The Precision Segmented Reflector (PSR) program currently under way at the Jet Propulsion Laboratory is a test bed and technology demonstration program designed to develop and study the structural and material technologies required for lightweight, precision segmented reflectors. A Figure Verification Sensor (FVS) which is designed to monitor the active control system of the segments is described, a best fit surface is defined, and an image or wavefront quality of the assembled array of reflecting panels is assesse

Macenka, Steve A.

Manhart, Paul K.

English

NASA Technical Reports Server

PRECISION SEGMENTED REFLF__TOR ,
FIGURE VERIFICATION SENSOR
Paul K. Manhart
Steve A. Macenka
Optical Sciences and Applications Section 385
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California'
Z.>,_ - /9
2J9 ¢_,7
N90-19272
369
https://ntrs.nasa.gov/search.jsp?R=19900009956 2020-03-19T22:55:04+00:00Z
ABSTRACT
The Precision Segmented Reflector (PSR) program currently under way at the Jet Propulsion
Laboratory is a test bed and technology demonstration program designed to develop and study the
structural and material technologies required for lightweight, precision segmented reflectors. This
paper describes a Figure Verification Sensor (FVS) which is designed to I) monitor the active control
system of the segments, 2) to define a 'best fit' surface and to 3) assess image or wavefront quality of
the assembled array of reflecting panels.
INTRODUCTION
The need for large diameter optical instruments for astronomical research has been recognized
since the invention of the telescope. Large diameter optical systems are attractive for their
increased light gathering capability and angular resolution in object space. Large diameter optics
allow one to see fainter objects, and to see them in more detail. Many future missions in
astrophysics and spacecraft optical communications will utilize very large diameter, precision
reflectors. However, there is a practical limit for ground-and spac.e-based optical instruments with
monolithic mirrors. With ground-based systems you have exponentially increasing cost for the
telescope mounts and support structures as the weight of the mirror goes up and Weight and mass scale
as the diameter Is cubed. Atmospheric cell s-ize and thermal equilibrium problems also tend to limit
the usefulness of very large diameter ground-based telescopes. For space-based telescopes, the
limiting factors are launch weight, serviceability, fabrication cost and risk. In short, weight,
fabrication difficulties, and high costs for high quality, large aperture mirrors are driving factors
which put an upper limit on aperture size for ground-based telescopes and space systems. A new
technology therefore is needed to make these missions feasible.
A variety of new technologies, such as lightweight graphite/epoxy reflective panels, and studies, such
as LDR, have opened new avenues toward being able to build affordable, large aperture reflectors.
However, additional technologies are required to make these segmented reflector systems feasible.
Many of these challenges are currently being addressed by the Precision Segmented Reflector (PSR)
program at the Jet Propulsion Laboratory.
r
37O
TRIPOD
SUPPq
|
PANELS I
I
J PANELS
I
FIGURE
VERIFICATION
SENSOR
SECONDARY
SUPPORT
DESIGN AND FABRICATE
TECHNOLOGY VALIDATION
DEMONSTRATION HARDWARE
DEMONSTRATE ACTIVE CONTROL
OF SIX PANELS
USE AN ABSOLUTE FIGURE
SENSOR TO VERIFY FIGURE
DEMONSTRATE FIGURE CONTROL
UNDER REALISTIC THERMAL
GRADIENTS
TEST VIBRATION SUPPRESSION
CAPABILITIES
MEASURE EFFECTS ON FIGURE
DUE TO VARIOUS VIBRATION
CON DI TI ONS
TEST
STAND
I
CONTROL
CONSOLE
FIGURE 1. PSR TEST BED
PSR is a Civilian Space Technology Initiative (CSTI) program at JPL serving as a test bed and
technology demonstration of a large diameter (5 meter), parabolic, actively controlled, segmented
mirror telescope. The I r- meter segments are hexagonal in shape and composed of epoxy/graphite.
The major objectives of the PSR effort are 1) to develop the enabling technologies for advanced,
large, lightweight segmented reflector systems for space and 2) to validate design concepts for
actively conffoIIed, multi-segmented, precision reflectors by means of a system demonstration.
The crKicalnew technology areas required to make space-base¢_, Segmented reflector systems
feasible include ,le lightweight reflector panels, the interface structure for connecting the panels,
the control ann measuremen[ system needed to maintain extremely precise alignment and dynamic
stability of the optical components, and advanced optics (such as two-stage optics and internal
wavefront sensing). Each of these technology areas must be addressed and advanced in order to
evaluate and develop lightweight segmented optics. The PSR test bed, (see Figure 1) concentrates on
the lightweight segmented reflector along with the interface structure and control system. To
evaluate and develop these new techno]ogies_ however, a method must be devised which can quantify
the behavior and performance of the system.
371
FIGURE VERIFICATION SENSOR (FVS)
The first set of requirements for PSR optics is to design a Figure Verification Sensor (FVS) to
monitor the optical quality of the wavefront and the behavior of the control system used to position
the lightweight reflectors. Since PSR is primarily a materials and structures development program,
the FVS should be responsive to structural changes due to thermal gradients and vibrations.
The requirements of the FVS are to i) quantify the behavior of the control system, and 2) quantify
the optical quality of the assembly of panels. The initial positioning of each panel is assumed to be
_+1 mm and the smallest increment in positional change that must be detected is 0.1 lJm. The
requirements for the FVS are difficult to achieve with a single instrument for the following reasons.
I) The dynamic range out to I mm precludes most conventional optical techniques that
