The design requirements of the polarimeter were established by the scientific optical objectives of the experiment to be launched aboard the Solar Max Mission which will study active solar regions. The polarization of the light is accomplished by a rotating magnesium fluoride quarter wave plate. The quarter wave plates are rotated in 22 1/2 degree steps about an axis coincidental with the light beam. As the light beam passes through the wave plate, the transformation that occurs can be expressed by mathematical equations. By having the wave plates calibrated, the data obtained from solar flares can be analyzed and meaningful information provided to the investigators. The polarimeter has two wave plates with different optical characteristics to provide both redundancy and versatility. A four mirror polarizer was added behind one wave plate to provide additional polarization. The mechanical design, testing, and operation of the polarimeter for the high resolution ultraviolet spectrometer/polarimeter are described

Calvert, J. A.

English

NASA Technical Reports Server

, N80 23520
A POLARIMETER FOR THE HICH RESOLUTION
ULTRAVIOLET SP_CTROMETER/POLARIMETER
John A. Cal_ert
George C. Marshall Space Flight Canter
INTRODUCTION
The High Resolution Ultraviolet Spectrometer/Polarimeter (HRUVSP) is an
experiment to be launched aboard the Solar Max Mission sometime in early 1980.
The HRUVSP will allow the study of active solar regions through ultraviolet
polarimetry amd spectroscopy. The polarization capability, along with other
modifications, was added to an e_isting engineering model of the OSO-8 High
Resolution SpeCtrometer.
The High Resolution Ultraviolet Spectrometer/Polarimeter isdepicted in
Figure i. The polarlmetcr ( X wave plate) is shown enclosed by a heavy line.
This paper will deal specifically with the mechanical d_sign, testing,
and operation of the polarimeter for the HRUVSP.
DESIGN REQUIREMENTS
The design requirements of the polarimeter were established by the
scientific optical objectives of the experiment. The polarization of the
light is accomplished by a rotating magnesium fluoride quarter wave plate.
The quarter wave plates are rotated in 22½-degree steps about an axis coinci-
dental with the light beam. As the light beam passes through the wave plate,
the transformation that occurs can be expressed by mathematical equations.
By having the wave plates calibrated, the data obtained from solar flares can
be analyzed and meaningful information provided to the investigators. The
pclarimeter has two wave plates with different optical characteristics to
provide both redundancy and versatility. A four-mlrror polarizer was added
behind one wave plate to provide additional polarization.
MECHANICAL DESIGN
The polarimeter mechanical design was required to provide three positions
for the wave plates: either one of the wave plates in the light path or both
wave plates out of the light path. When the wave plate is in the light path,
rotation in 22_-degree steps is necessary. To provide the quality of data
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required, the eenterli_e of rotation of the wave plates was not _o be over
1.8 mm from ceneorline of the light path; and _ha deviation from the desired
22½-degree rotation of the wave plates was to be less than ½ degree.
In addition to the above Optical criteria, the design was required to:
1. Provide mechanical and electrical redundancy;
2. Have low electrical power U_age;
3. Maintain cleanliness for wave plates and other optics is the
experiment;
4. Fit into an existing cavity (12.7 cm wide x 11.4 cm lo_g x 8.9
cm highS;
5. Operate intermittently for one year in space; _nd
6. Provide a data feedback system to indicate wave plate position
and rotation.
DESCRIPTION
The polarimeter is shown in artist's conception in Figures 2 and 3,
WAVE PLATE ROTATION SYSTEM
The quarter wave plates are two layers of magnesium fluoride optically
bonded together to a total thickness of .600 mm and an open diameter of 7.0mm.
The plates are fragile and cannot be easily handled alone. A structural
holding ring was designed to provide a safe method of handling and a means of
securing the wave plates into the barrel assembly. The complete wave plate-
holding ring assembly is shown in Figure 4. The holding ring was made of a
non-outgassing plastic material .051 mm thicker than the recess in the barrel.
This additional thickness insured that the quarter wave plates were securely
held, and vibration would not cause the plates to move. After initial inStal-
lation and rotational allgnmen= of the wave plotes, the plastic holding rings
and keeper rings were drilled for installation of an alignment pin so the wave
plate assemblies could be removed and reinstalled precisely.
The limited volume available to house the polarlmeter dictated the use of
small stepper motors (Figure 5). The available running torque of 44.6 cm-gm
