**Alignment Measurements of the Microwave Anisotropy Probe (MAP)**

**Instrument in a Thermal/Vacuum Chamber using Photogrammetry**

Michael D. Hill \*^a^, Acey A. Herrera ^b^, J. Allen Crane ^b^, Edward
A. Packard ^a^, Carlos Aviado ^c^**,**

Henry P. Sampler ^a^

^a^ Goddard Space Flight Center, Greenbelt, MD

^b^ Swales Aerospace, Inc., Beltsville, MD

^c^ ManTech, Greenbelt, MD

**ABSTRACT**

The Microwave Anisotropy Probe (MAP) Observatory, scheduled for a late
2000 launch, is designed to measure temperature fluctuations
(anisotropy) and produce a high sensitivity and high spatial resolution
(\< 0.3° at 90 GHz.) map of the cosmic microwave background (CMB)
radiation over the entire sky between 22 and 90 GHz. MAP utilizes
back-to-back Gregorian telescopes to focus the microwave signals into 10
differential microwave receivers, via 20 feed horns. Proper alignment of
the telescope reflectors and the feed horns at the operating temperature
of 90 K is a critical element to ensure mission success.

We describe the hardware and methods used to validate the
displacement/deformation predictions of the reflectors and the microwave
feed horns during thermal/vacuum testing of the reflectors and the
microwave instrument. The smallest deformations to be resolved by the
measurement system were on the order of [+]{.underline} 0.030 inches
(0.762 mm).

Performance of these alignment measurements inside a thermal/vacuum
chamber with conventional alignment equipment posed several limitations.
A photogrammetry (PG) system was chosen to perform the measurements
since it is a non-contact measurement system, the measurements can be
made relatively quickly and accurately, and the photogrammetric camera
can be operated remotely.

The hardware and methods developed to perform the MAP alignment
measurements using PG proved to be highly successful. The PG
measurements met the desired requirements, enabling the desired
deformations to be measured and even resolved to an order of magnitude
smaller than the imposed requirements. Viable data were provided to the
MAP Project for a full analysis of the on-orbit performance of the
Instrument\'s microwave system.

**KEY WORDS:** MAP, photogrammetry, thermal/vacuum, alignment

**1.0 INTRODUCTION**

The Microwave Anisotropy Probe (MAP) Observatory, designed and built by
NASA Goddard Space Flight Center in partnership with Princeton
University, will measure temperature fluctuations (anisotropy) and
produce a high sensitivity and high spatial resolution map of the cosmic
microwave background (CMB) radiation over the entire sky between 22 and
90 GHz. The CMB radiation is the remnant radiant heat left over from the
Big Bang.

MAP will measure the anisotropy of the CMB radiation over the full sky
with an angular resolution of at least 0.3 degrees, a sensitivity of 20
uK per 0.3 degree square pixel, and with systematic effects limited to 5
uK per pixel. MAP will obtain and process differential temperature data,
rather than sensing absolute temperatures, to produce a differential
temperature map of the sky. This will be accomplished by using two
back-to-back optical systems followed by a set of 10 differential
microwave receivers and associated signal processing electronics. The
Observatory will have a compound scan with a spin rate of \~0.454 rpm
and a precession rate of \~1 revolution per hour. This allows MAP to
collect data over a 45° x 180° swath of the sky every hour. The
microwave receivers will cool to \~95 K, while the reflectors will cool
to \~40 K.

Proper alignment of the telescope reflectors and the receiver feed horns
at the operating temperature of 90 K is a critical element to ensure
mission success. The instrument components, made of various materials
(aluminum, copper, composite, etc.), were designed based on deformations
predicted by thermal modeling. In order to ensure that the hardware
behaved as predicted at cold operating temperatures, alignment
measurements would have to be taken during the thermal/vacuum testing of
the prototype and flight hardware. These measurements would have to
resolve the deformations of the critical instrument components as the
hardware reached it\'s operating temperature.