measure wavefront quality.
2) The expected surface error of the epoxy/graphite panels is 3-5 fan RMS. Figure errors of
this magnitude limit the degree to which 'best focus' can be determined; thus, the figure
errors tend to mask the minute changes in panel position and put a limit on resolution.
Assuming perfectly made parabolic segments, it would be a simple task to bring all the panels into
the range of a visible interferometer with a star, or focus test, which would phase each panel segment
with respect to some common focal point. Conventional interferometric techniques in the visible
would then be used to finely tune the system and accurately monitor any change in panel position
due to structural or control changes. A 3-5 _ RMS surface error on each panel, however, gives 12-20
waves RMS optical path difference in the visible. This magnitude of error gives about a +0.5 mm
focus uncertainty when using a star test. It also means that there could easily be 2_ ambiguities at
the edges of the panels. These ambiguities preclude full aperture, single wavelength interferometry.
The precision of fitting the focus and tilt terms of Zemike polynomials on individual, off-axis,
parabolic segments gives at best a +50 l_n repeatability, provided the fringes can be analyzed.
BASELINE FIGURE VERIFICATION SENSOR DESIGN
The baseline FVS design consists of a Shack-Hartmann setup with a battery of integrated optical
tests all contained in a compact, versatile package which gives precise information on panel tilt,
piston and wavefront quality. In conjunction with the Shack-Hartmann test, the FVS utilizes a dual
wavelength laser distance measurement system to accurately monitor panel motion as described by
piston. Figure 2 shows a conceptual layout of the FVS testing the PSR primary mirror in a paraxial
center of curvature,(null lens') configuration. The laser distance measurement system is not shown
in the figure but is assumed to have one laser beam per panel which is retumed to the interferometer
via a small retro-reflector on the panel surface. The sensor consists of a Shack-Hartmann slope
error test with a multiple wavelength, Shack cube, moire lnterferometer capable of performing star
tests, knife edge tests, wire tests and multiple wavelength interferometry.
The Shack-Hartmann test provides accurate tilt information by monitoring centroid positions of
the test path with respect to those of the reference path. The centroids from the reference path define
the perfect system. Minimizing the RMS difference in centroid position between the reference path
and the test path for each panel also gives an indication of the "aest fit' parabolic surface. Note that
372
this test is very sensitive to changes in panel tilt but insensitive to panel focus. The multiple
wavelength interferometer is used to assess wavefront quality. The star test is used for rough
alignment and visual assessment of the image, and a wire, knife edge and Focault test are used for
slope measurements. A tunable laser operating in the visible is used with the interferometer. The
first interferogram at _'1 is recorded on a thermoplastic camera and developed in place. The second
interferogram at X2 is imaged on top of the first. The resulting moire pattem is that of an
interferogram at
X effective = (_'I*_2)/(_'I-X2)"
The advantages of this sensor over others that were studied are its sensitivity, versatility, dynamic
range and proven technology.
_ _ PSR PRIMARY AND
_ ENTRANCE PUPIL
Jl _ ,. _'_-.CUBE,_-_/ /" REFERENCE
_"'_"_ "_,_J_ ,NE,G / PATH
""'-_. IP_;_ "-_-LENS
".. LENS _k._ . - "-. /
_'I_ _ "_/ " "_'x "_ LENSLET ARRAY
_ %. _///_ "_ AND EXIT PUPIL
CUBE INTER FEROMETER,_/_"_ \\\)_/_
FOLD _ - CHOPPER / _ _ J
MIRROR _ Jjk_ENS MOTOR / _'t"_J'_ J
TEST _'_ i,,.. v -- _DETECTOR
PATH _ ulor, ARRAY
/_'_/B.S. CUBE
TUNABLE----/ /
LASER (j/
FIGURE 2. FIGURE VERIFICATION SENSOR
Figure 2 shows a tunable laser being split into two separate paths by a cube beamsplitter. The
reference path goes through the beamsplitter to a fold mirror and a beam diverger to define the
paraxial center of curvature focal point. The reference beam is collimated and directed into the
Shack-Hartmann lenslet array. The Shack-Hartmann array is represented by three or more
373
lenslets per panel (shown here as seven lenslets per panel). The light passing through the lenslets
focuses onto a detector array and the centroids for the reference path are noted. The energy in the
test path is focused on the paraxial center of curvature focal point, but the light is diverted to the PSR
primary mirror. Upon returning from the primary mirror, the energy passes through the
collimating lens and lenslet array, and focuses onto the detector array. The chopper blade located
near the first beamsplitter can be used to blink between the two paths. The Shack cube
interferometer, inserted in the test path, is used for initial alignment and assessment of wavefront
quality.
SUMMARY
The combined dynamic range and sensitivity requirements for the PSR Figure Verification Sensor
precluded any single conventional optical technique that measures wavefront quality. The initially
expected 3-5 _ RMS surface error of the panel figuremasl¢_ the minute changes in panel position
(focus) and puts a limit on resolution. The proposed FV'S design satisfies the initial requirements
and does not require any technology development to implement. A Shack-Hartmann setup with a
battery of integrated optical tests all contained in a compact, versatile package gives precise
information on panel tilt, wavefront quality and panel alignment. The two wavelength, moire
interferometry extends the dynamic range of the instrument in its ability to quantify optical
performance. The star tests and shadowgrarns give visual assessment and the laser distance
measurment system gives precise information on panel piston motion.
374


Precision segmented reflector, figure verification sensor

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server