During Integration and Testing of large optical systems the exposed optics can become contaminated with many particles from the various test and integration environments that the hardware is exposed to. The particles may cause degradation of mirror performance due to stray light scatter and optical throughput loss due to obscuration. Cleaning the optics can be a challenge depending on what type of particles are found on the optic, how extensive the coverage, how long they have been on the surface, the type (beryllium), size and concave structure of the optics, as well as the optical coating on the surface.
Common ways to clean optics include drag wiping the surface with various cloths and solvents; blowing off the optics with ionized gaseous nitrogen (GN2); blowing off the optics with carbon dioxide (CO2) (snow gun). With these techniques the surface could be damaged if hard particles such as metallic pieces, are pushed along with the gas or wipe. Drag wiping also had the potential to smear acrylic adhesive particles and other molecular contaminates that may be present on the surface. Ionized GN2 at low velocity does not remove enough particles to make it effective at cleaning. At high velocity GN2 can damage thin light-weighted mirror substrate as well as dislodge beryllium dust from the exposed machined surface causing a potential health hazard. It also dislodges particles in a chaotic fashion potentially causing them to be forced into other sensitive regions of the optics or system. CO2 snow similarly causes all dislodged particles to move in an uncontrolled fashion. CO2 snow also significantly changes the temperature of the optic, and in the case of light-weighted space telescope optics with a thin face it can cool the surface enough to cause condensation, which in turn can leave difficult-to-remove evaporation marks on the optical surface.
A new technique utilizing a fine bristle brush was developed for JWST to safely and effectively remove the majority of the particles. This technique removes particles in a controlled fashion making sure they do not migrate to other parts of the telescope, and uses very light pressure, which avoids damage to the coating from metallic particles. Not all particles are removed but the surface is left in a much cleaner state without loose particles present. Several of the individual types such as adhesive and large fibers can be removed later with a spot cleaning technique.
Potential damage from the brush to the surface was evaluated utilizing several different techniques.
The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) and Integrated Science Instrument Module (ISIM) completed their element level integration and test programs and were integrated to the next level of assembly called OTE/ISIM (OTIS) at Goddard Space Flight Center (GSFC) in Greenbelt, Maryland in 2016. Before shipping the OTIS to Johnson Space Center (JSC) for optical test at cryogenic temperature a series of vibration and acoustic tests were performed. To help ensure that the OTIS was ready to be shipped to JSC an optical center of curvature (CoC) test was performed to measure changes in the mirror’s optical performance to verify that the telescope’s primary mirror was not adversely impacted by the environmental testing and also help us in understanding potential anomalies identified during the JSC tests. The 6.5 meter diameter primary mirror consists of 18 individual hexagonal segments. Each segment is an off-axis asphere. There are a total of three prescriptions repeated six times each. As part of the CoC test each segment was individually measured using a high-speed interferometer (HSI) designed and built specifically for this test. This interferometer is capable of characterizing both static and dynamic characteristics of the mirrors. The latter capability was used, with the aid of a vibration stinger applying a low-level input force, to measure the dynamic characteristic changes of the PM backplane structure. This paper describes the CoC test setup and both static and dynamic test results.
The James Webb Space Telescope (JWST) recently saw the completion of the assembly process for the Optical Telescope Element and Integrated Science Instrument Module (OTIS). This integration effort was performed at Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. In conjunction with this assembly process a series of vibration and acoustic tests were performed. To help assure the telescope’s primary mirror was not adversely impacted by this environmental testing an optical center of curvature (CoC) test was performed to measure changes in the mirror’s optical performance. The primary is a 6.5 meter diameter mirror consisting of 18 individual hexagonal segments. Each segment is an off-axis asphere. There are a total of three prescriptions repeated six times each. As part of the CoC test each segment was individually measured using a high-speed interferometer (HSI) designed and built specifically for this test. This interferometer is capable of characterizing both static and dynamic characteristics of the mirrors. The latter capability was used, with the aid of a vibration stinger applying a low-level input force, to measure the dynamic characteristic changes of the PM backplane structure. This paper describes the CoC test setup, an innovative alignment method, and both static and dynamic test results.
The James Webb Space Telescope (JWST) primary mirror is 6.6 m in diameter and consists of 18 hexagonal mirror segments each approximately 1.5 m point-to-point. Each primary mirror segment assembly (PMSA) is constructed from a lightweight beryllium substrate with both a radius-of-curvature actuation system and a six degree-of-freedom hexapod actuation system. With the JWST being a near to mid-infrared observatory, the nominal operational temperature of a
PMSA is 45 K. Each PMSA must be optically tested at 45 K twice, first to measure the change in the surface figure & radius-of-curvature between ambient & cryogenic temperatures and then to verify performance at cryo following final polishing. This testing is conducted at Marshall Space Flight Center's (MSFC's) X-Ray & Cryogenic Facility (XRCF). The chamber & metrology system can accommodate up to six PMSAs per cryo test. This paper will describe the optical metrology system used during PMSA cryogenic testing. This system evolved from systems used during the JWST mirror technology development program. The main components include a high-speed interferometer, a computer-generated holographic null, an absolute distance meter, a tiltable window, and an imaging system for alignment. The optical metrology system is used to measure surface figure error, radius-of-curvature, conic constant, prescription
alignment, clear aperture, and the range & resolution of the PMSA actuation systems.
NASA's Technology Readiness Level (TRL)-6 is documented for the James Webb Space Telescope (JWST) Wavefront
Sensing and Control (WFSC) subsystem. The WFSC subsystem is needed to align the Optical Telescope Element
(OTE) after all deployments have occurred, and achieves that requirement through a robust commissioning sequence
consisting of unique commissioning algorithms, all of which are part of the WFSC algorithm suite. This paper identifies
the technology need, algorithm heritage, describes the finished TRL-6 design platform, and summarizes the TRL-6 test
results and compliance. Additionally, the performance requirements needed to satisfy JWST science goals as well as the
criterion that relate to the TRL-6 Testbed Telescope (TBT) performance requirements are discussed.
The primary mirror of the James Webb Space Telescope (JWST) consists of 18 segments and is 6.6 meters in diameter.
A sequence of commissioning steps is carried out at a single field point to align the segments. At that single field point,
though, the segmented primary mirror can compensate for aberrations caused by misalignments of the remaining
mirrors. The misalignments can be detected in the wavefronts of off-axis field points. The Multifield (MF) step in the
commissioning process surveys five field points and uses a simple matrix multiplication to calculate corrected positions
for the secondary and primary mirrors. A demonstration of the Multifield process was carried out on the JWST Testbed
Telescope (TBT). The results show that the Multifield algorithm is capable of reducing the field dependency of the TBT
to about 20 nm RMS, relative to the TBT design nominal field dependency.