We describe the hardware and methods used to validate the
displacement/deformation predictions of the reflectors and the microwave
feed horns during thermal/vacuum testing of the reflectors and the
microwave instrument. The smallest deformations to be resolved by the
measurement system were on the order of [+]{.underline} 0.030 inches
(0.762 mm). The system chosen to perform these alignment measurements
was the photogrammetric camera system.

**1.1 MAP Hardware Description**

MAP is essentially a dual-telescope differential microwave receiver. The
two telescopes are mounted back-to-back such that each telescope can
collect microwave signals simultaneously from different parts of the sky
as the Observatory spins about it\'s axis. The instrument is divided
into two receivers, an A-side and a B-side, each side having a telescope
and it\'s associated microwave sensing systems. Differencing assemblies
then combine the signals collected by the two sides.

The two major subsystems involved in the PG measurements are the Thermal
Reflector System (TRS) and the Microwave System (MS). The TRS and the MS
mount onto the spacecraft bus, as shown in the exploded view of the MAP
Observatory in Figure 1. The TRS and the MS, when assembled, comprise
the Instrument, as shown in Figure 2.

![](media/image1.wmf) ![](media/image2.wmf)

**Figure 1 - MAP Systems - Exploded View Figure 2 - MAP Instrument -
Assembled View**

The TRS, shown in Figure 3, consists of a truss structure that is
designed to support and maintain alignment of the telescope reflectors,
which focus the microwave energy into the MS. The reflectors were
manufactured by PCI of Anaheim, California, and are constructed of Korex
Honeycomb composite with a vapor deposited aluminum and SiOx coating.

The MS, shown in Figure 4, houses the microwave feed horns and the
microwave amplifiers. The MS consists of the Focal Plane Assembly (FPA)
and the Receiver/Transceiver Box (RXB). The FPA supports and aligns the
20 feed horns (10 feed horns/frequency bands per side) that collect the
microwave sky signals focused by the TRS telescopes, as well as the warm
microwave components that amplify and detect

![](media/image3.wmf)

**Figure 3 - TRS Structure**

![](media/image4.wmf)

**Figure 4 - Microwave System Assembly**

these signals. The RXB houses the warm microwave amplifiers. The FPA,
RXB, and feed horns are constructed of aluminum. The feed horns feed the
signal into copper waveguides and into the RXB.

**1.2 Test Overview**

The primary objective of these measurements was to validate the design
displacement predictions of the MAP hardware during thermal vacuum (T/V)
testing. Two different test articles, the MS and the TRS, would be
tested. This required two separate T/V chamber sessions so that the test
articles could be swapped-out. Both of the test sessions were designed
to perform the displacement measurements at the mission operational
temperature of \~ 60-90 K.

The primary objective of the MS test article configuration was to
validate the feed horn displacement predictions. Secondary objectives
were to verify that there were no permanent deformations of the FPA
structure, and to observe displacement of targets placed on the
Receiver/Transceiver Box (RXB) mounted below the FPA. Deformations on
the order of [+]{.underline} 0.100 inches (2.540 mm) were expected on
the MS test article.

The TRS test article configuration would attempt to validate the TRS
reflector displacement and distortion predictions. Secondary objectives
were to observe deformations of any FPA portions that were visible to
the camera, and to observe flange deformations on certain feed horns to
determine if a gap between the flange and inner bulkhead developed as
the structures got cold. Deformations on the order of [+]{.underline}
0.030 inches (0.762 mm) were expected on the TRS test article.

**1.3 Photogrammetry Description**

The PG system was chosen to perform these measurements because it could
be converted to operate in a hands-off capacity. Since the test articles
would be enclosed in the T/V chamber during the thermal tests, remote
operation was an important capability.

Photogrammetry (PG) is a metrology method that uses digital photographs
and the principals of triangulation to obtain its results^1,2^. The PG
system consists of the following components:

> \- [IN]{.underline}telligent digital [CA]{.underline}mera (INCA)
> (modified Kodak DCS series camera)
>
> \- Laptop with data analysis software (VSTARS) and a network
> connection to the INCA camera
>
> \- Retro-reflective targets & scale bars of various lengths with a
> known coefficient of thermal expansion (CTE)

PG measurements involve taking multiple digital photographs of the test
object (targeted with the retro-reflective targets) from various
positions around the object (to obtain proper triangulation of the
measured target points). Triangulation is performed by the software,
calculating the position of measured points. Resection is then
performed, calculating the position of the camera at the time each
picture was taken. The process of triangulation and resection by the
software is referred to as a software bundle.

