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Hari Subedi

Publications and source records attributed to Hari Subedi.

7 records
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

WFIRST CGI: Prism and Polarizer Characterization Modes

As part of its technology demonstration, the WFIRST 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 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 and is designed to accommodate a 15% bandpass spanning 610-785nm. 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, 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 stability based on thermal and structural predictions, 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.

Tyler D Groff↗
NASA NTRSDate not supplied

Verification and Calibration of Spectroscopy and Polarization modes for the Roman Coronagraph Instrument

As part of its technology demonstration, the Nancy Grace Roman Space Telescope Coronagraph Instrument (CGI) will demonstrate point source spectroscopy and polarization measurements of disks. The spectroscopy mode is a zero-deviation Amici prism and slit to a confirmed planet after an imaging detection. The Wollaston prism polarization optics allow for imaging two orthogonal polarization states simultaneously. The CGI spectral characterization modes, designed and built at Goddard Space Flight Center (GSFC), have a spectral resolution of R50 in two 15% bandpass centered at 660nm and 730nm intended to capture key methane absorption features. There are duplicate Wollaston prism channels, clocked 45 degrees with respect to one another covering 10% bandpasses at 575 and 825nm, but the optics meet performance requirements across the full CGI bandpass. The Wollaston design and optical elements are a contribution by the Japanese Aerospace Exploration Agency, with final alignment and testing being done at GSFC. We highlight the requirements for these modes, the ground-to-orbit calibration process, and the operations required to use a deployable slit on a CGI point source in the presence of pointing error. We also provide further detail on the optomechanical design, testing results from the final as-built flight units, verification process, and performance of the as-built flight assemblies.

Tyler D. Groff↗
NASA NTRSDate not supplied

The Parabolic Deformable Mirror Testbed at NASA Goddard Space Flight Center

One of the primary goals of future coronagraph space missions is to spectrally characterize Earth-like exoplanets. Observing efficiency is critical and directly related to how large of a field of view we can correct and the bandwidth of that correction. In addition to the coronagraph itself, the bandwidth and depth of contrast is related to the wavefront control system, which classically uses two deformable mirrors (DMs) in series. The minimum distance between these DMs is determined by the Talbot distance of the beam, meaning it scales as the square of the beam size. NASA Goddard is exploring an alternative design architecture to the two-DM approach in which the reimaging optics for the coronagraph are also deformable and we only retain a high-order pupil DM. With these off-axis parabolic elements being made deformable, the long propagation distance between the two DMs is eliminated. Goddard has procured an off-axis parabolic DM (PDM or OAP DM) from vendor ALPAO, France. A testbed is currently being built to test the PDM and demonstrate its utility for achieving high contrast in a coronagraph. The testbed is also being built with an integrated low-order wavefront sensing capability, allowing for integrated testing of a dynamic environment with multiple estimation and control loops.

Hari Subedi↗
NASA NTRSDate not supplied

Experimental Verification of the Parabolic Deformable Mirror for the ExoSpec Project

For the Habitable Worlds Observatory, it is essential to broaden the controllable wavelength bandwidth for high-contrast imaging and spectroscopy to increase the exoEarth yield and characterization. The Parabolic Deformable Mirrors (PDM) under the NASA Headquarters directed ExoSpec Work package is specifically tailored to do so. We have successfully procured a generation 1 (Gen 1) PDM device and completed in-depth characterization of the device. This robust evaluation has become instrumental in informing subsequent stages of development, particularly in shaping the design and specifying requirements for the next generation, Gen2, PDM device. We have built a testbed in an environmentally controlled cleanroom to experimentally demonstrate the use of a parabolic DM in a coronagraph instrument as well an integral field spectrograph (IFS). This versatile testbed is designed to test different DM architectures, various low-order wavefront schemes, and a lenslet based IFS. This provides us with a basis for comparison with different DM configurations – 1) flat DM, 2) parabolic DMs, and 3) a flat DM and parabolic DMs. In this communication, we will discuss the testbed design and updates, parabolic DM characterization, Gen 2 requirement definitions, and different tests planned for the testbed.

Hari Subedi↗