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.
The principal science goal of directly imaging exoplanets is spectroscopy. Future mission concepts such as LU- VOIR and HabEx have baselined their science instruments to use an integral field spectrograph (IFS) to maximize observing efficiency. Coronagraphic imaging with an IFS has strong heritage at ground observatories, having produced planet and brown dwarf near-infrared spectra at contrast floors as low as high 1 × 10-6 levels. As a technology demonstration for future missions the WFIRST coronagraph instrument (CGI) will demonstrate the ability to measure spectra in visible light with at contrast levels sufficient for detecting Earth-like planets. The spectrograph was originally meant to match that of future missions; an independent IFS channel for wavefront control and imaging spectroscopy. Due to a series of de-scopes as the project progressed, the baseline spectroscopic capability of CGI is no longer an IFS but a zero deviation optical prism and slit used in conjunction with the direct imaging camera. Regardless, CGI advanced a lenslet IFS through the preliminary design phase of a flight project. Here we present that development as the challenges and requirements of an IFS for coronagraphy are quite unique and relevant to future missions. With that development in place, we also present the initial design of the zero optical deviation prism, highlighting the relative challenges and performance of each concept.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.