The accuracy of the measurements, and the ability of the software to
bundle the shots, is a balance between the targeting scheme (number and
placement of targets on the test object), the number of shots
(photographs) taken, and the positions from which the shots were taken.
The software provides indications of the quality of the job after the
bundle is completed. By verifying that these quality values are within
an acceptable range, the quality of a PG job can be assured.

Since PG measurements are inherently dimensionless, at least one known
distance is required to scale the measurement. Scale bars are placed in
the PG targeting scheme to provide this known distance.

**2.0 DEVELOPMENT EFFORTS**

In order to operate the PG camera system remotely, various support
equipment had to be developed to allow the PG camera to interface with
the T/V chamber and the testing hardware. Remote systems needed to be
developed to power, control, house, and move the camera.

The test articles would be surrounded by the helium shroud, which cools
the test article during thermal testing. Since the helium shroud
completely encloses the test article, the modifications required to
accommodate the PG camera system would need to be made to the helium
shroud.

The helium shroud structure supports twenty-eight cyropanels, providing
a total area of \~ 300 square feet to radiatively cool the test article.
The structure is octagonal in shape, measuring 110 inches (2.79 m)
across and 182 inches (4.62 m) high. The cryopanels are plumbed into a
recirculating helium skid that is capable of providing over 1000 W of
cooling at 20 K. The shroud is flooded with LN2 during cryogenic
operations. The helium shroud crown is lowered onto the helium shroud
walls via a crane once the test article has been placed inside the
shroud. A cylindrical-shaped thermal blanket is used to closeout the
upper section of the helium shroud from the T/V chamber environment.

All support equipment developed had to be able to withstand and operate
under the cryogenic temperatures planned for the test.

**2.1 Camera Power Supply and Network Interface.**

Since the camera was to be enclosed inside the T/V chamber and operated
remotely during testing, external power and network capabilities were
required in order to operate the camera system from outside the T/V
chamber. The PG system vendor specially designed and built a power
supply/network module to provide an external interface with the camera.
The power interface fed the camera and the strobe-flash, while the
network connection established a link between the camera and a laptop
computer outside the T/V chamber.

**2.2 Camera Canister Development.**

Since the PG camera system was designed for operation at room
temperature, and measurements were to performed with the camera inside
the T/V chamber in vacuum and at temperatures of \~ 60-90 K, a
protective environment needed to be developed for the camera. The camera
canister was developed to house the camera and provide a stable
environment. Essentially a closed cylinder with a viewing window (see
Figure 5), the canister provides a pressurized, temperature-controlled
environment for the PG camera.

The canister is equipped with heaters and a fan to offset the cryogenic
temperatures in the T/V chamber. In order to prevent flashback from the
ring strobe (mounted on the front of the camera) off of the outer window
surface and back into the camera, the canister window was designed as a
two-part window with

![](media/image5.wmf){width="3.890972222222222in" height="2.6in"}

**Figure 5 - Camera Canister**

conical-shaped bonding. A bulkhead connector on the canister allows the
power and network connections to be made to the camera.

The camera canister also provided the ability to rotate the canister 90
degrees about the pointing angle of the camera (shots at 0 and 90
degrees were required to satisfy camera self -calibration requirements).
A stepper motor mounted to the canister support frame provided the
canister rotation capability.

**2.3 Camera Carousel Development.**

In order to measure a test article with PG, a method for moving the
camera to various stations around the test article while inside the T/V
chamber was required. The camera carousel, shown in Figure 6, was
designed to allow the camera canister to move \~ 350 degrees around the
perimeter of a test article.

![](media/image6.wmf){width="5.045138888888889in"
height="3.4479166666666665in"}

**Figure 6 - Canister/Carousel Configuration**

Essentially a circular chain-driven track (measuring \~ 110 inches (2.79
m) in diameter), the carousel would be mounted on top of the helium
shroud below the shroud crown. The camera canister would be mounted to
the track and transported around the test article by the carousel. The
carousel would be remotely operated from outside the T/V chamber via
software control.

