In-situ monitoring of the Wide-Field/Planetary Camera, a Hubble Space Telescope science instrument, was performed in a vacuum environment to better understand the formation of ice on cooled optical detectors. Several diagnostic instruments were mounted on an access plate to view the interior of the instrument housing and the graphite epoxy optical bench. The instrumentation chosen and the rationale for choosing the instrumentation are discussed. In addition, the performance of the instrumentation during monitoring operations is discussed

Hansen, Patricia A.

Jenkins, Teresa K.

Maag, Carl R.

Taylor, Daniel M.

English

NASA Technical Reports Server

N89-12585
CHARACTERIZATION OF A GRAPHITE EPOXY OPTICAL BENCH
DURING THERMAL VACUUM CYCLING 1
Patricia A.Hansen, Teresa K. Jenkins,
Carl R. Maag, and Daniel M. Taylor
Jet Propulsion Laboratory
California Institute of Technology
ABSTRACT
In-situ monitoring of the Wide-Field/Planetary Camera, a Hubble Space
Telescope science instrument, was performed in a vacuum environment to better
understand the formation of ice on cooled optical detectors. Several
diagnostic instruments were mounted on an access plate and viewed the interior
of the instrument housing and the graphite epoxy optical bench. The
diagnostic instruments were comprised of the following: a TQCM (Temperature-
Controlled Quartz Crystal Microbalance), a Pressure Gauge and an Optical
Witness Sample. This paper describes the instrumentation and the rationale
for choosing this instrumentation. In addition, the performance of the
instrumentation during monitoring operations will be presented.
INTRODUCTION
An in-depth contamination study of the Wide-Field/Planetary Camera
(WF/PC) was conducted by the Jet Propulsion Laboratory (JPL). 2 As part of
this study the formation of ice on the cooled detectors was investigated.
During previous system thermal vacuum tests, a contaminant, thought to be ice,
was observed on the detectors. This contaminant was easily removed when the
detectors were warmed up while under vacuum. After this period, the detectors
were cooled and no contaminant, or ice, was seen on the detectors during
imaging.
This phenomena was thought to be the result of water desorption from the
graphite epoxy optical bench material (ref. i). To better understand this
phenomena, several diagnostic instruments were proposed for in-situ monitoring
i This work has been carried out by the Applied Technologies Section,
Jet Propulsion Laboratory, California Institute of Technology and was
sponsored by the National Aeronautics and Space Administration.
2 Carl R. Maag: Results/Findings of the WF/PC Contamination Team, Draft.
JPL Internal Document, Jet Propulsion Laboratory, Pasadena, CA, May 1987.
12
https://ntrs.nasa.gov/search.jsp?R=19890003214 2020-03-20T05:50:21+00:00Z
of the WF/PCduring a system thermal vacuumtest. The diagnostic instruments
included a Temperature-Controlled Quartz Crystal Microbalance (TQCM),a
Pressure Gaugeand an Optical Witness Sample (OWS). These instruments were
mounted on an access plate and viewed the interior of the instrument housing
and the thermally blanketed graphite epoxy optical bench.
One of the principal ground rules was that the diagnostic instrumentation
could not introduce contaminants into the WF/PChousing and could not change
the existing heat flow paths. Thus the instrumentation was subjected to
rigorous preconditioning prior to installation on the WF/PChousing. In
addition, only low outgassing materials were used on the access plate.
During the system thermal vacuumtest additional diagnostic
instrumentation and contamination monitoring devices were used to measure the
test environment. Only the access plate instrumentation will be discussed in
this paper. A summaryof the access plate instrumentation results is also
presented. An in-depth discussion of the TQCMmeasurementtechniques and data
reduction is the subject of another paper included in this conference
(ref. 2).
WIDE-FIELD/PLANETARY CAMERA
The WF/PC is the radial science instrument on the Hubble Space Telescope
(HST) developed and built by the JPL and the California Institute of
Technology. The scientific objectives of the WF/PC are to provide
photometrically and geometrically accurate, multiband observations of stars
and extended sources over a wide field-of-view (FOV) on the HST, and to
provide very high angular resolution photometrically and geometrically
accurate, multiband images of the solar system and its astronomical objects.
The WF/PC contains two complete optical relay anddetector systems as
shown in Figure i. Each system is capable of producing a four-part image
mosaic. One relay system operates for wide-field work and the other operates
for planetary or high resolution work. Only one optical system provides
images at any given time.
Once the relay optics intercept the light, they reflect it through the
