The mechanical design and fabrication are discussed of the imaging photopolarimeter (IPP), a multifunction space-qualified instrument used on the Jupiter Pioneer missions. The extreme environmental requirements for the structural design, optical system, and mechanisms are described with detailed discussion of some of the design and fabrication problems encountered

Kodak, J. C.

English

NASA Technical Reports Server

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_ 15. THE MECHANICAL DESIGN OF AN L_4AGING PHOTOPOLARIMETER
_/ FOR THE JUPITER MISSIONS
:+=, (PIONEER 10 AND 11)
h_ By James C. Kodak
_ Santa Barbara Research CenterGoleta, California
+_ SUMMARY
_:_:[. This paper discusses the mechanical design and fabrication of the
_. Imaging Photopolarimeter (IPP), a multi[unction space-qualified instrument
_: used on the Jupiter Pioneer Missions. The extreme environmental require-
_ ments for the structural design, optical system, and mechanisms are de-
_. scribed with detailed discussion of some of the design and fabrication prob-
!i'r' lems encountered.
~
:. INT ROD UC T ION
_r
_:; An Imaging Photopolarimeter (IPP)was launched aboard Pioneer I0 in
i March of 197Z and, after Zl months of travel through space, passed by the!
.? planet Jupiter in December of 1973. During this time, the instrument passed
through the asteroid belt and the intense radiation and magnetic fields of Ju-
: piter to successfully fulfill its mission. The i_terplanetary portion of the
_ mission was utilized to generate sky maps of brightness and polarization of
! the zodiacal light. During the Jovian encounter, the IPP produced two-color
images of Jupiter with resolution and at viewing angles unobtainable from
; earth. In addition, it measured the brightness and polarization of light re-
_ flected from the planet from which, for example, details regarding the size
_' and shape of the particles composing the atmosphere can be deduced.
A second IPP launched aboard Pioneer II in April of 1973 will pass by
_; Jupiter in December of 1974 to accumulate additional data at viewing and ap-
_./ proach angles different from those of Pioneer 10. Since Pioneer 10 fulfilled
C
_. its main objectives and successfully survived the Jovian radiation belts,
_" Pioneer 11 has been retargeted so as to pass by Saturn nearly _ive years _.
_ after the December 1974 encounter with Jupiter. This will provide signifi- ._
I cant bonuses with only relatively minor compromises, e.g., somewhat lessfavorable imaging geometry at Jupiter. %
,RECEDINGPAGZ B_K NO_ _
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'[",eIPP is a multifunction instrument designed to: I) map the zodiacal
light (faintlight measurement); Z) measure the brightnes s and polarization of
light in two spectral bands (colors) reflected from the planet Jupiter; and
3) produce two-color images of Jupiter. The instrument (see Figure 1) con-
sists of an electronics module and a scanning electro-optics assembly. The
instrument is mounted aboard a spinning spacecraft {see Figure Z)and uti-
lizes the 4.9-rpm spin of the spacecraft to scan a narrow strip across the
planet (or across the sky in the case of the zodiacal light mode) during each
revolution. The electro-optics assembly may also be positioned in 0.5-mrad
" steps in the direction of flightfrom Z9 ° from the earthward direction to 170 °
from the earthward direction. This stepping allows scanning of contiguous
strips which can be recornbined during ground data processing. The various
optical modes are achieved by inserting various fleld-defInlng apertures, po-
larization-analyzing optics, and calibration sources into the optical path at
the focal plane. The optical system is a complex combination of mirrors,
lenses, prisms, multilayer coatings, and filters that separate the viewed
scene into four beams, two red and two blue, with each colored pair orthogo-
nally polarized. The four beams are then individually imaged onto individual
cathodes of two dual-channel detectors. The red and blue light is quantized
into 64 intensity levels (imaging mode) and 10Z4 levels (photopolarimeter and
: zodiacal light modes) and then transmitted to earth as coded numbers. The
signals are reconstructed and enhanced by computer to provide two-dimen-
sional displays of the data (spectacular color images and brightness and po-
i larization maps ).
