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.
KEYWORDS: Photogrammetry, Nondestructive evaluation, Cameras, Metrology, James Webb Space Telescope, Cryogenics, Temperature metrology, Optical alignment, Space telescopes, Interfaces
The James Webb Space Telescope (JWST) is a 6.6m diameter, segmented, deployable telescope for cryogenic IR space
astronomy (~40K). 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 ISIM structure must meet
its requirements at the ~40K cryogenic operating temperature.
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 for stress relief and in order to measure temperature-induced mechanical,
structural changes. These ambient-to-cryogenic changes in the alignment of SI and OTE-related interfaces are an important
component in the JWST Observatory alignment plan and must be verified.
We report on the planning for and preliminary testing of a cryogenic metrology system for ISIM based on photogrammetry.
Photogrammetry is the measurement of the location of custom targets via triangulation using images obtained at a suite of digital
camera locations and orientations. We describe metrology system requirements, plans, and ambient photogrammetric
measurements of a mock-up of the ISIM structure to design targeting and obtain resolution estimates. We compare these
measurements with those taken from a well known ambient metrology system, namely, the Leica laser tracker system.
We are reporting our progress in the measurements of thin glass optics under development for the soft X-ray telescope for the Constellation-X space observatory. We are using a Non-Contact laser probe (which uses triangulation techniques to measure displacement) to determine the surface shape of our ultra-lightweight mirrors. If this technique meets technical specifications we will for the first time have mapped the 3 dimensional surfaces of ultra-lightweight optics. As a secondary project, we are also automating this entire process which will give us better repeatability.
We present the metrology requirements and metrology implementation necessary to prove out the mirror technology for the Constellation-X (C-X) soft x-ray telescope (SXT). This segmented, 1.6m diameter highly nested Wolter-1 telescope presents many metrology and alignment challenges. A variety of contact and non-contact optical shape measurement, profiling and interferometric methods are combined to test the forming mandrels, some of the replication mandrels, the formed glass substrates before replication and the replicated mirror segments. The mirror segments are tested both stand-alone and in-situ in mirror assemblies. Some of these methods have not been used on prior x-ray telescopes and some are feasible only because of the segmented approach used on the SXT. Methods to be discussed include axial interferometric profiling, azimuthal circularity profiling, midfrequency error profiling, and axial roughness profiling. The most critical measurement is axial profiling, and we compare the method in use to previous methods such as the long trace profilometer (LTP). A companion paper discusses the method of non-contact 3D profiling using a laser sensor and distance measuring interferometers.
In support of the Constellation X mission the Optics Branch at Goddard Space Flight Center is developing technology for precision figuring and polishing of mandrels used to produce replicated mirrors that will be used in X-Ray telescopes. Employing a specially built machine controlled in 2 axes by a computer, we are doing automated polishing/figuring of 15 cm long, 20 cm diameter cylindrical, conical and Wolter mandrels. A battery of tests allow us to fully characterize all important aspects of the mandrels, including surface figure and finish, mid-frequency errors, diameters and cone angle. Parts are currently being produced with surface roughnesses at the 0.4nm RMS level, and half-power diameter 0.6 arcseconds.
In support of Goddard Space Flight Center's (GSFC) Constellation-X mandrel manufacturing effort, a series of fabrication experiments are being performed to determine a best approach, and to supply the project with precision mandrels. Currently, polishing immediately after diamond turning produces a RMS surface roughness of 0.3 nm, on an electroless nickel-plated aluminum mandrel. Studies currently under way will incorporate an abrasive-figuring step to be followed by this polishing operation. The current diamond turning, figuring and polishing procedures will be described and the results presented.
We have superpolished a diamond-turned aluminum mandrel to an axial roughness of 0.34 nm rms. The mandrel is made to the Astro-E secondary mirror design for the 81st shell. Precision metrology at 100 mm to submicron scales has established the power spectral density of the mandrel and ultralightweight gold coated replicated segments. Predicted image quality of a set of optimally aligned replicated segments of this and a matching primary is substantially improved as compared to the flight mirrors for Astro-E. This approach using metal mandrels, superpolishing, and replicated ultralightweight foil mirrors, may represent a cost-effective approach to meeting the 15 arcsec half-energy width and weight requirements for the Constellation-X mission. Descriptions of the polishing apparatus, the precision metrology instruments, and the surface data analysis are presented. The general methods describe dare applicable to precision optics for both normal incidence and grazing incidence optics.
The second servicing mission for the Hubble Space Telescope (HST), scheduled for early 1997, will be the first change in the spectroscopic capabilities of HST since its initial deployment. The Space Telescope Imaging Spectrograph (STIS) is a multipurpose instrument covering the far ultraviolet (FUV) through near infrared spectral range. It acquires spectra at several spectral resolutions, which facilitates observations at many distances and brightnesses. STIS will replace both of the first generation spectrographs, the Goddard High Resolution Spectrograph and the Faint Object Spectrograph. This will allow the addition of a Near- Infrared Camera. STIS required the development and testing of many high quality diffraction gratings, including several very difficult echelles for the FUV. The methods and results of this grating development program are presented. The results serve as a snapshot of industry capabilities for producing high quality spaceborne diffraction gratings.
Advances in optical coating and materials technology have made possible the development of instruments with substantially improved efficiency in the extreme ultraviolet (EUV). For example, the development of chemical vapor deposited (CVD) SiC mirrors provides an opportunity to extend the range of normal incidence instruments down to 60 nm. CVD-SiC is a highly polishable material yielding low scatter surfaces. High UV reflectivity and desirable mechanical and thermal properties make CVD-SiC an attractive mirror and/or coating material for EUV applications. The EUV performance of SiC mirrors as well as some strengths and problem areas are discussed.
Advances in optical coating and materials technology have made possible the development of instruments with substantially improved efficiency in the extreme ultraviolet/far ultraviolet (EUV/FUV) spectral region. For example, the development of chemical vapor deposited (CVD) SiC mirrors provides an opportunity to extend the range of normal incidence instruments down to 60 nm. The EUV performance and some applications of optical coatings including MgG2 protected aluminum, CVD- SiC, SiC films, boron carbide films, and multilayer coatings will be discussed. Contamination sensitivity and cleaning will be addressed.
Low level particulate contamination of some of the COSTAR optics during environmental testing necessitated the development of a short duration, low pressure CO2 jet spray cleaning technique. The technique was proven to be compatible with the high reflectivity UV magnesium fluoride protected aluminum coatings applied to the COSTAR optics at the Goddard Space Flight Center (GSFC). These coatings are easily damaged by solvent flush techniques in the presence of this contamination and by conventional CO2 jet spray cleaning methods. In addition, the technique adopted here was compatible with the geometry of the fully integrated and aligned optics on the COSTAR. Eventually, thorough testing of intentionally contaminated test mirrors and a final UV throughput test of the flight optics indicated that the instrument would still meet specifications, therefore the COSTAR flight optics were not cleaned. However, we proceeded with the qualification of the process so that it would be available for use if cleaning became necessary prior to launch. Success criteria for the cleaning included minimal degradation of reflectivity, surface roughness, near-angle scattering, minimal production of pinholes in the coating, compatibility with the polyurethane bond applied to the optics and high efficiency of particulate removal. The technique was qualified for COSTAR specific coatings and particulate contamination and should be recertified for conditions departing from those baselined in our study.
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