The polarizer mechanism for the Space Telescope Faint Object Spectrograph is described. This device will allow spectropolarimetric measurements of faint astronomical objects. The mechanism employs a unique arrangement to meet functional requirements in a compact package and with only one actuator. Detailed tolerance analysis and a variety of tests indicate that the polarizer is capable of accurate and reliable performance

Thulson, M. D.

English

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POLARIZER MECHANISM FOR THE
SPACE TELESCOPE FAINT OBJECT SPECTROGRAPH
Mark D. Thulson*
2488? !
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ABSTRACT
This paper describes the polarizer mechanism for the Space Telescope
Faint Object Spectrograph. This device will allow spectropolarlmetrlc
measurements of faint astronomical objects. The mechanism employs a
unique arrangement to meet functional requirements in a compact package
and with only one actuator. Detailed tolerance analysls and a variety of
tests indicate that the polarizer is capable of accurate and reliable
performance.
INTRODUCTION
The polarizer mechanism for the Faint Object Spectrograph (FOS) will
allow the Space Telescope to obtain spectropolarlmetrlc data on faint
astronomical objects. Measurements made using this device will be quite
useful to astronomers studying a wide variety of phenomena, especially
with the extension into the far ultraviolet made possible by the Space
Telescope.
The FOS, shown in Figure i, has two dls::inct light paths and dlgicon
photon counting detectors. One detector is red sensitive (1800-8000 A)
and the other is blue sensitive (1150-5000 _).
Spectropolarimetrlc data is obtained by introducing a Wollaston
prism and waveplate into either of the light paths. The Wollaston prism
forms two dispersed images in opposite senses of polarization at the
detector. The waveplate is rotated with respect to the stationary prism
to analyze for linear and circular polarization. For better coverage of
the spectrum, two waveplates with different retardations are used, each
with its own Wollaston prism.
}_CHANISM DESCRIPTION
The polarizer mechanism must be able to do the following:
- place either of the Wollaston/waveplate pairs in either of the
light paths;
- rotate each waveplate with respect to its Wollaston prism; and
- leave both light paths simultaneously clear when the polarizer is
nc t in use.
The mechanism uses an arrangement which accomplishes these
requirements with a slagle actuator and in a compact package. The two
Wollaston prisms are fixed to a drum which also contains two holes to
* ,lartln Marietta Denver Aerospace, Denver, Colorado
97
https://ntrs.nasa.gov/search.jsp?R=19830016616 2020-03-22T00:52:23+00:00Z
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clear the optical paths (see Figure 2). The waveplates are mounted in
rotating cylinders inside the drum and ahead of the prisms. Each of
these cylinders has a 16 tooth gear cut into its outside diameter. Both
of these gears mesh with a stationary 17 tooth gear located on the drum's
axis of rotation.
Rotating the drum about its centerline allows either Wollaston/
waveplate pair to be inserted into either optical path, or both paths to
be cleared simultaneously. Turning the drum 360o rotates the Wollaston
prism 360o, and the waveplate 17/16 times as much, or 382.5 ° . The net
effect of one rotation of the drum, then, is to rotate the waveplate
22.5o. Sixteen revolutions of the drum returns the optical elements to
their original positions.
The drum is driven through a two stage, 105:1 gear train by a 90o
permanent magnet stepper motor as shown in Figure 3. The motor is driven
at 50 pulses per second (750 RPM). This means that one 22.5 ° increment
of the waveplate takes 8.4 seconds.
Two eight-bit pin contact encoders provide position feedback. The
encoders are geared in such a way (see Figure 4) that a motor step
advances encoder A 3.03 counts and encoder B 3.09 counts. In 16
revolutions of the drum, enc_der A rotates 80 times and encoder B 81
times, bringing both back to their starting point. As a result, each of
the 6,720 steps of the motor required to complete 16 revolutions of the
drum is associated with a unique combination of bits from the encoders.
The fact that motor steps are separated by slightly more than three
counts guarantees that the outputs are unambiguous.
Positional accuracy of the optical elements is determined by the
accuracy of the motor's position plus any error in the gear train. The
design includes several features to minimize this error. First, spring
loaded antibacklash gears are used wherever possible. The 210 tooth gear
is a conventional antibacklash gear with two extension springs. The 18
tooth pinion in the second stage is a special antibacklash gear which is
preloaded by a small torsion spring mounted inside the gear.
