The Near Infrared Camera (NIRCam) instrument for NASA's James Webb Space Telescope (JWST) has an optical
prescription which includes numerous fold mirror assemblies. The instrument will operate at 35K after experiencing
launch loads at ~293K. The optic mounts must accommodate all associated thermal and mechanical stresses, plus
maintain exceptional optical quality during operation. Lockheed Martin Space Systems Company (LMSSC) conceived,
designed, analyzed, assembled, tested, and integrated the mirror assemblies for the NIRCam instrument. This paper
covers the design, analysis, assembly, and test of two of the instruments key fold mirrors.
The NIRCam instrument on the James Webb Space Telescope (JWST) will provide a coronagraphic
imaging capability to search for extrasolar planets in the 2 - 5 microns wavelength range. This capability is
realized by a set of Lyot pupil stops with patterns matching the occulting mask located in the JWST
intermediate focal plane in the NIRCam optical system. The complex patterns with transparent apertures
are made by photolithographic process using a metal coating in the opaque region. The optical density
needs to be high for the opaque region, and transmission needs to be high at the aperture. In addition, the
Lyot stop needs to operate under cryogenic conditions. We will report on the Lyot stop design, fabrication
and testing in this paper.
The Pick Off Mirror (POM) is the business end of the Focus and Alignment Mechanism (FAM) of NIRCam. The POM
harnesses the light delivered by the telescope and steers it into the Near Infrared Camera. At strategic points during the
build and test of the Pick Off Mirror and its mechanism (the FAM) the surface figure error (SFE) of the mirror was
monitored. This metric was used to track the health of the mirror throughout this testing regime. For example, the team
ran an SFE test before and after Vibration testing the FAM. In this paper, we will provide an overview of the testing
regime and the results of these periodic SFE tests. These results lead to the qualification of the POM and FAM designs
for flight on the James Webb Space Telescope.
The Bandpass Filters in the NIRCam instrument are required to have high throughput in bandpass spectral region and excellent
out-of-band blocking over the entire region of detector spectral response. The high throughput is needed for the instrument to have high sensitivity for detecting distant galaxies, and the out-of-band
blocking is needed for accurate calibration on James Webb Space Telescope. The operating temperature of the instrument is at cryogenic temperature from 32 Kelvin to 39.5 Kelvin. We have performed spectral measurement of NIRCam bandpass filters at cryogenic temperature after three cryo-to-ambient cycles. We will report the experiment and results in this paper. This work was performed and funded by NASA Goddard Space Flight Center under Prime Contract NAS5-02105.
The Focus and Alignment Mechanism (FAM) is an opto-mechanical, cryogenic mechanism that positions the Pick-Off
Mirror (POM) for the Near Infrared Camera of the James Webb Space Telescope. The POM is used to direct the light
collected by the telescope into the Near Infrared Camera. The POM is a spherical, fused silica mirror. In order to retain
high surface quality at cryogenic temperatures, the POM is attached to the mechanism via a titanium flexure-mount
assembly. Three linear actuators are employed to position the POM in tip, tilt and piston. These linear actuators are
stepper motor driven, with harmonic drive gear reduction. In this paper, we will summarize the design and role of this
opto-mechanical mechanism and present the results of the environmental testing of the Engineering Test Unit. The tests
performed were thermal-vacuum cryogenic cycling, and vibration testing.
The Dichroic Beam Splitter (DBS) in the NIRCam instrument is required to have small reflected wavefront error and
high throughput in order for the instrument to view the images of first light in the Universe in the James Webb Space
Telescope (JWST). The operating temperature of the instrument is from 32 Kelvin to 39.5 Kelvin. We have performed
NIRCam prototype DBS (fabricated by JDS Uniphase) spectral and reflected wavefront error measurements at cryogenic
temperatures. We report the experiment and the results in this paper.
An example is given of how cryogenic optical testing is being performed for the NIRCam instrument.
A 94 mm diameter Lithium Fluoride lens was mounted and thermally cycled between room temperature
and approximately 60 K. Interferometric measurements were taken before, during, and after the cycling
to determine the effects of temperature on the optical performance. We found that the net distortion of
the surface of the lens decreased with temperature. We also found that that the distortion did not
increase as the temperature rose again, and that the transmitted wavefront quality remained unchanged
before and after thermal cycling.
The Near Infrared Camera (NIRCam) instrument for NASA's James Webb Space Telescope (JWST) is one of the four science instruments to be installed into the Integrated Science Instrument Module (ISIM) on JWST. NIRCam's requirements include operation at 37 Kelvin to produce high-resolution images in two wave bands encompassing the range from 0.6 microns to 5 microns. In addition, NIRCam is to be used as a metrology instrument during the JWST observatory commissioning on orbit, during the precise alignment of the observatory's multiple-segment primary mirror. This paper will present the optical analyses performed in the development of the NIRCam optical system. The Compound Reflectance concept to specify coating on optics for ghost image reduction is introduced in this paper.
