The James Webb Space Telescope (JWST) is a general astrophysics mission which consists of a 6.6m diameter,
segmented, deployable telescope for cryogenic IR space astronomy (~35K). The JWST Observatory architecture
includes the Optical Telescope Element and the Integrated Science Instrument Module (ISIM) element that contains four
science instruments (SI) including a Guider.
The alignment philosophy of ISIM is such that the cryogenic changes in the alignment of the SI interfaces are captured in
the ISIM alignment error budget. The SIs are aligned to the structure's coordinate system under ambient, clean room
conditions using laser tracker and theodolite metrology. The ISIM structure is thermally cycled and temperature-induced
structural changes are concurrently measured with a photogrammetry metrology system to ensure they are within
requirements.
We compare the ISIM photogrammetry system performance to the ISIM metrology requirements and describe the
cryogenic data acquired to verify photogrammetry system level requirements, including measurement uncertainty. The
ISIM photogrammetry system is the baseline concept for future tests involving the Optical Telescope Element (OTE) and
Observatory level testing at Johnson Space Flight Center.
A warm window surface with a relatively high (>50%) surface emittance can add significant undesired heat loading into a cryogenic test chamber. However, a front surface coating that consists of a very thin adherent layer of evaporated Cr that is overcoated with about 7nm of evaporated Au has been demonstrated to
reduce the inherently high emittance of a glass or sapphire window surface down to about 14%, while maintaining a visible transmittance in excess of 55%. The coating possesses reasonably good adhesion and cleaning durability when deposited onto glass or sapphire substrates and has survived multiple temperature cycles between 316K and 20K. The addition of a single layer anti-reflection coating, such as reactively evaporated SiOx, to the otherwise uncoated exterior surface of a cryogenic window produced a further increase in visible wavelength transmittance without altering window emittance. This paper will present measured reflectance, transmittance, and emittance data for the Cr + Au window surface coating relevant to
a cryogenic window application.
The James Webb Space Telescope is a large infrared observatory with a segmented primary mirror, part of the
Optical Telescope Element (OTE), and four science instruments supported by the Integrated Science Instrument Module
(ISIM). We present the calibration plan for the ISIM Test Platform (ITP) which replicates the ISIM-to-OTE interface: to
calibrate the location and orientation of metrology features at ambient and cryogenic environmental conditions, to verify
that ITP behavior (deflection under load, warm-to-cold alignment shift) can be modeled, predicted, and tested, to prove
that the ITP is stable (upon repeated cryogenic cycles, and after loading and handling), and to calibrate the relationship
between the Master Alignment Target Fixture and the ITP at ambient and cryogenic conditions.
KEYWORDS: Cameras, Cryogenics, Photogrammetry, James Webb Space Telescope, Distortion, Error analysis, Metrology, Calibration, Received signal strength, Optical alignment
The alignment philosophy of the James Webb Space Telescope (JWST) Integrated Science Instrument
Module (ISIM) is such that the cryogenic changes in the alignment of the science instruments (SIs) and
telescope-related interfaces are captured in an alignment error budget. The SIs are aligned to the structure's
coordinate system under ambient, clean room conditions using laser tracker and theodolite metrology. The
ISIM structure is thermally cycled and temperature-induced mechanical and structural changes are
concurrently measured to ensure they are within the predicted boundaries.
We report on the ISIM photogrammetry system and its role in the cryogenic verification of the ISIM
structure. We describe the cryogenic metrology error budget and the analysis and testing that was
performed on the ISIM mockup, a full scale aluminum model of the ISIM structure, to ensure that the
system design allows the metrology goals to be met, including measurement repeatability and distortion
introduced from the camera canister windows.
The James Webb Space Telescope (JWST) is an infrared space telescope scheduled for launch in 2013. JWST has a 6.5 meter diameter deployable and segmented primary mirror, a deployable secondary mirror, and a deployable sun-shade. The optical train of JWST consists of the Optical Telescope Element (OTE), and the Integrated Science Instrument Module (ISIM), which contains four science instruments. When the four science instruments are integrated to ISIM at NASA Goddard Space Flight Center, the structure becomes the ISIM Element. The ISIM Element is assembled at ambient cleanroom conditions using theodolite, photogrammetry, and laser tracker metrology, but it operates at cryogenic temperature, and temperature-induced mechanical and alignment changes are measured using photogrammetry. The OTE simulator (OSIM) is a high-fidelity, cryogenic, telescope simulator that features a ~1.5 meter diameter powered mirror. OSIM is used to test the optical performance of the science instruments in the ISIM Element, including focus, pupil shear, and wavefront error. OSIM is aligned to the flight coordinate system in six degrees of freedom via OSIM-internal cryogenic mechanisms and feedback from alignment sensors. We highlight optical metrology methods, introduce the ISIM and the Science Instruments, describe the ambient alignment and test plan, the cryogenic test plan, and verification of optical performance of the ISIM Element in cryo-vacuum environment.