(.62 oz-in) and precision o!_tical alignment requirements made a precision
gear mesh, low frlction system mandatory. Four motors, as depicted by
Figure 5, were used in the polarimeter.
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00000008-TSC04
Each of the two wave plata rotation systems consisted of a s_¢pper moto_
driving through a 96 pitch, 31 tooth spur gear plnnad to its shaft and the
barrel assembly (Figure 4) with a 96 pitch, 124 tooth spur gear. Both barrel
• assOmblies and their respective drive motors wore hold by a single machined
alutnlnum housing. This housing gave the best opportunity _or maintaining thO
tolerances required for proper gear tooth operation and also for maintaining
the alignment required for the optics. This design approach required CloSe
tOlerancing and precision machining operations; but because of the limited
volume available for the poZarimeter, there was no space available for adjust-
manes if separate asSemblieshad been used.
The wave plates had to be rotated Cone direction only) in 22½-degree
steps a_ 7.8 steps/seCond with data being taken while the wave plates were at
rest. Zt was required that the plates complete one rotation every two seconds.
The step pulse width of the wave plate rotatloa motor was 23 milliseconds,
and oscillation of the plate was to be less than ½ degree after 20 milli-
seconds. The usual procedure of sending a pulse to the motors and allowing
the friction to damp out the oscillations of the wave plate requlred too ntuCh
of the time allocated for each step. A special control circuit was designed
so that after the motor steps the motor Windings were shorted to make use of
viscuous damping caused by the generated EMF. This gave tNO improvements to
the design: it shortened the step-oscillation time and also gave a more
: repeatable positioning of the wave plates. During design of the barrel
assembly, the inertia was kept as low as possible to help in the damping of
the oscillations and to decrease the time to step the wave plates. During
assembly of the bagrel assembly into the machined housing, care was taken to
have the minimum preload possible on rotati_)nal bearings to give stability but
have minimum friction possible.
WAVE PLATE POSITIONING SYSTEM
As indicated previously, the wave plate positioning system has three
, positions: wave plate "A" in the light pathj both wave plates out of the light
path, and wave plate "B" in the light path. The i_sertlon-retraction system
is capable of placing either wave plate into Or out of the light beam.
The system has two stepper motors (Figure 5) identical to the wave plate
rotation motors, both driving through a 31 tooth spur gear to a 124 tooth
spur gear. One stepper motor is fixed to the support plate, the other is
movable about the worm gear (Figures 2 and 3). For the movable motor to
operate, the worm gear and supporting shaft are held fixed by the magnetic
detent of the stepper motor and the non-backdrlve characteristic of a single
thread worm. The movable motor is pulsed 140 steps at a rate of 15.625 steps/
second and, through a 280:1 total gear reduction (4:1 spur gear, 70:1 worm
gear), the movable motor and worm move _ 45° around the worm gear. Depending
upon motor rotation direction, either wave plate will be placed into the
light path. The outer assembly rotates about the supporting shaft on four
bearings. For the fixed motor to operate, the outer rotational assembly was
fixed to the worm gear; and the fixed motor rotated the worm gear, supporting
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00000008-TSC05
shaft, and rotational assembly as a unit. The supporting shaft rotated in
hearings in tho support posts at olthor end of the shaft.
This arrangement had the advantage of moehanleal redundancy (either motor
could plae_ elth_ wave plato into the light path without having to backdrlve
the other; and completely _eparate sets of boarlngs were provided for each
mode of operation) without having =o have a differential in the drLve system.
The method of operation of the Inserglon-retraCtlon motors was tO select
either motor for operation, designate direction of rotation, and the elec-
tronics would send 140 pulses at a rate of 15.6 steps/s_Cond to the designated
mOto_. This would place the selected wave plate into the light p_th. A feed-
back system, to be discussed later, would then send a signal to indicate the
wave plate was in the proper position. The signal also served as a redundant
cutoff in case of electronic malfunction, and more t:_an 140 Step commands
Were given.
The Imsertion-retraction mOtorS (IRM) did not have the non-oscillating
requirements of the wave plate rotation motors, but more torque was required.
Also the IRM was to be operated only a short period of time, so electrical
power consumption was not as important a consideration. For these reasons a