primary and secondary optics to the detectors located in the rear of the
optical assemblies (in the camera heads). The eight (8) detectors (one in
each of the four camera heads for either the wide-field or planetary system)
contain charge coupled devices (CCDs). The CCDs are cooled to a nominal
temperature of -90°C to suppress their dark current and to improve their
performance.
WF/PC CONTAMINATION STUDY
The WF/PC contamination study investigated the formation of ice on
critical surfaces such as the CCD windows and the Thermoelectric Coolers
13
(TEC). The goal of this investigation was to prevent the formation of ice on
optical surfaces and to develop operational strategies to respond to ice
formation if it reoccurred during flight operations.
Models predicting the formation of ice were developed to define and
quantify the parameters associated with this phenomena. As this phenomenawas
thought to be the result of water desorption from the graphite epoxy optical
bench material, a model of the dewpoint inside the housing was derived for
external vacuumconditions. 3 Whenthe temperature of a surface such as the
CCDwindows or TECsare below the dewpoint temperature (and below the
freezing point of water) ice will form on these surfaces. This dewpoint
model was then used to develop operational strategies for cooling the
detectors.
Several layers of MLI (multilayer insulation) blanket were removedfrom
the housing to decrease the nominal operating temperature of the WF/PCby
several degrees. Scenarios were developed to predict the time delay after
launch for the dew point temperature of the out_assing water, or contaminant,
to be below the temperature of the CCDwindows._ The operating temperature of
the detectors was raised ten degrees to allow early operation of the
detectors. It is believed that these changeswill result in ice free
operation of the cooled detectors 30 days after launch.
The diagnostic instrumentation was proposed for in-situ monitoring of the
optical bench and housing to quantify the initial parameters of the derived
models. A TQCMwas employed to measure the adsorption and desorption of
volatile condensible material (VCM)on the surfaces in the WF/PC,primarily
the graphite epoxy optical bench and the electronics. The pressure gaugewas
employed to measure the initial water content of the optical bench (i.e. the
partial pressure of water). The OWSwas used to measure any change in
reflectance of an optical surface due to the deposition of outgassed material
on that surface. The access plate was a structural memberonly.
DIAGNOSTIC INSTRUMENTATION
INSTRUMENT CONFIGURATION
The diagnostic instrumentation was mounted on the access plate as shown
in Figure 2. The operational constraints of the pressure gauge required it be
mounted on an extended inlet tube outside the WF/PC shrouds. The inlet tube
3 Jack B. Barengoltz: A Model of the WF/PC Dew Point. JPL Interoffice
Memorandum No. 3543:87:0010, Jet Propulsion Laboratory, Pasadena, CA,
January 15, 1987.
4 Jack B. Barengoltz: WF/PC Thermal Blankets Revisited. JPL Interoffice
Memorandum No. 3543:87:0084, Jet Propulsion Laboratory, Pasadena, CA, May 14,
1987.
14
was mounted on the access plate as shownin Figure 2. The access plate was
mounted on the WF/PChousing as shownin Figures 3 and 4.
ACCESSPLATE
The goal of the monitoring was to better understand the formation of ice
on the detectors using "passive" measurementtechniques. As such, it was
imperative that the access plate and diagnostic instrumentation did not change
the existing WF/PCheat flow paths or introduce volatile condensible material
(VCM)into the housing.
VESPEL(SP-I) was chosen as the material for the access plate as it
provided good thermal isolation from the WF/PChousing and the vacuumchamber
shrouds, was easily machined and was a low outgassing material. The access
plate was subjected to a 80°C ± 2°C vacuumbakeout for 36 hours at i x 10.6
torr prior to installation onto the WF/PChousing. Swabsampling was
performed prior to installation to verify that the access Elate would not
contaminate the interior housing of the WF/PCduring test. J
Freon TF (Burdick-Jackson HPLCpurity l,l,2-Trifluorotrichloroethane) was
used to clean the access plate and the external surfaces of the diagnostic
instrumentation prior to installation onto the WF/PChousing. The NVR(nonvolatile residue) was removedfrom these surfaces and thus prevented the
introduction of organic contaminants into the WF/PChousing.
TEMPERATURE-CONTROLLEDQUARTZCRYSTALMICROBALANCE
A temperature-controlled quartz crystal microbalance (TQCM)performed in-
situ measurementsof adsorbed and desorbed VCMon surfaces in the WF/PC,
primarily the graphite epoxy optical bench and electronics. The TQCMwas
instrumented with 15 MHzoptically polished crystals (ref. 3). The crystals
were coated with aluminum to be representative of a reflective optical
