Measuring a target’s radial velocity is usually achieved using high-resolution spectroscopy; however, higher signal to noise ratios can be obtained using direct correlation spectrometers (DCSs). In our system, a liquid crystal spatial light modulator serves as the mask against which the incident spectrum is correlated, and the polarization is controlled to enable both in- and out-of-band light to be captured simultaneously. This offers enhanced performance against atmospheric scintillation and may also enable single-shot radial velocity measurements. In this paper, we describe the design and implementation of our polarization-DCS and experimental validation is performed by acquiring radial velocity measurements of Venus.
The design of a Fraunhofer line optical correlator is detailed. The instrument described herein correlates a reflected solar
spectrum against multiple Fraunhofer absorption lines to estimate the radial velocity of the reflecting body. By using a
spatial light modulator (SLM) as a photomask for known solar absorption lines in the visible spectrum, the ratio of
Doppler shifted solar energy to the total received energy can be calculated. Although the reflected light from targets in
high orbit is weak, signal-to-noise ratio (SNR) is enhanced by the measurement of multiple Fraunhofer lines in a single
snapshot image. Simulations indicate that prediction of orbital parameters is improved by incorporation of this velocity
information, and in some cases the number of line-of-sight measurements can be reduced from three to two.
Ultraspectral sensing has been investigated as a way to resolve terrestrial chemical fluorescence within solar Fraunhofer lines. Referred to as Fraunhofer Line Discriminators (FLDs), these sensors attempt to measure "band filling" of terrestrial fluorescence within these naturally dark regions of the spectrum. However, the method has challenging signal to noise ratio limitations due to the low fluorescence emission signal of the target, which is exacerbated by the high spectral resolution required by the sensor (<0.1 nm). To now, many Fraunhofer line discriminators have been scanning sensors; either pushbroom or whiskbroom, which require temporal and/or spatial scanning to acquire an image. In this paper, we attempt to quantify the snapshot throughput advantage in ultraspectral imaging for FLD. This is followed by preliminary results of our snapshot FLD sensor. The system has a spatial resolution of 280x280 pixels and a spectral resolving power of approximately 10,000 at a 658 nm operating wavelength.
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