pulse width of .030 second Was used. This gave more assurance that a step
would not be missed, and the waVe plates would be in position _or data taking.
Both the wave plate rotational system and the insertlon-retraction system
had a method of position feedback (reference Figures 2 and 3). Each wave
plate had rotational sensors that indicated once per revolution. The sensors
were photodiodes and phototransistors. A hole was provided in the spur gear
on the barrel assembly for the phototransistor to sense the light emitted by
the photodiode. The photodiode is pulsed rather than powered constantly to
provide additional electrical power savings. The insertion-retraction posi-
tion sensors were required to provide a one motor step resolution so the wave
plates could be positioned to within 1.8 mm (one step accuracy) about the
optical light path. The position plate is fixed to the wave plate holder
machining. It has one slot .2 mm wide in the center to correspond to the
out of light path position. On each side of the position plate a sharp edge
is used for reference. An edge was used as it allowed fitting at assembly to
achieve the minimum possible deviation from the light path. The low light
level of the photodiode and the small hole (.2 mm d_ameter) in the mask re-
quired the phototransistor be placed as close as possible to the photodiode.
LUBRICATION
The cleanliness requirements of the optics made the lubrication an
important design consideration. Any coating of the optics due to outgasslng
would reduce the ultraviolet light throughput of the wave plates. Both gears
and bearings were lightly loaded, but the bearings and spur gears of the wave
plate rotational system had a possible 6.3 x 107 cycle lifetime requirement.
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00000008-TSC06
Tests wore conducted _o Compare low outgassing greas_ a_d also dry lubri-
cant. It was found _h_ grease contamina_Od th_ optles and could no_ bo used.
The dry _ubricaat was applied to all bearings and all spur _oars. It was
s_ll n_eessary to shlold the optics from lln_ of sight relatlonshlp to the
open spur g_ars to prevent any d_y lubrlcant which might separate during
operation from being deposited on_o the optics.
The worm-worm gear interface was a sp_al consld_ration. Dry lub=i-
carlos was not Conside=ed satisfactory because of the sliding action of the
gears. The worm _ear was manufactured from polyimide so no iubrlca_ion would
be required. An antl-backlash worm was considered, but because of =he added
friction and low torque available f=om the motors, it was not used.
PROBLEMS ENCOUNTERED DURING DEVELOPMENT TESTING
i. It was fOun_ that the insertion-retraction system would not operate
co_slstently. The unit would operate satisfactorily one afternoon, then the
next mo_ning with the same el_ctrlcal configuration _he unit would not give
consistent operatlon. After investigation it was found the polyimi<le worm
gear was hydr_scoplc, and an increase of humidity over 50% would cause inter-
ference between the Norm and worm wheel. The qualification unit was installed
into a thermal-humidity Chamber and tested for both temperature and humidity
tolerance. It was found that within _ 20° C. (specified design temperatures)
and below 30% relative humidity, no problems were encountered With the
Insertlon-retractlon system. Humidity in-fllght will not be a problem.
2. The thickness of the dry lubricant applied to the spur gears was
also a problem. The added thickness of the lubricant and the build-up of
dry luhrlcant between the spur gear teeth caused binding between the spur
gears. This problem was solved by brushing the spur gears with a soft wire
brush and removing all dry lubricant except a thin film. Due to the light
loading of the gears, no breakdown of the lubricant was experienced.
CONCLUSION
Three complete units of the polarlmeter have been built, one prototype
and two flight units. The prototype unit was used for development and
qualification testing, One flight unit was dellve_ed to GSFC in January of
1978 for integration into the flight experiment. The flight polarlmeter has
undergone system testing (vibration, vacuum, and optical alignment) and has
met design requirements to this date.
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_'igure 2,- Polarimeter, showing movable motor side.
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OR|GINAL PAGE IS
OF P(X)R QUALITY
00000008-TSC08
Figure 4.- Barrel assembly.
3O9
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00000008-TSC09
"" ' L m
(1) 90 ° Stop Angle
._! (2) 44.6 cm-gm Running Torque
?, (. 62 oz-.iaa)
-_ (3) W_ight 96 g,1 (3.4 oz)
27 mm
_,, 29 rnm _ _
-_" Figure 5 - Stepper motor.
=!y •
!
j: ,
3:
310
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A polarimeter for the high resolution ultraviolet spectrometer/polarimeter

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