space telescopes
isoand HerscHel, our Institute was well prepared
for these high-risk challenges and became involved
in the development of wheel mechanisms for Miri and
Nirspec, which have even more demanding requirements
than those of previous missions.
The JWST wheel mechanisms are based on a ratchet
principle (see Fig. IV.5.3): At the periphery of the wheel
small ball bearings are mounted. Their number corresponds
to the quantity of optical elements. A wedgeshaped
element on a moving lever latches between two
ball bearings, thus locating the position of the wheel with
a repeatability of about one second of arc. The central
motor is a torque motor without transmission. The exact
position is carried out mechanically without electric power
by the spring torsion of the ratchet system. This drive
Positioning is performed using a detent latching into the index
bearings outer race. The mechanism has a diameter of 28 cm
and a total mass of 3 kg.
Magnet Ring
Integrated Bearing
with KIP
Ratchet
Subassembly
Support
Structure
Stator Interface
Plate
C84-Motor