John P. Hackett

The JWST fine guidance sensor

The science instrumentation for the James Webb Space Telescope (JWST) has concluded its Phase A definition stage. We have developed a concept for the JWST Fine Guidance Sensor (FGS), which will form the Canadian contribution to the mission. As part of the JWST re-plan in early 2003, the FGS design was recast to incorporate a narrow-band (R~100) science-imaging mode. This capability was previously resident in the NIRCam instrument. This FGS science mode makes use of tunable filters and filter wheels containing blocking filters, calibration sources and aperture masks. The science function of the FGS Tunable Filters (FGS-TF) remains complementary to the NIRCam science goals. Narrow-band FGS-TF imaging will be employed during many of the JWST deep imaging surveys to take advantage of the sensitivity to emission line objects. The FGS-TF will also provide a coronagraphic capability for the characterization of host galaxies of active galactic nuclei and for the characterization of extra solar planets. The primary function of the FGS remains to provide the sensor data for the JWST Observatory line-of-sight stabilization system. We report here on the overall configuration of the FGS and we indicate how the concept meets the performance and interface requirements.

MOPITT On-Orbit Stirling Cycle Cooler Performance

The Measurements of Pollution In The Troposphere (MOPITT) instrument was launched aboard the Terra spacecraft (formerly known as EOS AM-1), from Vandenburg Air Force Base, California on Dec 18th 1999. At present the instrument is in normal operations mode, having undergone its outgas and activation phases.

MOPITT Stirling Cycle Cooler Vibration Performance Results

Both of the Measurements Of Pollution In The Troposphere’s (MOPITT) instrument detector subassemblies require cooling to 90 K with 850 mW head load per subassembly. The stringent spacecraft level mechanical disturbance specification requires compressor forces to be ≤ 200 mNrms and displacer forces to be ≤ 50 mNrms For the first seven (7) harmonics, an acceleration feedback system using two Matra Marconi Space (MMS) 50–80 K Stirling Cycle Coolers (SCCs) and Lockheed-Martin Low Vibration Drive Electronics (LVDE) was used to meet these requirements.

Layout and packaging of the MOPITT instrument

The Measurements Of Pollution In The Troposphere (MOPITT) instrument is one of five instruments flying on NASAs Terra (formerly EOS-AM1) spacecraft that was launched in December 1999. This paper describes the MOPITT instrument mechanical configuration and how it was derived based on system considerations and spacecraft interfaces. These system level considerations include contamination control, EMC/EMI, thermal, optical and structural behavior. The key spacecraft interfaces include mechanical mounting, optical field-of-view (FOV), and thermal transfer. In addition, a detailed discussion is provided for the cryogenic region of the instrument that contains detectors, cold optics, warm optics, and active coolers. Special test fixtures were designed and incorporated in this region of the instrument to permit cooling of the detectors during ambient atmospheric conditions. Some of these test fixtures were designed to fly due to the difficulty in removing them. This utility of operating the instruments cryogenic detectors within the laboratory environment was extremely beneficial during the instrument optical alignment, EMC testing, and special optical system tests. Final configuring (or closeout) of the instruments cryogenic region for flight was performed to balance contamination and EMC risks. On-orbit data about the effectiveness of this closeout is provided.

Timing control and signal processing design of the MOPITT instrument

The MOPITT instrument operates on the principle of correlation spectroscopy where the incoming signal is modulated by gas filter and chopper mechanisms and synchronously demodulated within the signal processing system. The performance and flexibility required by the MOPITT instrument resulted in the development of a novel timing control and signal processing design. This design synchronizes modulation and demodulation from a central programmable timing control unit. The data collection system performs a highly linear sigma-delta analog-to-digital conversion prior to signal demodulation. The demodulation operation includes data averaging which reduces the sampled signal bandwidth and extends the signal to noise ratio of the data to in excess of the analog-to-digital converters rated 16-bit dynamic range.

Flight electronics for vibration cancellation in cryogenic refrigerators: performance and environmental testing results

Space flight optical instruments and their support hardware must reliably operate in stressing environments for the duration of their mission. They must also survive the mechanical and thermal stresses of transportation, storage and launch. It is necessary to qualify the hardware design through environmental testing and to verify the hardwares ability to perform properly during and/or after some selected environmental tests on the ground. As a rule, flight electronics are subjected to thermal, mechanical and electromagnetic environmental testing. Thermal testing takes the form of temperature cycling over a temperature difference range (Delta) T of up to 100 degrees C for a minimum of six cycles, with additional performance verification testing at the hot and cold extremes. Mechanical testing takes the form of exposure to random vibration, sine sweep vibration, shock spectra and static loading on a centrifuge or by sine burst on a vibration table. A standard series of electromagnetic interference and electromagnetic compatibility testing is also performed.