**3.0 PG MEASUREMENT METHODOLOGY**

Once PG was chosen as the metrology method and the support GSE was
developed to allow PG to be used remotely from outside the T/V chamber,
efforts could be focused on how the measurements would be taken.
Significant efforts were expended to streamline the metrology procedures
and ensure that the measurement methodology was sound, since the
measurements would be taken remotely during the T/V chamber operations.
Interruption or delay of a T/V test to make changes to the camera system
or the targeting would be relatively expensive.

**3.1 Photogrammetric Targeting.**

PG targets are used to define coordinate systems, help the PG software
bundle the job, and provide measurement values for points of interest.
Development of a good targeting scheme design is driven by the shape and
size of the object itself, and the number of stations from which the PG
photos will be taken. For complex structures, various viewing angles
(elevations) are usually required to see the targets. In the case of the
MS and TRS test articles, the camera was restricted to a single plane
(defined by the canister on the carousel), and the structures were quite
complex. The number of possible carousel stations around the objects
were nearly unlimited, but changes in elevation could not be achieved.
Proper design of the targeting scheme for the T/V test was even more
important since access to the test articles could not be gained to add
or adjust targets once the T/V chamber doors were closed.

The types of targets used in the MS and TRS test article targeting
schemes are described below:

**- Coded Targets -** A coded target consists of a series of
retro-reflective dots that are uniquely patterned to make it immediately
identifiable to the PG software. The software uses the coded targets to
correlate the positions of the targets between photographs, helping it
to identify the non-coded targets during the bundling process.

**- Retro-Reflective Dot Targets -** These targets, approximately 0.25
inches (6.35 mm) in diameter, were used to target various points of
interest on structures.

> ![](media/image7.wmf){width="2.7131944444444445in"
> height="2.0458333333333334in"}

**Figure 7 - 45 Degree Angled Target**

**- 45 Degree Angled Targets -** These targets, shown in Figure 7,
consist of a retro-reflective dot placed on a surface angled 45 degrees
from the shank centerline. The angled target and offset these targets
provide allow for greater visibility than surface targets. A tooling
hole is required in the structure being measured for these targets to be
installed.

**- Tooling Hole Bosses (THB\'s) -** Not a target itself, a THB is a
target holder that has been bonded to the structure being measured. Any
variety of targets can be installed into the THB.

Analyses/simulations performed by the PG system vendor helped establish
the best regions for targeting in terms of expected accuracy. The final
number and location of targets within those regions were determined by
GSFC. Test shots of the test article setups were performed to verify the
targeting schemes and to build the software template for the two test
articles. Portions of the final targeting schemes for the MS and TRS
test articles are shown in Figures 8 and 9.

![](media/image8.wmf){width="2.959722222222222in"
height="2.129861111111111in"}

**Figure 8 - MS Targeting Scheme**

![](media/image9.wmf){width="2.8020833333333335in"
height="2.7215277777777778in"}

**Figure 9 - TRS Targeting Scheme**

Two aluminum scale bars were incorporated into each of the test article
setups, which the PG software uses to scale the job. Each scale bar
provided three scaling distances (six targets). The effects of thermal
expansion and contraction of the scale bars needed to be accounted for
since the T/V test environment involved radical changes in temperature.
A scale factor was calculated for each PG run, knowing the temperature
of the scale bars and the CTE of the scale bar material. The test
article/scale bars could not be deforming at the time of the PG
measurement. A temperature change rate of [\<]{.underline} 1 ^o^ C per
hour was required before a PG run (series of PG shots) could be made.
Thermal instrumentation (platinum resistance thermometers were placed on
various portions of the test article and on the scale bars) provided
real-time temperature data of the test article and the scale bars.. The
rate of temperature change was verified before each PG measurement. In
addition, the scale bars were thermally isolated from the structure with
blocks of G-10 isolation material.