_ ENVIRONMENTAL AND INTERFACE REQUIREMENTS
: The IPP was designed to withstand the normal launch environment of
most spaceborne instruments and to survive the almost two-year journey
through space and passage through the Jovian radiation belts. In addition,
the presence of a magnetometer aboard the spacecraft made it necessary to
produce a virtually nonmagnetic instrument.
The instrument is located on the spacecraft to take advantage of the
4.9-rpm spin and to provide as optically clear a field of view as possible.
The IPP is mounted in the spacecraft instrument compartment with the
electro-optics assembly extending through the side of the compartment wall.
As a result, the electronics are maintained at close to ambient temperature
and the electro-optics assembly is exposed to the sub-zero temperatures of
deep space. This mounting configuration caused some concern _mong the
experimenters involved because the exposed telescope could readily collect
dust and debris during launch. Dust would create extensive light scattering
and could jeopardize the faint light experiment. Maintaining cleanliness of
the optics was mandatory during fabrication, assembly, and launch as well
as in flight to Jupiter.
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= ; The total weight of the IPP is less than 10 pounds and the power re-
quirement is approximately Z watts.
_ MATERIAL CONSIDERATIONS
_ The IPP was designed to be constructed almost entirely of beryllium.
__ This material was selected primarily to minimize weight. All m_terials
_ used were considered for their radiation resistance and nonmagnetic charac-
_i. teristics. Mater.ials considered suspect were exposed to specif;: doses of
_: radiation and evaluated for damage. Radiation problems had their most sig-
_: nificant impact in the electronics and optics areas. The most serious prob-
_ lem in the optics area was discoloration or phosphorescence. In the initial
_ design concept, the detectors were mounted with the electronics, close to the
high-voltage supply, and the optical beams were relayed to the detectors
_.__:- through fiber optics. However, all the fiber optics materials that were flex-
_:', ible enough to allow the telescope scanning would discolor when exposed to
_: the levels of radiation anticipated during the Jovian encounter. As a result,
_ the fiber optics were eliminated and the detectors mounted directly on the
_" electro-optics assembly. The high voltage was then routed from the elec-
_ tronics module through a specially fabricated flat cable to the detectors on
_ the electro-optics assembly.
:_ Another problem was encountered with the calcite material selected for
_: the three-segment Wollaston prism. Because of its large size, artificially
grown calcite was not available and material had to be obtained from natu-
'_: rally occurring crystals. Natural crystals generally contain impurity ions
_ which discolor or phosphoresce when exposed to high energy radiation. As a
_= result, it was necessary to irradiate various small samples cut from large
_: crystals to find a sufficient quantity of material unaffected by the radiation.
_
<
_ Most mechanical materials are not affected significantly by radiation.
_ However, the use of organic materials was potentially a problem. While
most epoxies tend to harden a minor amount, teflon and nylon, for example_
are severely affected. All materials and components utilized were checked
!_' for magnetic characteristics to ensure that some magnetic item was not in-
i advertently assembled. One magnetic screw or washer could make the sys-
tem so "hot" thatdisassembly would be necessary. The glass feedthroughs
I on the detectors and other electronic components were another problem.
These feedthroughs were fabricated of low nickel alloys so that their thermal3
.,:, expansion coefficients closely matched that of glass. It was necessary to clip
the magnetic leads off as close to the glass as possible and weld nonmagnetic
i: wire in place of them.
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Gimbal bearings were f_bricated of Composition 12-5 and Berylco Z5.
Small instxument bearings did not cause any significant problems and were
therefore screened and used as is.
INSTRUMENT CONFIGURATION '
: The instrument (see Figure I) has two major components, the elec-
tronics module and the scanning electro-optics assembly. An integral mount-
ing platformwas configured to provide a direct thermal path from the
: electronics and from the scanning optical telescope to the spacecraft without
; interference from other components or one another. Spacecraft thermal con- :
trol therefore exerts a direct effect in controlling the enviromnent of these
assemblies. The mounting platform was fabricated of black anodized beryl- ;
lium. Critical interfaces were gold-plated to enhance thermal and electrical
: transfer. The mounting platform was fabricated of four separate structures
and screwed and bonded together to form an assembly. The bonded joints
were located such that they do not interrupt the thermal path nor are they
solely relied on to carry the structural loading. This four-piece assembly
was designed to allow fabrication of the structure from small sections of be-
ryllium plate in lieu of the less available and more costly large beryllium
block.