surface. -_
The heat generated by the TQCMelectronics was removedby a gold plated
copper heat sink which was thermally coupled to a heat exchanger maintained at
10°C ± 5°C. This provision, suggested by the manufacturer, provided increased
accuracy of the TQCMmeasurements(ref. 3). In addition, this provision
allowed the contamination loading from the WF/PCoptical bench and housing to
be monitored without changing the existing heat flow paths.
The TQCMmonitored the access plate vacuumbakeout. As a monitoring
device, the TQCMwas subjected to a 80°C ± 2°C vacuumbakeout for 36 hours.
5 Patricia A. Hansen: Results of the WF/PCAccess Plate Bakeout.
Interoffice MemorandumNo. 354C:88:0012 (ST-DFM1140), Jet Propulsion
Laboratory, Pasadena, CA, January 21. 1988.
JPL
15
Prior to installation on the access plate, the exterior surfaces of the TQCM
were cleaned.
PRESSUREGAUGE
The pressure gaugewas a Series 275 Convectron gauge from Granville-
Phillips Company(ref. 4). It was capable of providing a pressure measurement
from i000 torr to 10-3 torr. The gauge tube contained a temperature
compensatedheat loss sensor which utilized conduction cooling to sense
pressure at lower pressures. At higher pressures, it utilized convection
cooling in which gas molecules were circulated through the gauge tube by
gravitational force.
Prior to installation on the access plate, the pressure gaugewas
calibrated to extend the nominal useful range from 1 x 10.3 torr to 2 x i0 -4
torr The pressure gau_e was also subjected to a 80°C ± 2°C vacuumbakeout
for 24 hours at 1 x i0 "Q torr. The exterior surfaces of the pressure gauge
were cleaned prior to installation on the access plate.
The pressure gauge inlet tube was extended to allow the pressure gauge to
be mounted on the access plate but reside several inches away from the access
plate due to operational constraints. During operation, the pressure gauge
was mounted to remain in a horizontal position and at ambient temperatures.
Although not shownmountedon the access plate in Figure 4, the pressure gauge
is presented in Figure 5.
OPTICALWITNESS AMPLE
The optical witness sample (OWS)was an optically polished aluminum
sample. The sample was coated with MgF2 to simulate an optical surface. The
OWSwas installed in a holder for ease of handling prior to installation. An
example of the OWSinstalled in a holder is shownmounted on the external test
fixture in Figure 6.
The reflectance of the OWSwas measuredat several wavelengths (1216 A,
1608 A, 2200 A, 2537 A and 3130 A) prior to installation on the access plate.
The OWSholder was vacuumbaked with the access plate. 5 The exterior surfaces
of the holder were cleaned prior to the OWSinstallation.
THERMAL VACUUM TEST MONITORING
The TQCM and pressure gauge continuously monitored the WF/PC optical
bench and housing during the system thermal vacuum test. The WF/PC test
profile simulated three flight operational conditions: nominal operational
conditions, a flight hot case and a flight cold case. During these profiles
16
the temperature of the TQCMwas varied to measurethe relative volatility of
the surface contamination from the optical bench and housing.
The OWSwas continuously exposed to the WF/PChousing and optical bench.
The OWSwas not temperature controlled and was coupled by radiation to the
housing and optical bench. Thus the OWSfollowed the temperature profile of
the housing and optical bench and varied from approximately 35°C to -5°C.
RESULTS
The temperature of the TQCM was varied to measure the relative volatility
of the surface contamination from the optical bench and housing which was held
at a constant temperature during the test profiles. Self-contamination of the
TQCM was considered highly improbable due to the rigorous preconditioning
program of the access plate and diagnostic instrumentation. Therefore, the
contamination source was internal to the WF/PC. The TQCM measured two groups
of contaminants as a result of varying its temperature and the optical bench
and housing temperature. As a result of these measurements, it was concluded
that the TQCM collected an organic contaminant, not ice, when cold. The
source of this contamination was not identified as the optical bench and
housing are composed of many subassemblies (e.g. electronics, camera head,
mechanisms, etc). However, additional measurements indicated the
contamination source was strongly correlated to the operation of the
electronics bays. Although surface contamination was detected by the TQCM,
additional temperature cases showed that the detectors could be outgassed
without detector performance loss. An in-depth discussion of the TQCM results
is the subject of another paper included in this conference (ref. 2).
During operation, the TQCM was not operated at a temperature cold enough
to collect ice. For operational temperatures below -60°C, the manufacturer