**3.2 PG Software Template.**

Due to the large number of PG runs planned for each test article,
attempts were made to minimize the processing time for each run and
streamline the bundling process. To accomplish this, a template job was
developed for both the MS and TRS test article setups. A template file
provides the following advantages:

1\. [Automation of Bundling Process]{.underline}:

> When a job is bundled, the software looks for points in the
> photographs and assigns its own labels to the target points it
> identifies. Each of these labels must be manually changed to reflect
> the labels defined by the targeting scheme. Normally, this process
> would be required for each set of pictures that are bundled. Use of a
> template job limits this renaming process to only the first bundle.
> The template job automates the bundling process by storing the name
> and location of desired targets. When using a template job on a new
> set of pictures, the software looks for these targets stored in the
> template, and automatically names them. The template job is created
> after the labels on the first set of pictures has been renamed to be
> compatible with the targeting scheme. For measurements requiring
> multiple PG runs of the same setup (as is the case with the MS and TRS
> test articles), the use of a template file saves time and effort on
> the part of the PG operator.
>
> Another advantage of a template is that it forces the software to look
> only for those points stored in the template file. Without a template,
> the software searches for all possible targets in the pictures. This
> can be a problem if there are non-targets such as glints or
> reflections in the pictures. Any errant/mistaken targets would have to
> be manually removed during the bundling process. Again, use of the
> template saves time and effort on the part of the operator.

2\. [Establishing a Coordinate Reference Frame (CRF)]{.underline}:

> When a set of pictures is bundled, a default CRF is generated by the
> software. The bundle data needs to be transformed from the local CRF
> to the test article CRF before data can be analyzed. The test article
> CRF is defined by a subset of the measured points (that have known
> coordinates in the test article CRF). Once collected, this design data
> is imported into the software template file. The software then
> determines a best fit between the measured points and the design data.
> The end result is a bundled job containing measured points that have
> been best fit and scaled to the design data.
>
> In the case of the MS test article, 12 points along the sides of the
> FPA (6 per side) were used as design data and were imported into the
> software template to define the instrument CRF. These same points had
> previously been measured and valued in the CRF using two other
> metrology systems. When the MS PG runs were bundled, the FPA targets
> were best-fit to the design data. The remaining targets (coded,
> aperture, RXB, etc.) were allowed to "float" (the software did not
> include them in the best fit).
>
> For the TRS test article the TRS CRF was defined by a subset of the
> THB targets mounted on the truss structure. These targets had been
> previously measured and valued in the CRF by the manufacturer of the
> TRS structure and GSFC using various metrology systems. The
> manufacturer data was imported into the software template for the TRS
> test article as design data. As with the FPA test article
> measurements, the remaining targets (coded, FPA, reflector, etc.) were
> allowed to float.

The development and use of a software template file for the MS and TRS
test articles successfully automated the processing of data for each of
the PG runs, significantly reducing the amount of time required to
bundle each run.

**3.3 Error Analysis.**

An error analysis was required to ensure that the measurement
requirements for the two parts of the T/V test were met. It was
determined that the camera system itself and the targeting were the
major sources of error for these measurements.

The measurement precision of the photogrammetric camera is
[+]{.underline} 0.002 inches (0.05 mm) at a range of 8 ft. (2.44 m) (2
σ). The separation distance between the camera and the targets was less
than 8 ft. for all of the pictures taken in the T/V chamber.

The PG camera is self-calibrated as a byproduct of the measurement.
There are three requirements to achieve self-calibration. The first is
to roll the camera 90 degrees for half of the pictures taken. Second,
there is a minimum number of pictures that must be taken from a minimum
number of locations. Third, there is a minimum number of
well-distributed targets visible in each picture and for the entire test
article. All of these requirements were addressed during the development
of GSE and the targeting scheme for the T/V tests.

Through the process of bundling, the PG software provides indications of
job quality and the occurrence of errors. By carefully examining the
software quality/error indicators and through proper use of the camera,
the worst-case error contribution from the PG camera itself is
[\<]{.underline} [+]{.underline} 0.002 inches (0.051 mm)

Target placement and target repeatability errors can be attributed to
the PG targets used on the FPA during the T/V test. The overall target
error contribution (a combination of the these two errors) was
calculates to be [\<]{.underline} [+]{.underline} 0.0015 inches (0.038
mm).