The Near Infrared Camera (NIRCam) for NASA's James Webb Space Telescope (JWST) is one of the four science instruments installed into the Integrated Science Instrument Module (ISIM) on JWST intended to conduct scientific observations over a five year mission lifetime. NIRCam's requirements include operation at 32 to 37 K to produce high resolution images in two wave bands encompassing the range from λ = 0.6 to 5.0 microns. In addition NIRCam is used as a metrology instrument for the JWST observatory, providing critical data for alignment of the observatory's multiple-segment 6.3 meter primary mirror. JWST is scheduled for launch and deployment in 2012. This paper is an overview of the NIRCam instrument's optical hardware and performance. Detailed discussions of specific subassemblies will be presented in other papers in the same conference.
The James Webb Space Telescope (JWST) Observatory, the follow-on mission to the Hubble Space Telescope and to the Spitzer Space Facility, will yield astounding breakthroughs in the realms of infrared space science. The science instrument suite for this Observatory will consist of a Near-Infrared Camera, a Near-Infrared Spectrograph, a Mid-Infrared Instrument with imager, coronagraph and integral field spectroscopy modes, and a Fine Guider System Instrument with both a Guider module and a Tunable Filter Module. In this paper we present an overview of the optical designs of the telescope and instruments.
We present the performance results of the as-built Pointing Calibration and Reference Sensor (PCRS) for the Space Infrared Telescope Facility (SIRTF). A cryogenic optical (center wavelength 0.55 microns) imager, the PCRS serves as the Observatory's fine guidance sensor by providing an alignment reference between the telescope boresight and the external spacecraft attitude determination system. The PCRS makes precision measurements of the positions of known guide stars; these are used to calibrate measurements from SIRTF's star trackers and gyroscopes to obtain the actual pointing of the SIRTF telescope. The PCRS calibrates out thermomechanical drifts between the 300 K spacecraft bus and the 5.5 K telescope. We have demonstrated that the PCRS meets its centroiding accuracy requirement of 0.14 arcsec 1-σ radial. The PCRS was installed inside the SIRTF Cryo-Telescope Assembly in July, 2000 and has logged over 1000 hours of failure-free operation ever since. We have verified that the PCRS has survived all box-level environmental requirements, including the 1.4 K operating temperature, random vibration, pyroshock, and EMI/EMC, necessary to survive launch and operations over SIRTF's 2.5 year lifetime. Currently, the PCRS is undergoing testing as part of the recently integrated Observatory in preparation for a January, 2003 launch.
This paper extends the previously reported results of cryogenic optical testing (SPIE Volume 2543, 1995) by including the results of further reduction of the test data for the 170-mm-diameter silicon carbide mirror and the 178-mm- diameter aluminum mirror. Both mirrors were manufactured by the Vavilov State Optical Institute, St. Petersburg, Russia, for infrared applications and were loaned to LMMS for these tests. Optical tests were performed in the Lockheed Martin cryogenic optical test facility at liquid helium temperatures, using a Zygo Mark II interferometer. The initial surface figures were 0.18 waves and 0.08 waves for the aluminum and the SiC mirrors, respectively, with figure error being given as rms wavefront error at 0.6328-micron wavelength at room temperature. It was found that the maximum change in shape after cooling was between 0.007 and 0.036 waves for the SiC mirror and between 0.017 and 0.062 waves for the aluminum mirror.
Mark Sullivan, Matthew Bye, Paul Ehrensberger, Enrique Romero, Howard Demroff, Scott Fletcher, Daniel DeBra, John Goebel, Paul Limtiaco, Donald Davidson, Lynn Huff, Ada Jefferson, Ali Kashani, Dale Gill, Kenneth Triebes, Jeff Grant
This paper presents the results of interferometric tests of two silicon carbide mirrors tested at room temperature and 6 K. The first mirror has a spherical f/1.73 surface, a diameter of 170 mm, and is of solid, plano-concave construction. The other mirror, a plano measuring 308 mm by 210 mm, is of lightweighted, closed-back construction. The mirrors were manufactured by the Vavilov State Optical Institute, St. Petersburg, Russia, and were loaned to Lockheed for these tests. Optical tests on both mirrors were performed using the Lockheed cryogenic optical test facility at liquid helium temperature and a Zygo Mark II interferometer. There was no change in the surface figure of the mirrors, within the test uncertainty of approximately plus or minus 0.02 waves at 0.6328-micrometer wavelength.
The properties of silicon carbide (low CTE, high modulus, high conductivity, low density) are ideal for mirrors performing at cryogenic temperatures. Test data at cryogenic temperatures indicate high thermal strain homogeneity as well as low hysteresis (critical properties for high quality optical performance). Until recently, the largest SiC mirrors tested at liquid helium temperatures have been only a few centimeters in diameter. Recently a lightweighted (6 kg) 20-inch-diameter SiC mirror manufactured by United Technologies Optical Systems was tested for figure change at 10 K. Hysteresis was quantified upon return to room temperature. The results indicate high thermal strain homogeneity and low hysteresis. These optical results are applied to a parametric model developed from numerous previous cryogenic tests to estimate the thermal strain variability. Quantitative comparisons to other cryogenic materials are made based on reported test data.
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