<title>SABER instrument design update</title>

Abstract

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...

Recent progress in the Elegant Breadboard supra-THz activities for LOCUS and a view to an astronomy application

“…Scientific definition of the THz lines of interest identified the micro-windows as shown in Table 1 together with requirements on the IR channels were derived from those used in the NASA SABER [3], [4] instrument currently in orbit. For each band of interest a single THz receiver front end comprising a Schottky barrier diode mixer, local-oscillator, low noise amplifier (LNA) and a subsequent stage of room temperature of IF amplification.…”

“…Previous missions (SABER-TIMED) have in the past indirectly measured the main constituent (atomic oxygen) of the upper MLT dominates from IR measurements [2], [3] of ozone chemi-luminescence which in itself relies on theoretical reaction rates. The LOCUS mission goal is to directly measure atomic oxygen by heterodyne detection, as well as simultaneously measuring the thematically linked trace gases CO, CO2, NO, O3 and OH, several of which present novel measurements in their own right.…”

The LOw Cost Upper atmosphere Sounder: The "elegant breadboard" programme

“…The necessary attributes of the THz frequency channels were evaluated in detail against the scientific requirements, and resulted in the identification of micro-windows as shown in Table 1. The requirements on the IR channels were derived from those used in the NASA SABER [3], [4] instrument which is currently in orbit: these are given in table 2.…”

Contamination control of the SABER cryogenic infrared telescope

“…Altitude profiles in the earthlimb are acquired using a single-axis scan mirror mounted in the baffle assembly. 1,2 High off-axis rejection performance was obtained by extensive straylight analyses and optimization during design and the required use of nearly pristine, superpolished fore-optics. 3,4 Furthermore, an intermediate field stop and an inner Lyot stop are incorporated to eliminate the stray light and diffraction effects that are typically associated with on-axis telescope designs.…”