Taking all of the above errors into consideration, the total error for a
given PG run can be established to be [\<]{.underline} [+]{.underline}
0.0035 inches (0.089 mm). This ensures that all measurements will be
able to resolve the [+]{.underline} 0.100 inch (2.54 mm) and
[+]{.underline} 0.030 inch (0.762 mm) deformations that were predicted
for the MS and TRS test articles, respectively.

**4.0 PG MEASUREMENTS/ANALYSIS**

A PG run consisted of a series of pictures taken at various carousel
stations around the test article. Two pictures were required at each
station to facilitate the camera self calibration - one with the
canister at 0 degrees and the other with the canister at 90 degrees. The
number and location of carousel stations was predicted and tested during
the pre-chamber targeting verification tests.

For each test article, the first PG run in the T/V chamber was performed
at ambient temperature and pressure to establish a pre-test baseline and
to verify the carousel stations. As the first few PG runs progressed,
the number of carousel stations and their locations around the test
article were modified slightly to optimize the camera\'s view of the
test article targeting.

**4.1 PG Runs.**

The final carousel station layout for the MS and TRS test articles are
shown in Figures 10 and 11. A total of 44 stations were used during the
MS test article PG runs, resulting in 88 PG shots (camera at 0 and 90
degrees at each station). The TRS test article PG runs consisted of 42
carousel stations, resulting in 84 PG shots. It took the camera operator
and carousel operator \~ 30 minutes to obtain all of the shots in a PG
run.

![](media/image10.wmf){width="2.902083333333333in"
height="3.9590277777777776in"}

**Figure 10 - MS Carousel Layout**

![](media/image11.wmf){width="2.984027777777778in"
height="3.9541666666666666in"}

**Figure 11 - TRS Carousel Layout**

Once the PG shots for a given PG run were taken, the run was bundled
using the template job. Once the bundle was deemed successful (\~ 20
minutes of processing and analysis time by the operator on the laptop
computer), the PG operator would give concurrence for the test to
proceed to the next test (temperature) point.

A series of PG runs had been planned for the T/V testing as a baseline.
However, the number of PG runs and their timing was changed as the T/V
testing progressed. A total of six PG runs were made for the MS test
article, while five PG runs were made for the TRS test article.

Each MS and TRS test article PG run was bundled using the template to
automate the job bundle. Each job was transformed into the appropriate
coordinate reference frame via a best fit with design data (present
within the job template). An EXCEL™ program was used to determine the
scale factors and CTE for the scale bars at each temperature case. Once
the job was scaled, the measured points were exported to an EXCEL™
program in order to compare the runs. Data tables were developed to
present the results of the measurements. For each of the two parts of
the test, each of the PG runs was compared to all of the other runs to
determine the target displacements from case-to-case.

**5.0 PG MEASUREMENT RESULTS.**

Both the MS and TRS test articles were thermally tested in the T/V
chamber at cryogenic (\~ 60-90 K) temperatures. PG measurements were
successfully made using the PG system remotely from outside the T/V
chamber. Brief summaries of the results of these measurements are
provided in the sections below.

**5.1 MS Test Article Results.**

The primary measurement objective of the MS test article configuration
was to validate the feed horn displacement predictions. Secondary
objectives were to observe any displacement/rotation of the RXB and to
verify that there were no permanent deformations of the FPA structure.

The following result summaries were derived from the data obtained
during the six PG runs of the MS test article:

**[Entrance Aperture Translations]{.underline}:**

The feed entrance aperture translations were all within 10 percent of
predictions. Translations were on the order of + 0.100 inches.

**[RXB Motion]{.underline}:**

RXB motions were determined by least squares fitting the warm positions
to the cold positions. The resulting translations and rotations of the
RXB were as expected, based on hand calculations.

