KEYWORDS: Prisms, Calibration, Polarizers, Polarization, Spectroscopes, Spectral resolution, Equipment, Target detection, Space telescopes, Signal to noise ratio
The Nancy Grace Roman Space Telescope Coronagraph Instrument (CGI) will demonstrate spectroscopy of planets and polarization measurements of disks. The spectroscopy and polarization modes utilize Amici and Wollaston prism designs. The spectroscopy mode, designed and built and Goddard Space Flight Center (GSFC), has a resolution of R50 in 15% bands centered at 660nm and 730nm. The Wollaston design and optics are contributed by the Japanese Aerospace Exploration Agency, with final alignment and testing at GSFC. We present the requirements, ground-to-orbit calibration, and deployable slit operations. We also detail on the design, results from the as-built flight assemblies.
As part of its technology demonstration, the Nancy Grace Roman Space Telescope (RST) Coronagraph Instrument (CGI) will demonstrate point source spectroscopy and polarization measurements of disks. The specific implementation of spectroscopy is a zero-deviation Amici prism and a slit to be placed on the planet after high contrast has been achieved by CGI. The polarization optics are a set of Wollaston prisms so that orthogonal polarization states can be measured simultaneously. The CGI spectral characterization mode, being designed and built and Goddard Space Flight Center (GSFC), has a spectral resolution of R50 at its central wavelength and is designed to accommodate a 15% bandpass spanning 610785nm. In order to recover Stokes information, there are two sets of Wollaston prisms clocked 45 degrees with respect to one another with each measurement taken in series. The Wollaston design and optical elements are a contribution by the Japanese Aerospace Exploration Agency (JAXA), with final alignment and testing being done at GSFC. The spectroscopy mode is designed to target Methane absorption features around 730nm, keeping the spectral resolution as low as possible to improve the signal-to-noise ratio and hence reduce detection time. We highlight the requirements for these modes and address the challenge of on-orbit spectral calibration for a deployable slit in the presence of pointing drifts. Of unique interest is how the observatory error budget couples into good stellar spectrum calibration and subsequent speckle subtraction. We also provide further detail on the optomechanical design, its modeled performance, and operations concept. These performance metrics are simulated to demonstrate how a slit located at an arbitrary field point is homed onto the planet and converted to a calibrated spectrum.
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