Space does not permit the 16 tooth waveplate gears to be antibsck-
lash. The fixed 17 tooth gear cannot be, because it is engaged by both
waveplate gears. The angular position of the waveplate, however, is not
as critical as that of the Wollaston prism. Control of center distance
and runout to minimize backlash limits this error to an accepta_!e range.
Second, all gear ratios are integral. As a result, when the
mechanism returns to a previous position, all the same gear teeth are in
contact and any errors resulting from gear tolerances, bearing runout,
and so on are repeatable. This also means that extremely high precision
gears are not required - AGMA QIO gears are sufflclently accurate.
Third, ABEC 7P bearings are used throughout and comparable
tolerances are maintained on all housings and shafts. This minimizes
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Bclearances between bearings and housings or shafts and the resulting
angular errors.
Finally, the bearings are preloaded axially to eliminate clearance
within the bearings themselves. This is done with belleville washers for
the rotating drum bearings, a wave spring washer for the intermediate
shaft, and compression springs for the waveplate bearings. The use of
springs accomodates thermal changes without large changes in preload.
In the event of a failure, activation of a hot wire pinpuller allows
two torsion springs to rotate the mechanism completely out of the optical
paths to the position shown in Figure 5, allowing contl-lued use of the
remainder of the FOS. The holes engaged by the pin are match drilled at
assembly to guarantee proper alignment. As a further precaution against
binding of the pin, the bolts which attach the plnpuller to the mechanism
are left .025 to .076 mm (.001 to .003 inch) loose to allow the pin to
freely align Itself with the hole.
The entire assembly is attached by six #6-32 screws to a fitting
which is, in turn, bonded to two graphite epoxy tubes that are part of
the FOS optical bench. Laminated shims at these six points allow any
adjustment required for initial alignment. Figure 6 shows the polarizer
after installation into the FOS.
,MATERIALS AND LUBRICATION
All gears are 416 stainless steel, heat treated to a hardness of
Rockwell C32. After final machining, the gears are case hardened to a
depth of 5 to I0 _m (.0002 to .0004 inch) by a nltriding process,
resulting in a surface hardness of Rockwell RC 60 to 70. The hardened
surface will minimize gear wear during the life of the mechanism. Any
wear would result in positional errors as well as the generation of
particle contamination.
By using the nltrldlng process, the gears can be cut, then hardened,
thus eliminating the need for final grinding of the teeth. Dimensional
changes are well within the tolerances for AGMA QIO gears. The fact that
the hardened case is so shallow allows the gears to be drilled and reamed
where required for pinning at assembly.
The bearings are 440C stainless steel. All other major parts are
aluminum to save weight. All non-functlonal surfaces are coated with
flat black polyurethane paint to mimlze optical reflection.
The bearings and gears are lubricated with Braycote 3L-38RP grease.
The shaft about which the mechanism rotates when the pinpuller is
activated is lubricated by Mo52 with an impinged binder.
REPEATABILITY ANALYSIS
Some of the more critical requirements for the polarizer have to do
with the repeatability of the angular position of the optical elements.
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oTwo kinds of angular error are possible - i) rotation about the optical
axis, or @z; and 2) rotation about an axis perpendicular to the optical
axis, or ex and @y (tilt). Either of thes_ can result in both motion of
the image at the detector and errors in polarization measurements.
To insure that these requirements would be met, a detailed tolerance
analysis was performed prior to fabrication of the mechanism. For a
particular requirement, all of the potential error sources were
Ident_fled and their effect on angular position calculated. Root sum
square addition of these coutrlbutors resulted in a predicted maximum
error.
The largest singl_ source of error in @z for either the Wollaston
prisms or waveplates i_ the stepper motor. Its actual position is
specified to be within +SZ of a step, or +4.5o of its nominal
position. The resulting e_ror in @z for the Wollaston prisms, then, is
_4.5°/I05 = +2.57 arc mln.
Most of the remainder of the error in @z arises from bearing to
housing and bearing to shaft clearance. This clearance can induce error
in two ways. First, motion of a gear along a llne passing through the
centers of the mating gears changes the center distance. This results in
rotation of the gear which is related to the pressure angle. Motion in a
direction perpendicular to a llne through the centers also results in
rotation of the gear. The resultant of these two components cannot
exceed an amount determined by the maximum total clearance in the
supporting bearings.
The analysis divides the total possible motion of each gear between
these two components in such a way as to maximize the rotational error in
question. Bearing runout is also included in the same way to account for
the possibility that the outer race rotates with respect to the housing
or the inner race with respect to the shaft.
