SAGE IV Pathfinder multi-spectral imaging spectrometer telescope paves the way for semi-custom CubeSat imaging missions

Abstract

The SAGE IV (Stratospheric Aerosol and Gas Experiment) Pathfinder looks towards ushering in the next generation of the SAGE family of instruments, leveraging solar occultation to retrieve vertical profiles of aerosols and gases in the stratosphere, providing high precision calibration data for other instruments. A development funded through the NASA Earth Science Technology Office (ESTO) Instrument Incubator Program (IIP) SAGE IV Pathfinder is designed to extend the data record from the SAGE III scanning grating spectrometer with a multispectral imaging approach. Solar disk imaging improves the data collected by providing: (1) absolute pointing information; (2) measurements of atmospheric refraction effects; and (3) measurements of solar disk anisotropy. This additional information relaxes traditionally tight constraints on attitude knowledge, stability, and pointing control making a free-flying 6U CubeSat instrument feasible. Early estimates show this approach might reduce the cost of SAGE continuity missions by as much as 90%. A key benefit of the SAGE IV Pathfinder design to future missions is the versatility of the resultant telescope subsystem. The F/5.25 telescope resulted in <90% encircled energy within a 30 μm/28 arcsecond pixel and point source normalized irradiance transmittance (PSNIT) of <1E-4 0.5° outside of the field of view (FOV). The baseline design can be adapted to accommodate changes to layout, aperture, focal lengths, filters, and/or detectors in various CubeSat form factors. The telescope was designed to be thermally agnostic, with STOP analysis results indicating negligible performance variation as thermal gradients fluctuate on orbit. Once thermal validation of STOP analysis is completed, proven micron-level alignment, mounting, and analyses can then be leveraged for new high performance, semi-custom instruments, saving significant development cost for future science missions.