The LOng-Range Reconnaissance Imager (LORRI) is a panchromatic imager for the New Horizons Pluto/Kuiper belt mission. New Horizons is being prepared for launch in January 2006 as the inaugural mission in NASA's New Frontiers program. This paper discusses the calibration and characterization of LORRI.
LORRI consists of a Ritchey-Chretien telescope and CCD detector. It provides a narrow field of view (0.29°), high resolution (pixel FOV = 5 μrad) image at f/12.6 with a 20.8~cm diameter primary mirror. The image is acquired with a 1024 x 1024 pixel CCD detector (model CCD 47-20 from E2V). LORRI was calibrated in vacuum at three temperatures covering the extremes of its operating range (-100°C to +40°C for various parts of the system) and its predicted nominal temperature in-flight. A high pressure xenon arc lamp, selected for its solar-like spectrum, provided the light source for the calibration. The lamp was fiber-optically coupled into the vacuum chamber and monitored by a calibrated photodiode. Neutral density and bandpass filters controlled source intensity and provided measurements of the wavelength dependence of LORRI's performance. This paper will describe the calibration facility and design, as well as summarize the results on point spread function, flat field, radiometric response, detector noise, and focus stability over the operating temperature range.
LORRI was designed and fabricated by a combined effort of The Johns Hopkins University Applied Physics Laboratory (APL) and SSG Precision Optronics.
Calibration was conducted at the Diffraction Grating Evaluation Facility at NASA/Goddard Space Flight Center with additional characterization measurements at APL.
The development of high quantum efficiency photemissive detectors is recognized as a significant advancement for astronomical missions requiring photon-counting detection. For solar-blind NUV detection, current missions (GALEX, STIS) using Cs2Te detectors are limited to ~10% DQE. Emphasis in recent years has been to develop high QE (>50%) GaN and AlGaN photocathodes (among a few others) that can then be integrated into imaging detectors suitable for future UV missions. We report on progress we have made in developing GaN photocathodes and discuss our observations related to parameters that effect efficiency and stability, including intrinsic material properties, surface preparation, and vacuum environment. We have achieved a QE in one case of 65% at 185 nm and are evaluating the stability of these high QEs. We also discuss plans for incorporating photocathodes into imaging and non-imaging sealed devices in order to demonstrate long term stability.
The Ultraviolet and Optical Telescope (UVOT) is one of the three
astronomical instruments onboard the SWIFT spacecraft. The optical
calibration of this instrument, which was done prior to integration
to the SWIFT spacecraft optical bench, is key to determine if UVOT
will meet its science objectives. In this paper, we describe
the optical ground support equipment (GSE) used for the
calibration of UVOT. These tests, which were carried out in the
Diffraction Grating Evaluation Facility (DGEF), at NASA Goddard
Space Flight Center, required building an optical stimulus. We
report the radiometric measurements of all the optical components
used in putting together this stimulus. This includes a vacuum
collimator with a Cassegrain design, a Pt/Cr-Ne light source, a
complete set of neutral density filters spanning 6 orders of
magnitude in transmission levels, a set of narrow-band filters
matching the center of each of the six bands of UVOT, a set of pinholes of various sizes, flat fielding diffusers, and a set of parabolic mirrors.
The UV/optical telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (~1 minute) UV and optical photons from the afterglow of gamma-ray bursts as well as long term observations of these afterglows. This is accomplished through the use of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey-Chretien design with micro-channel plate intensified charged-coupled device detectors that provide sub-arcsecond imaging. Unlike most UV/optical telescopes the UVOT can operate in a photon-counting mode as well as an imaging mode. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument.
The Swift/UVOT is a 30-cm aperture imaging telescope that is sensitive to photons in the wavelength range 170nm-600nm and is designed to provide near-ultraviolet and optical measurements of γ-ray bursts and other targets that the Swift observatory observes. The performance of the telescope and its photon counting detectors has been assessed in a series of calibration measurements made under vacuum conditions in a test facility at the Goddard Space Flight Center. We describe some of the results of this campaign, including measurements of the instrument throughput, image quality and distortion, and linearity of response. We also describe the spectroscopic capability of the instrument, which is enabled by the use of two grisms operating in the UV and optical bands respectively. The results from the ground calibration activities will form the basis for establishing the full calibration matrix of the instrument once on orbit.
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