John C. Kemp

SOFIE instrument overview

Space Dynamics Laboratory (SDL) recently designed, built, and delivered the Solar Occultation for Ice Experiment (SOFIE) instrument as the primary sensor in the NASA Aeronomy of Ice in the Mesosphere (AIM) instrument suite. AIMs mission is to study polar mesospheric clouds (PMCs). SOFIE will make measurements in 16 separate spectral bands, arranged in eight pairs between 0.29 and 5.3 μm. Each band pair will provide differential absorption limb-path transmission profiles for an atmospheric component of interest, by observing the sun through the limb of the atmsophere during solar occulation as AIM orbits Earth. A pointing mirror and imaging sun sensor coaligned with the detectors are used to track the sun during occulation events and maintain stable alignment of the sun on the detectors. This paper outlines the mission requirements and goals, gives an overview of the instrument design, fabrication, testing and calibration results, and discusses lessons learned in the process.

Wide-field Infrared Explorer (WIRE)

The wide-field infrared explorer (WIRE) is a small spaceborne telescope specifically designed to study the evolution of starburst galaxies. This powerful astronomical instrument will be capable of detecting typical starburst galaxies at a redshift of 0.5, ultraluminous infrared galaxies beyond a redshift of 2, and luminous protogalaxies beyond a redshift of 5. The WIRE survey, to be conducted during a four month period during 1998, will cover over 100 deg2 of high galactic latitude sky at 12 and 25 micrometer. WIRE will measure the ratio of 12 and 25 micrometer flux of detected sources, which is a powerful statistical luminosity indicator. The distribution of starburst galaxy 12-25 micrometer colors as a function of flux density will reveal their evolutionary history and perhaps the presence of protogalaxies at high redshifts. This mission, which is part of the NASA Small Explorer program, takes advantage of recent advances in infrared array detector technology to provide a large sensitivity gain over previously flown missions. During its four-month mission lifetime, WIRE will amass a catalog exceeding the size of the 1983 Infrared Astronomy Satellite (IRAS) Point Source Catalog at flux levels over 500 times fainter than the IRAS Faint Source Catalog. WIRE has been designed to maximize detections of high-redshift starburst galaxies using an extremely small and simple instrument. The 30 cm aperture Cassegrain telescope has no moving parts, no reimaging optics and a wide 33 by 33 arcminute field of view. The optics and detectors are cooled during the mission using a lightweight two-stage solid hydrogen cryostat. The three-axis stabilized spacecraft bus is provided by the Goddard Space Flight Center Small Explorer Project Team. The mission, to be launched in September 1998 using an Orbital Sciences Corporation Pegasus XL Launch Vehicle, is managed by GSFC.

Terrestrial Measurement of the Performance Of High-Rejection Optical Baffling Systems

Spatial rejection is critical when exo-atmospheric or in situ radiometric measurements of faint sources, such as air glow emissions, are made in the presence of relatively intense sources such as the sun, moon, or earth. Scattering from atmospheric molecules and aerosols during evaluation in an earth-bound laboratory makes the high-rejection baffle appear to be less effective than it actually is in the measurement situation. A Monte Carlo computer program SCAT was written to predict the effects of atmospheric scattering upon field-of-view calibration measurements of optical baffling systems. It was found that Mie scattering could be reduced in a clean room environment and Rayleigh scattering was determined to be the limiting mechanism in a special chamber designed for off-axis measurements. The background flux at 40° off axis was found to be 1 x 10-9 of the on-axis incident flux for a 5° full-angle baffle when illuminated by a 10.2 cm diameter collimated beam. A two-step process was used to measure a baffle response at 40° off axis down to 1 x 10-11 of the on-axis incident flux.

Measurement Of High Out-Of-Band Filter Rejection Characteristics

The high dynamic range of naturally occurring atmospheric emissions can present a serious challenge to infrared instrumentation designers. The requisite optical filters have out-of-band rejection requirements that are beyond the capability of the standard measuring techniques. While the filter manufacturers can predict their filter performance to more than seven orders of magnitude, their measurement capability is limited to about four orders. The Space Dynamics Laboratory has recently tested a new measurement method which employs a cascaded test filter and a Michelson interferometer spectrometer.

Cryogenic Michelson Interferometer Spectrometer For Space Shuttle Application

A Michelson interferometer spectrometer using a flexural pivot suspension for the moving mirror was fabricated for use at 20° K as part of the CIRRIS 1A experiment. The spectral range 2.5 to 25 μm is achieved using a potassium bromide beamsplitter. The softness of the beamsplitter material required special mounting care to preserve beamsplitter flatness while undergoing the strain of being cooled and the shock and vibration of the shuttle launch. Five various sized elements in the arsenic-doped-silicon focal plane provide for tradeoff of sensitivity, spatial and spectral resolution capabilities. A redundant position reference system uses optical fibers to couple optical power from HeNe lasers through the vacuum/cryogenic interface into the interferometer optics where it travels antiparallel to the infrared signal. An alignment system using geared stepper motors provides capability of realignment in space. An eight-position filter wheel is used to enhance the out-of-band signal rejection to enhance high sensitivity in the presence of strong infrared emitters. The interferometer is mounted inside of a high-off-axis rejection telescope to enable measurement of the earth limb emissions. The telescope is cooled to 20° K using supercritical liquid helium.