suggests using a CQCM (Cryogenic Quartz Crystal Microbalance) (ref. 3).
However, ice formation on the detectors (i.e. the CCD windows) was not
expected as the amount of adsorbed water in the graphite epoxy optical bench
and housing had been limited by the use of an ultradry nitrogen purge prior to
test and the use of operational strategies during the test. The ultra-dry
nitrogen purge system and the operational strategies are the subject of
another paper included in this conference (ref. 5).
The TQCM measurements identified that water (ice) was not the only
contaminant inside the WF/PC housing and optical bench. Flat field imaging
was performed to verify that the operational strategies did indeed result in
ice free operation of the cooled detectors (ref. 6). These images were not
obscured as in previous tests. Therefore, these operational strategies
resulted in ice free operation of the cooled detectors_
The pressure gauge was initially recommended so that its measurements
could be used to derive the partial pressure of water inside the WF/PC, it was
found that these measurements needed to be extended into the 10 -5 torr range.
In the event of ice formation on the CCDs, the pressure gauge would have been
used to determine the actual vapor pressure of the ice, or contaminant. In
17
addition, the pressure gauge would have been used to determine the vapor
pressure of the ice during the detector warmup/ice removal sequences.
As the detectors did not collect ice, the pressure gauge was only used to
measure the pressure differential between the WF/PC housing and the vacuum
chamber during chamber pump down and purging sequences. The rate of diffusion
of the purge gas out of the camera and the effectiveness of the vent tube were
measured. These measurements were correlated with the previous predictions. 6
The OWS was measured immediately after test. The results of these
measurements showed the reflectance of the OWS did not degrade beyond three
(3) percent at Lyman-Alpha wavelengths. The OWS reflectance measurements
indicated that if warmed to room temperature and atmospheric pressure, the
contaminant did not cause a loss in reflectance on an optical surface (i.e.
the CCD windows).
CONCLUSIONS
Significant information was obtained with in-situ monitoring of the WF/PC
optical bench and housing during a system thermal vacuum test. The diagnostic
instrumentation was an invaluable tool in monitoring the internal
contamination. The instrumentation allowed the internal environment of the
WF/PC to be monitored without altering the functional requirements or nominal
operating requirements of the WF/PC.
The TQCM performed in-situ measurements of adsorbed and desorbed VCM on
the internal surfaces of the WF/PC. The TQCM measured an organic contaminant,
less volatile than water, correlated to the operation of the electronics bays
within the WF/PC. The OWS reflectance measurements indicated that if warmed
to room temperature and atmospheric pressure, the contaminant did not cause a
loss in reflectance on an optical surface.
Although the TQCM temperature was not cold enough to simulate the cooled
detectors, verification of ice free operation of the cooled detectors was
performed with flat field imaging. The flat field images were not obscured,
thus verifying ice free operation of the cooled detectors.
6 Jack B. Barengoltz: Actual WF/PC Venting and Implications for Water
Modeling. JPL Interoffice Memorandum No. 354:JB:88:0129, Jet Propulsion
Laboratory, Pasadena, CA, May 2, 1988.
18
REFERENCES
. Springer, G. S.; et. al.: Environmental Effects on Composite Materials.
Westport, CT:Technomic, 1981.
. Taylor, D.; Barengoltz, J; Jenkins, T.; Leschly, K.; and Triolo, J.:
WF/PC Internal Molecular Contamination During System Thermal-Vacuum Test.
15th Space Simulation Conference, NASA CP-, 1988. (Paper of
this compilation.)
. Extended Range TQCM Engineering Report, Revision C. FAR°78-012, Faraday
Laboratories Inc., La Jolla, CA, April 24, 1986.
. Wide Range Convectron Vacuum Gauges from Granville-Phillips. Product
Literature No. 275 177 I00, Granville-Phillips Co., Boulder, CO, February
1987.
° Hansen, P.; Maag, C.; Argoud, M.; Barengoltz, J.; and Taylor, D." Wide-
Field/Planetary Camera Contamination Control Assessment. 15th Space
Simulation Conference, NASA CP-, 1988. (Paper __ of this
compilation.)
. Westphal, J.: Science Performance Summary. JPL Document No. D-5322
(CM-02), WF/PC I Delta Pre°Ship Review, April 6, 1988.
19
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UV F LOOD
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22
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24
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Session II
CONTAMINATION II
PREOED1NO PAGl_ BLANK NOT F'ILMED


Characterization of a graphite epoxy optical bench during thermal vacuum cycling

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