The error in @z for the waveplates is somewhat larger than that for
the Wollaston prisms. This is because of the extra pair of bearings
involved and the backlash in the waveplate gear meshes.
Repeatability error for @x and @7 (tilt) of the Wollaston prisms are
calculated from the clearances and runout of the bearings which support
the rotating drum. Tilt of the waveplates also includes the effect of
the waveplate bearings and manufacturing tolerances for perpendicularity
between the waveplate mounting shoulder and bearing lands.
Table 1 summarizes the required, predicted, and measured values for
Wx, ey and @z for the Wollaston prisms and waveplates. Predicted and
measured values compare favorably, and both are well within the
requirements.
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|PROBLEMS AND SOLUTIONS
Minimal problems were encountered during assembly, but one became
obvious. Interference between two of the motor mounting screws and the
mounting bracket prevented full rotation of the mechanism when the
plnpuller was operated (see Figure 4). Two small modifications solved
this problem. First, stress analysis showed that three screws were more
than adequate to mount the motor, so one was removed. Second, a small
area was machined from the mountlng bracket to clear the other
interfering screws.
TESTING
Functional tests were performed to verify that the polarizer
performed as expected. These tests included repeatedly operating the
motor a specified number of steps, then reading the two encoders. The
encoders consistently produced reliable and repeatable data.
Further testing verified that the polarizer could be operated closed
loop, as it will be in the FOS. Here the mechanism is commanded to a
position and the encoders are read to determine when it has arrived.
This also worked reliably.
Optical tests were performed to measure image motion Ind
polarization measurement errors. These tests indicate that errors are
well within required values and that the polarizer is capable of
excellent performance.l
The mechanism was also tested under random vibration with no
problems.
CONCLUSION
At this writing (December 1982) the polarizer has been installed
(along with the other mechanisms, optics, and detectors) Into the FOS.
The entire FOS Is presently undergoing thermal vacuum testing.
ACKNOWLEDGEMENT
This work was performed under NASA Goddard contract NAS5-24463 and
purchase contract EO7121-3143 from the Regents of the University of
California (Faint Object Spectrograph for the Space Telescope).
|
I Allen, R.G., and Angel, J.R.P., "Performance of tbe Spectropolarimeter
for the Space Telescope Faint Object Spectrograph", Proceedings of the
S.P.I.E., Instrumentation and Astronomy IV, 1982.
101
ELE CTRONI CS
O
• °
OPTICAL BENCH
DIGICON DETECTORS
FILTER/GRATINGWHEEL
oc
GRAZING MIRROR
P OLARI ZER
ENTRANCE APERTURE
ENTRANCE PORTS
Figure 1. - Faint Object Spectrograph
• _.| tUp'l,_RF ....
: _l_.*
I i
O,.,,b
o
A
CLEAR APERTURES ..
--- , - PRISM
WAVEPLATE
WOLLAS TON/WAVEPLATE PAIRS
-- 16 TOOTH
WAVEPLATE GEAR
SECTION A-A
Figure 2. - Rotating Drum
)
018 TEETH (ANTIBACKLASH) --.
/
10 TEETH
210 TEETH
(ANTIBACKLASH)
90 TEETH
S STEPPER MOTOR
i
/
_._ _ ROTATING DRUM
O_ ¸
o _:,
Figure 3. - Gear Train
,i .
0 MOUNTINGBRACKET
o
om ENCODER A
81 TEETH
(ANTZBACKLASH)
ENCODER B
80 TEETH
(&NTIBACKLASH)
81 TEETH
Oo
o_
O _,
:Dr-
ROTATING DRUM
Figure 4. - Encoder Gears
STOP
0
o
TeRSION SPRINGS
PIN'PULLER
o _
Figure 5. - Fail-safe Mode
'<
o
O0
"_;_
o_
Figure 6. - Po!arlzer Installed in FOS
Table i.
[¢ J
OF POOR QUALr'f"J,
Repeatabillty Tolerances
|
Wollaston
Prism Waveplate
ex, ey
(arc mln)
Required
Predicted
Measured
+8
+i. 1
+I. 7
m
+8
+3.2
+3.0
_z
(arc min)
Required
Predicted
Measured
+6
+3.0
+2.8
+21
+12.6
+I0.0
108
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Polarizer mechanism for the space telescope faint object spectrograph

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