The electro-optics assembly (see Figure 3)is supported from the
: mounting platform by two gxmbal bearings specially fabricated of nonmagnetic
i materials and dry lubricated with molybdenum disulfide. One gimbal bearing
is installed directly between the optics assembly and the mounting platform i),
and the other is installed in the platform-mounted telescope drive assembly
which positions the optics along its 160 ° scan. A flexible braided thermal
strap was connected between the optics assembly and the mounting platform
to enhance thermal balance between the cold optics assembly and internally
heated electronics housing. Thermal conductivity of the braid is considerably
greater than that of the bearing assembly, i
The electro-optics assembly consists of a 1Maksutov telescope, a six-
position focal plane aperture actuator, a Wollaston prism, various optical
elements, and two dual-channel Channeltron detectors (see Figure 4).
The Maksutov telescope consists of a primary mirror, seconaary mir- i
rot, and spacer installed between the mirrors. The spacer is fabricated of
the same fused silica material as the mirrors to minimize focal plane shift
as a result of thermal expansion. The entrance aperture is Z. 5 cm in diam-
eter and is located at the primary mirror. Contact pressure is maintained
between these components by a Viton O-ring installed on the outside of the
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secondary mirror (coi rector ). The entire telescope is housed in a machined
black anodized beryllium tube which contains integral 'Knife-edge" baffles
: for stray light suppression.
The six-position aperture actuator is driven by a stepper motor spe- '
cially designed to minimize static and dynamic magnetic fields. The actuator
design integrates the aperture carrier into the basic structure and provides
a receptacle for bonding in place the aperture plate and the focal plane optics.
: _ The aperture plate contains the fleld-defiaing apertures which vary in
7; size from a 0.5-mrad square (0. 038-ram square) to a 40-mrad square. The
i aperture is fabricated of beryllium copper, plated with black chrome and
, photoetched. Because of the design change that required that the detectors
_ be mounted on the moving optics, the space allocated for the aperture actu-
: _.' ator was limited and resulted in an actuator configuration with a limited bi-
: directional rotation (±50 °). AS a result, the damping and calibration
: _: required to achieve desired detenting within the specified response times and
voltage range, and over the required temperature range, were extremely
difficult. It is felt that this type of design should be discarded on future de-
: signs in favor of unidirectional continuous rotation configurations where space
: permits.
The Wollaston prism (see Figure 5) is constructed of three segments of
calcite. Calcite is a material possessing an anisotropic thermal expansion
coefficient, Z5 × 10-6/°C parallel to its optical axis and -6 ,< 10-6/°C per-
pendicular to its optical axis. It is necessarily (optically) configured such
that the thermal expansion coefficient of each segment differs from other ad-
jacent segments along the same axis. The three segments of the prisms are
bonded to each other and suspended in an aluminum housing by an elastomeric
encapsulant. Aluminum was selected for the housing because the combined
thermal expansion coefficients of the calcite and the elastomer closely match
the expansion coefficient of aluminum when used in the subject configuration.
As a result, the prism remains essentially stress free. Calcite is a very
fragile material and must be handled with special precautions. Care must be
exercised to shroud the material when thermal cycling, as even mild thermal
shocks were shown to produce fractures in the raw material.
Two dual-channel detectors were mounted on the optics assembly in an
aluminum housing with a semi-flexible adhesive. Aluminum was selected for
this housing as it provided more radiation protection for the detectors than
beryllium.
The various mirrors and lenses in the optics assembly were fabricated
of radiation-resistant glass and bonded to the beryllium housings by a semi-
flexible adhesive. All mirror and lens positions were individually adjustable
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: by replacement of premachined shims to allow optimum positioning of the
! optical images on each detector.
The completed optics assembly is shrouded by a low emissivity pol-
ished aluminum drum which eliminates the excessive heat loss that would
result from direct exposure to deep space. It is gimballed between the plat-
form-mounted telescope drive motor and platform-mounted bearing
a s s embly.