**[Post-Test Residuals]{.underline}:**

The final ambient measurement data was compared to the initial ambient
measurement data to identify residual deformation of the structure.
There were no apparent residual deformations due to thermal testing,
based on a comparison of 164 targets. The standard deviations for
residuals in each coordinate axis were 0.001 inches (0.025 mm). The
maximum residual was 0.004 inches (0.102 mm). The estimated PG
measurement error is [+]{.underline} 0.0035 inches (0.089 mm).

**5.2 TRS Test Article Results.**

The primary measurement objectives of the TRS test article configuration
were to validate the TRS reflector displacement and distortion
predictions and derive a CTE value for the composite truss structure.
Secondary objectives were to validate the gapping between the feed
flanges and the FPA inner bulkhead and to verify that there were no
permanent deformations of the TRS structure.

The following result summaries were derived from the data obtained
during the five PG runs of the TRS test article:

**[Reflector Deformations/Position]{.underline}:**

The measured reflector deformations confirmed measurements that were
made previously on the Reflector Engineering Unit (REU). Due to
difficulties in accurately modeling the complex curved honeycomb
composite reflectors, measurements at temperature were relied on to
determine the shape deformations. Consistency in the results provided
confidence in the shape values. Overall deformations, removing rigid
body motion, were on the order of [+]{.underline} 0.030 inches for the
primary reflectors and [+]{.underline} 0.015 inches for the secondary
reflectors.

Reflector position changes due to cool-down revealed a discrepancy in
the truss CTE values. The secondary reflectors translated inward, where
they were predicted to move outward, due to a negative truss CTE.

**[Truss CTE]{.underline}:**

The apparent CTE of the truss structure was +4.9E-7 in/in/K, based on
the PG measurements. The predicted CTE based on laminate properties was
--5.1E-7 in/in/K. Although uncertainty was later placed on both CTE
values due to simplifying assumptions, the test revealed secondary
reflector translations to be opposite of that predicted and ultimately
resulted in slight repositioning of replacement secondary reflectors.

**[Flange gapping]{.underline}:**

Targets were added to the FPA structure for measuring flange gapping
during the TRS testing. Gaps of 0.002 - 0.006 inches were measured. The
values compared well with the 0.005 inches predicted for the 4 feeds
measured.

**[Post-Test Residuals]{.underline}:**

The final ambient measurement data was compared to the initial ambient
measurement data to identify residual deformation of the structure.
There were no apparent residual deformations due to thermal testing,
based on a comparison of 185 targets. The standard deviations for
residuals in each coordinate axis were 0.002 inches (0.051 mm). The
maximum residual was 0.007 inches (0.178 mm). The estimated PG
measurement error is [+]{.underline} 0.0035 inches (0.089 mm).

**6.0 CONCLUSION**

The PG measurements under cryogenic T/V chamber conditions were a
success based on the data that has been analyzed and presented. The
concept and design of the camera canister and carousel GSE worked well,
providing a flexible metrology platform for the PG camera. The targeting
schemes that were developed for each test article provided visibility of
the regions in-question, as well as the accuracy required to measure
their distortions.

With the combined MS and TRS PG runs measurements were obtained for feed
translations, flange bulkhead gapping, RXB motion, reflector
deformations, and reflector motion resulting from transition to
cryogenic temperatures. These measurements were successfully made in
parallel with the thermal performance test objectives.

[]{.mark}

Overall, the MAP optical measurement test objectives were met using
these techniques in the thermal/vacuum environment.

**ACKNOWLEDGEMENTS**

The authors would like to thank the PG System vendor, Geodetic Services
Inc., for their hardware design and development effort, as well as their
support in getting the most out of the measurements. Our thanks also go
out to Mantech Services Inc. for the technical support in preparing and
24 hour operation of the T/V chamber, to Dean Osgood of Mantech Services
Inc. for the theodolite, laser tracker, and photogrammetry measurement
support, and to our Mechanical Systems Analysis & Simulation group for
mechanical systems analysis.

**REFERENCES**

1\. Elements of Photogrammetry, Paul R. Wolf, McGraw Hill 1983

2\. Automation in Digital Close-Range Photogrammetry, Clive S. Fraser,
First Trans Tasman Surveyors Conference, 12-18 April, 1997, New Castle