The stray light analysis of the sounding of the atmosphere using broadband emission radiometry (SABER) instrument on the thermosphere-ionosphere-mesosphere energetics and dynamics (TIMED) mission is discussed. Relevant mission objectives and operating conditions are stated to define the stray light problem. Since SABER is an earth limb viewing sensor, the telescope must be designed for large off-axis rejection. Described are the key design features which make the instrument well suited for its mission. Representative point source transmittance (PST) curves computed using the commercial stray light program APART are presented. Non-rejected radiance (NRR) values computed using APART generated PST curves and LINEPACK generated curves for the total radiance from the earth aixi the afinosphere are given. A method for computing NRR from the earth and the atmosphere using lineof-sight radiance profiles versus tangent height is described. Computed NRR values demonstrate that the effect of stray light on SABER's measurement capability is negligible.
This paper describes the design of a 10-channel infrared (1 .27 to 16.9 jim) radiometer instrument known as SABER (sounding of the atmosphere usingbroadband emissionradiometry) that will measure earth-limb emissions from the TiMED (thermosphere-ionospheremesosphere energetics and dynamics) satellite. The instrument telescope, designed to reject stray. light from the earth and the atmosphere, is an on-axis Cassegrain design with a clam shell reimager and a one-axis scan mirror. The telescope is cooled below 210 K by a dedicated radiator. The focal plane assembly (consisting of a filter array, a detector array, a Lyot stop and a window) is cooled to 75 Kby a miniature cryogenic refrigerator. The conductive heat load on the refrigerator is minimized by a Keviar support system that thermally isolates the focal plane assembly from the telescope. Kevlar is also used to thermally isolate the telescope from the spacecraft. Instrument responsivity drifts due to changes in telescope and focal plane temperatures as well as other causes are neutralized by an in-flight calibration system. The detectOr airay consists ofdiscrete IJgCdTe, JnSb and InGaAS detectors. Two InGaAS detectors are a new long wavelength type, made by EG&G, that have a long wavelength cutoffof2.33 im at 77 K.1. IPTRODUCTION SABER (sounding ofthe atmosphere using broadband emissionradiomelry) is an earth-limb-scanning radiometer that has been selected as one of the four payload instruments on TIMED (thermosphere-ionosphere-mesosphere energetics and dynamics) satellite to be launched in OctOber 1998. The TIMED orbit altitude is 600 km and the orbit inclination is 74.4 degrees. SABER will look 90 degrees to the ram. The mission life is 2 years.The SABER systems requirement review(SRR) and the conceptual design review (CoDR) were held in April 1 995, and the preliminaiy design review is scheduled for April 1996. Significant modifications to the SABER design described in the literature3 have been made in the last year. Many ofthese modifications resulted because the previous optical design required filters that were impractically thick to correct for chromatic focal shifts across the very wide spectral band covered by SABER. A new optical design that solved this problem and resulted in a much more rugged instrument was developed and is described in this paper. The stray light performance of this new design is excellent. This paper is intended to provide a comprehensive overview ofthe new SABER design. SYSTEM DESIGN -A functional diagram ofthe SABER instrument is shown in Figure 1. A high off-axis rejection telescope collects wanted light and discriminates against unwanted light. The scan mirror scans the instrument field ofview vertically across the earth limb. In orbit the telescope is oriented so the nadir_zenith line is vertical in Figure 1 and local depression angles are measured relative to the horizontal. The baffle opening allows the center of SABER's 1 .4 degree wide field ofview to be scanned across depression angles from 11.148 to 26.168 degrees. Thi...
The Sounding of the Atmosphere using Broadband Emission Radiometiy (SABER) instrument is a 10-channel earth limbviewing sensor that is to measure atmospheric emissions in the spectral range of 1.27 jm to 16.9 iim. Presented in this paper is the stray light design and analysis of SABER Unwanted radiation from the eaith and atmosphere are suppressed by the use of stiay light features that are Critical to mission success. These include the use ofan intermediate field stop, an inner and outer Lyot stop, and super-polished mirrors. The point source normalized irradiance transmission (PSNIT) curve, which characterizes the sensor's off-axis response, was computed using the stray light analysis program APART. An initial calculation of the non-rejected radiance (NRR) due to emissions and scatter from the earth and atmosphere was made using the PSNTT data. The results indicate that stray light will not impede the mission objectives.
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