Alignment of the James Webb Space Telescope (JWST) Optical Telescope Element (OTE) requires a multitude of demanding and exacting dimensional and positional measurements. Many of the alignment requirements are in the range of hundreds of microns over significant distances (up to 8 m) on a flexible structure, which creates stringent accuracy demands on the alignment measurements. Furthermore, to optimize the performance of the system, the telescope is aligned to a relatively small (<1 m) structure in the center, creating the potential for coordinate system errors. Measurements have been performed using laser trackers (predominantly), photogrammetry, coordinate measurement machine (CMM), and laser radar instruments. Measurements from different instruments/ stations are combined and processed within SpatialAnalyzer (SA) commercial software using the Unified Spatial Metrology Network (USMN) feature. While this approach should yield the best possible accuracies (hopefully in the tens of microns range), our experience has been that there can be significant errors in the data based on the details of how SA is set up and how the measurements are conducted. As a result of our experience, we have developed analytical tools and processes that allow us to test the data veracity in near real time using, for example, Excel spreadsheet calculations. These tools combine measurements made at various levels of assembly, measurements of cross check points, and finite element analysis to determine the correlated and uncorrelated discrepancies in the measured data. This provides a detailed understanding of systematic and random measurement errors and has allowed us to quickly uncover issues with placement, measurement, and modeling, as well as to quantify our measurement performance.
This paper describes a novel gap gauge tool that is used to provide an independent check of the James Webb Space Telescope (JWST) Optical Telescope Element (OTE) primary mirror alignment. Making accurate measurements of the mechanical gaps between the OTE mirror segments is needed to verify that the segments were properly aligned relative to each other throughout the integration and test of the 6.6 meter telescope. The gap between the Primary Mirror Segment Assemblies (PMSA) is a sensitive indicator of the relative clocking and decenter. Further, the gap measurements are completely independent of all the other measurements use in the alignment process (e.g. laser trackers and laser radar). The gap measurement is a challenge, however, that required a new approach. Commercial gap measurements tools were investigated; however no suitable solution is available. The challenge of this measurement is due to the required 0.1 mm accuracy, the close spacing of the mirrors segments (approximately 3-9mm), the acute angle between the segment sides (approximately 4 degrees), and the difficult access to the blind gap. Several techniques were considered and tested before selecting the gauge presented here. This paper presents the theory, construction and calibration of the JWST gap gauge that is being used to measure and verify alignment of the OTE primary mirror segments.
Proc. SPIE. 9904, Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave
KEYWORDS: Optical components, Telescopes, Mirrors, Data modeling, Adaptive optics, Space telescopes, Space telescopes, Optical alignment, Optical telescopes, James Webb Space Telescope, Shape memory alloys
The optical telescope element (OTE) of the James Webb Space Telescope has now been integrated and aligned. The OTE comprises the flight mirrors and the structure that supports them – 18 primary mirror segments, the secondary mirror, and the tertiary and fine steering mirrors (both housed in the aft optics subsystem). The primary mirror segments and the secondary mirror have actuators to actively control their positions during operations. This allows the requirements for aligning the OTE subsystems to be in the range of microns rather than nanometers. During OTE integration, the alignment of the major subsystems of the OTE structure and optics were controlled to ensure that, when the telescope is on orbit and at cryogenic temperatures, the active mirrors will be within the adjustment range of the actuators. Though the alignment of this flagship mission was complex and intricate, the key to a successful integration process turned out to be very basic: a clear, concise series of steps employing advanced planning, backup measurements, and cross checks that this multi-organizational team executed with a careful and methodical approach. This approach was not only critical to our own success but has implications for future space observatories.
A practical method for introducing stray light for testing the James Webb Space Telescope (JWST) at cryogenic temperatures using hollow shell spherical reflectors is described. Several alternate approaches to stray light testing are compared, including fiber sources, diffuse panels, and curved specular reflectors. Alignment of the sources can pose special difficulties when cooling to cryogenic temperatures, since the shape of the reflector, mounts, and support structure can all change. The hollow shell spherical reflectors do not have any of these difficulties, and can be mounted
so that they are automatically aligned by gravity. This also makes them insensitive to vibration, so they can be used with long detector integration times to provide adequate stray light signal to noise without interfering with other optical tests. The reflectors are positioned so as to work with test source(s) already in the cryo-vac chamber. The spherical reflectors operate at all wavelengths, reducing the number of reflectors required and providing operational flexibility in reflector placement. Electroplated stainless steel hollow shell reflectors are inherently compatible with cryogenic and vacuum environments. The reflectors are passive and have no thermal dissipation, eliminating impact on sensitive thermal tests.
Their light weight and single point suspension mounting minimize the dynamic and static loads. Finally, the reflector's simple geometry is inherently compatible with optical alignment metrology (e.g. LIDAR), making position measurements both more accurate and simpler to document.
A point symmetric design approach for creating a practical cat's eye retro-reflector (CERR) anastigmat lens with a wide
field of regard (FOR), uniform reflectance and wide wavelength range is described. An anastigmat design is presented
that demonstrates the performance capability of the design approach. The lens design is diffraction limited in double
pass at F/3, has a "working distance" between lens and reflector, wide wavelength range of operation, and uniform
reflectivity over a 120 deg FOR. An anastigmat fabricated from the design is presented; however, the design approach is
generally useful for any application requiring a high performance retro-reflector. The design uses only spherical
surfaces, thereby avoiding the fabrication expense of aspheric surfaces.
The location and installation of mid-infrared missile warning receiver sensors is limited by the mechanical constraints of the detector/dewar assembly and the associated cryogenic cooler assembly. The size, shape, and weight of these assemblies limit the installation alternatives, and prevent placing the missile warning receiver system in the optimum locations. Hence, their coverage and detection performance is limited. A micro-lens array coupled to a coherent fiber optic bundle and an infrared focal plane array were designed and experimentally implemented, to allow the mid-wave sensor and cryogenic devices to be located remotely from the receiver aperture. This eliminates the receiver aperture placement restrictions while easing the integration and maintenance of the sensor/dewar and cooler. Modulation transfer function and noise equivalent temperature difference measurements were performed to determine the performance of the imaging system.
Accessing a large field-of-regard (FOR) from an aircraft-mounted infrared system imposes significant structural and aerodynamic penalties. A novel conformal infrared (IR) transceiver concept is presented which is currently under development. A trial design of this concept can access a 160 deg FOR without a gimbal mirror or 'fish eye' lens. A fiber optic bundle is used to allow a wide range of beamsteering technologies with small steering angles (i.e., +/- 5 degree(s)) to access the large FOR (+/- 80 deg) through a single, conformal aperture. The output lens size is less than a factor of three times larger than the input/output IR beam, yet provides near diffraction limited polychromatic collimation over the full FOR. The concept is applicable over a wide spectral band (ultraviolet to far IR), however, it is being developed for the mid-IR (2 - 6 micron) band. The challenging technical aspects of the fiber optics in this spectral band are discussed.
An experimental system is presented for measuring a pulsed laser beam's atmospherically-induced phase and amplitude distortions, using a fixed receiver and mobile transmitter. The noninterferometric measurement method operates over ground paths up to 2 km, and has as its primary advantage over MTF and scintillometric methods the gathering of explicit phase information. System operation involves the transmission of 10-20 nsec laser pulses, at either 532 or 1064 nm, from the 200-m aperture of the transmitter to the 200-mm aperture of the receiver. The pupil and focal planes are related by the two-dimensional Fourier transform; by iteratively analyzing the irradiance distributions in these planes, the computer can ascertain the phase and amplitude of the entering wavefront.