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10.1117/2.1200607.0260
Sounding-rocket telescope uses
new technology
ultra-light-weight mirrors
David Content, Scott Antonille, Douglas Rabin, and Thomas
Wallace
Two NASA-funded programs will image the Sun and then an
exoplanet: both at high angular resolution and both using the same
mirror.
To discover still-unknown celestial objects and to study known
objects more accurately, high angular resolution telescope
mirrors that do not increase payload weights are increasingly
required for telescope-based experiments conducted outside the
Earth’s atmosphere. Sounding rockets—suborbital rockets that
carry payloads beyond Earth’s atmosphere but not into orbit—
are the fastest and least expensive transport devices for such
missions. However, ultra-light-weight, high-precision mirrors
necessary for the success of such missions are expensive and
have long production schedules beyond the scope of typical
sounding-rocket programs.
Two such programs require an ultra-light-weight, precisionpolished 0.5m aperture telescope mirror. By developing a
common design for the telescope, the expense of the mirror
can be shared by two programs: PICTURE (planet imaging
concept testbed using a rocket experiment) to image the Sun and
SHARPI (solar high angular resolution photometric imager) to
image an exoplanet (a planet that revolves around a star other
than the Sun).
The first sounding-rocket mission, PICTURE, flies in 2007
during periods when the exoplanet is at its brightest, and is
led by Dr. Supriya Chakrabarti of Boston University along
with members from the Jet Propulsion Laboratory (JPL),
NASA, and Boston Micromachines. The program is being
carried out in collaboration with a second sounding-rocket
program, SHARPI. The planet-imaging instrument uses a
visible nulling coronagraph built by JPL1 that incorporates
a MEMS (micro-electro-mechanical Systems) deformable mirror array. This array provides substantial wavefront-error
Figure 1. The primary mirror undergoes inspection at the NASA
Goddard Space Flight Center after receipt from ITT Space Systems.
correction to allow the high contrast necessary for exoplanet
imaging.
The researchers funded the telescope by a combination of
programs involving various sounding-rocket projects, NASA
light-weight mirror technology, small business research and
development, and independent research and development
from vendor ITT Space Systems (formerly Kodak Remote
Sensing Systems). The agreed-upon telescope uses a Gregorian
design (a reflecting telescope with two concave mirrors) and
a high-magnification tertiary mirror (similar to a three-mirror
anastigmat, a system designed to be free of astigmatism). The design produces large angular magnification with an intermediate
image field stop that eliminates most of the solar heat flux
affecting the mirror during the mission.
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SPIE Newsroom
Figure 2. The breakdown of the overall figure errors on the primary
mirror (top left) at nm level into radially symmetric figure error (top
right), radially asymmetric (bottom left), and cell print-through (bottom right).
The primary mirror (see Figure 1) has a clear aperture of
50cm, full size of 55cm, and is light weighted 92% (that is,
mass is reduced without significant degradation to rigidity)
so that it weighs only 4.5kg. ITT Space Systems polished the
primary mirror to approximately 7nm root mean square (rms)
figure error. Figure 2 shows a decomposition of the residual
errors at nanometer scale in terms of radially symmetric, radially
asymmetric, and cell quilting (cell print-through).2 At an areal
density of 20kg/m2 , the mirror is nine times lighter per unit
area and has better surface accuracy (that is, it has similar
overall surface errors but with a correct conic constant) than the
primary mirror of the Hubble Space Telescope. As required for
use in the ultraviolet region, the primary mirror is a precisely
polished (to 1.1nm rms microroughness) diffraction-limited
mirror.
The 12cm-diameter concave secondary mirror was produced
on a Small Business Innovative Research contract with SSG
Precision Optronics Inc./Tinsley Laboratories. In addition, the
mirror’s substrate, made by SSG/Tinsley, is composed of
reaction-bonded silicon carbide (SiC) and weighs less than
0.25kg. Partly because of its greater stiffness, the surface figure
accuracy of the secondary mirror is 3nm rms, twice as accurate
as the primary mirror. The optics are treated with visible,
Figure 3. The image of the primary mirror (with UV-protected aluminum coating) in a horizontal figure test setup, reflecting interferometric fringes from the test off a video monitor.
polarization-controlled coatings that are required for the high
contrasts involved in imaging exoplanets.3 During mounting,
the researchers will monitor the primary mirror’s structure
to minimize strains. Planned for the summer of 2006, final
alignment and integration of the telescope will occur at the
Goddard Space Flight Center. These mirrors are among the most
precise ultra-light-weight mirrors made.
The researchers verified the vendor’s figure data and
predictions of figure change that the mirror is expected to
experience under zero-gravity conditions during the flight.
Figure 3 shows the mirror reflecting interferometric fringes from
the video monitor during the test.
The mission goal of PICTURE is to take the first direct image
of the brightest exoplanet detected indirectly to date: a Joviansize planet orbiting the star Epsilon Eridani.4, 5 Upon completion
of this mission, the researchers will recoat the optics with
UV-protected aluminum and reintegrate the telescope with a
new tertiary mirror, UV detector, and filters to accommodate
the SHARPI payload. With such equipment, researchers hope
to acquire the highest-resolution solar ultraviolet images ever
obtained.
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Author Information
David Content, Scott Antonille, and Thomas Wallace
Optics Branch, MC551
NASA Goddard Space Flight Center
Greenbelt, MD
David Content is a scientist at NASA. He develops new
technology optical components for space- and earth-science
missions. In addition, Content has chaired conferences on optical
materials and written articles on light-weight mirrors and
diffractive optics.
Douglas Rabin
Heliospheric Physics Branch, Code 612
NASA Goddard Space Flight Center
Greenbelt, MD
References
1. M. Shao, J. K. Wallace, B. M. Levine, and D. T. Liu, Visible nulling interferometer,
Proc. SPIE 5487, pp. 1296–1303, 2004. doi:10.1117/12.552527
2. K. Balasubramanian, P. Z. Mouroulis, L. F. Marchen, and S. B. Shaklan,
Polarization-compensating protective coatings for TPF-Coronagraph optics to control
contrast degrading cross-polarization leakage, Proc. SPIE 5905, p. 59050H, 2005.
doi:10.1117/12.618753
3. B. Campbell, G. A. H. Walker, and S. Yang, A Search for Substellar Companions to
Solar-Type Stars, Astrophys. J. 331 (902-921), 1988. doi:10.1086/166608
4. S. Antonille, D. Content, D. Rabin, T. Wallace, and C. Stevens, High precision
metrology on the ultralightweight UV 50.8cm f/1.25 parabolic SHARPI primary mirror
using a CGH null lens, Proc. SPIE 5494, pp. 132–140, 2004. doi:10.1117/12.552614
5. A. Cumming, G. Marcy, and R. P. Butler, The Lick Planet Search: Detectability and
Mass Thresholds, Astrophys. J. 526, pp. 890–915, 1999. doi:10.1086/308020
c 2006 SPIE—The International Society for Optical Engineering