The telescope drive actuator (see Figure 6) which produces the 160 °
scan is a special sealed drive system designed to transmit rotational n_otion
through a welded hermetic seal. Thus, suitable lubricants were completely
contained within the active mechanical portion cf the drive to assure reliable
operation over the lengthy Jupiter mission.
The gear configuration is divided into two stages such that it converts
15° steps at the rotor to O.5-mrad steps at the output. The first-stagegear
reduction of 7.3:1 is accomplished in two passes of conventional gearing.
The second-stage gear reduction of 72:I is accomplished through a harmonic
drive. * The harmonic drive consists of _ree basic parts: the wave gener-
ator, flexspline, and rigid spline. The wave generator is an elliptical cam
which deforms the flexspline into an ellipse causing it to engage with the cir-
cular rigid spline at two points of contact. The flexspline is fixed and the
wave generator is rotated by the first-stage gearing. As the wave generator
rotates, tooth engagement between the flex and rigid spline follows the rots-
tion of the wave generator. The number of teeth in the flexspline differs
from the number of teeth in the rigid sp!ine by two. As the wave generator
makes one rgtation, the flex and rigid spline are displaced with respect to
each other by two teeth. With a 144-tooth flexspline, this produces a 144:Z
reduction.
The stepping accuracy (0.5 + 0. ! rnrad) required was pushing the state
of the art for this size actuator. The windup in the flexible spline (variation
in resist_.ng torque as a result of discontinuities in the deflection member) re-
sulted in somewhat worse accuracy than required in the initial unit. This
problem was minimized by matching flexsplines to wave generators and by
the ultimate addition of a torsional spring to the output shaft of the actuator.
This spring loading put a higher torque on the drive and made the variation in
torque caused by flexspline windup less significant.
•Gear Systems Divi_ ion, United Shoe Machinery Corporation, Beverly, _i_
Ma s sachusetts, i!
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_ An input encoder was built into the motor to indicate stepper motor
_ rotor position; _his is used in conjunction with an output shaft encoder to in-
i./ dicate telescope po_tion. The input encoder thus acts as a vernier position
_ readout. An output (bruch type) position encoder is mounted directly on the ,
_ motor output shaft and is indexed to the main mounting platform and the tele-
_ scope. This encoder provides the coarse positional readout.
An important actuator design consideration was that of magnetic field
suppression so as not to interfere with the extremely sensitive spacecraft
magnetometer. This was accomplished by special design of tig._tlycoupled
flux paths and the use of magnetic mater_.als of very low remane.lce charac- !
teristics. * Both motor-driven actuators in the IPP exhibit magnetic clean-
liness c]_aracteristics of approximately I0°70of the allowable Pioneer
_ specified values, thus confirming the validity of these techniques. For ex-
i ample, the aperture actuator which contains no shielding measured less than
0.1 gamma at 0.9 meter (3 feet)staticand 0.9.gamma in the worst axis while stepping
at maximum rate. The telescope drive assembly with a mu-metal shield
measured 0.3 gamma at 0.9 meter {3 feet)depermed, 1.9.gamma at 0.9 meter (3 feet_
aRer 25 gauss exposure and 0.06 gamma depermed and operating at maximum rate.
_ ACKNOWLEDGMENTS
The work reported in this paper was conducted at Santa Barbara Re-
i: search Center, Goleta, California, a subsidiary of Hughes A'rcraft Company
for NASA/Ames Research Center under NAS-2-5605.
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*Designed under subcontract by Schaeffer Magnetics, Inc., Chatsworth,
ii California. _
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Figure Z. Imaging Photopolarimeter Orientation on
Fioneer lO and II Spacecraft
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Figure 4. Optical System of the Pioneer Jupiter Imaging Photopolarimeter
(Al_alysis of two orthogonally polarized beams into two colors,
red and blue, is shown. )
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1976012084-205
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OPTI_ALAXIS OF
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ALUMINUM.qOUSING
CALCITEPR
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'ELASTOMERIC ENCAPSULANT ,_
Figure 5. Wollaston P_ism Mounting Configuration ii
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1976012084-206
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SPLINE
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Figure 6. Teloscope Drive Actuator
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1976012084-207


The mechanical design of an imaging photopolarimeter for the Jupiter missions (Pioneer 10 and 11)

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