Rocket-Borne Cryogenic Michelson Interferometer

A nitrogen-cooled Michelson interferometer was fabricated using a unique flexural pivot mirror translation system which allows a proportionally large aperture and is ideally suited for operation at cryogenic temperatures. Cooling the entire interferometer yields a sensitivity sufficient to measure weak atmospheric emissions from an electron-gun induced artificial aurora. The spectral range 2.0 to 5.6 µm is scanned at a repetition rate of 1.8 seconds with an apodized resolution of 2 cm-1. Piezoelectric elements in the fixed mirror mount allow realignment at cryogenic temperatures. Laser (sampling) and white light (absolute position) reference channels are run antiparallel to the main channel using the same optics. When launched aboard a Talos Castor rocket as part of the EXCEDE payload the interferometer maintained alignment within 20 percent of launch modulation efficiency.

SOFIE Instrument Model and Performance Comparison

Space Dynamics Laboratory (SDL), in partnership with GATS, Inc., designed, built, and calibrated an instrument to conduct the Solar Occultation for Ice Experiment (SOFIE). SOFIE is the primary infrared sensor in the NASA Aeronomy of Ice in the Mesosphere (AIM) instrument suite. AIMs mission is to study polar mesospheric clouds (PMCs). SOFIE will make measurements in 16 separate spectral bands, arranged in 8 pairs between 0.29 and 5.3 μm. Each band pair will provide differential absorption limb-path transmission profiles for an atmospheric component of interest, by observing the sun through the limb of the atmosphere during solar occultation as AIM orbits Earth. A fast steering mirror and imaging sun sensor coaligned with the detectors will track the sun during occultation events and maintain stable alignment of the Sun on the detectors. This paper outlines the instrument specifications and resulting design. The success of the design process followed at SDL is illustrated by comparison of instrument model calculations to calibration results, and lessons learned during the SOFIE program are discussed. Relative spectral response predictions based on component measurements are compared to end-to-end spectral response measurements. Field-of-view measurements are compared to design expectations, and radiometric predictions are compared to results from blackbody and solar measurements. Measurements of SOFIE detector response non-linearity are presented, and compared to expectations based on simple detector models.

Terrestrial black hole for measuring high-rejection off-axis response

An off-axis scatter facility was developed to support the Space Dynamics Laboratory in a number of earth limb measurement programs where the off-axis performance of the sensors was critical to the validity of the data. The facility was developed from three fundamental assumptions. (1) Careful control of any light scattered from the optical system being measured to make certain that it did not return to re-enter the system and corrupt the measurement. (2) Use of black specular surfaces in a unique shape to direct and attenuate scattered light. (3) Utilization of clean room technology to filter air to reduce scattering from particulates in the air and to prevent dust from degrading the specularity of the special surfaces. Therewithal analyses of atmospheric and surface scattering showed that surface scattering effects could be suppressed below atmospheric scattering limits by use of properly shaped specular walls. Analysis of measurements made in the facility demonstrated that the measurements were limited by Rayleigh scattering from the air molecules in the facility and not from dust or water droplets in the air nor from scattering from any chamber surfaces. Measurements of the Cassini narrow field camera showed a noise floor at 2.8E-12 of on-axis response.

Rocket-Borne Cryogenic Interferometer Spectrometer Used For An Artificial Auroral Measurement

A unique flexural pivot mirror translation system was used in the fabrication of a cryogenic Michelson interferometer spectrometer. Uniform cooling was obtained by placing the interferometer inside a convective atmosphere within a liquid nitrogen dewar. The interferometer was flown as part of the EXCEDE SPECTRAL rocket payload which also included electron accelerators to produce artificial auroras. The interferometer successfully measured infrared upper-atmospheric emissions within the spectral range 2.0 to 5.6 μm as induced by the pulsed electron accelerators. Observed emissions include the N2 Wu Benesch infrared system, the N2 McFarlane infrared system and CO2. In addition, H2O outgassing from the payload was observed. The 2 cm-1 spectral resolution of the interferometer clearly assisted in the unique identification of the dominant emissions based on their characteristic molecular band profiles.

Focal planes and mount assemblies for the WIRE program

The wide-field infrared explorer (WIRE) is a small spaceborne cryogenic telescope specifically designed to study the evolution of starburst galaxies. The use of advanced, large format, infrared hybrid focal plane array technology provides a large sensitivity gain over previously flown missions. The hybrid focal plane arrays (HFPAs) used in this instrument are 128 by 128-element arsenic-doped-silicon blocked impurity band infrared detector arrays connected via indium column interconnects to matching cryogenic multiplexers. The WIRE instrument includes two focal plane mount assemblies (FPMAs), each of which includes a HFPA optimized for a particular wavelength band. Details concerning design, fabrication and performance of the critical components of the WIRE FPMAs are described.