We establish a viable laser payload design for the Orbiting Configurable Artificial Star (ORCAS) mission. We share observational considerations and derive the engineering requirements for the laser payload. Developed by general Atomics Electromagnetic Systems, the dual-wavelength laser will operate at 1064 nm and can be frequency-doubled to 532 nm, with two possible beam divergence modes and tunable power. The laser payload can be operated at pulse repetition rates greater than 10 kHz to enable compatibility with Adaptive Optics systems and to maintain pointing requirements. We show that such a laser payload can be constructed based upon a high-TRL amplified fiber laser Communication Terminal modified to meet the mission requirements.
KEYWORDS: Stars, Exoplanets, Telescopes, Signal to noise ratio, Sun, Space operations, Planets, Optimization (mathematics), Large telescopes, Detection and tracking algorithms
We present optimized observation schedules for a distributed configuration of the Remote Occulter Mission. Accounting for refueling rounds, we show that an Earth-orbiting Remote Occulter could enable up to 158 ground-based observations of 80 exoplanetary targets in a mission lifetime. We develop two target lists, provide exposure time estimates for each potential target star, present an analytic approach for determining target observability, and estimate the cost of station-keeping and retargeting maneuvers required to maintain such a mission. We optimize the mission observation schedule over these cost and science delivery estimates using deterministic and metaheuristic optimization methods with varying degrees of operator intervention and conclude by assessing mission profile sensitivity to both isolated and accumulated cost and design perturbations.
The Remote Occulter (Orbiting Starshade) is a proposed 100-meter class starshade working with a ground-based telescope, designed for visible-band imaging and spectroscopy of temperate planets around sun-like stars. With advanced adaptive optics and the largest telescopes like the 39 m ELT, it would enable the study of planetary systems and a wide variety of exoplanets. In this paper, we describe the geometrical constraints and establish which parts of the sky are observable.
Circular polarization of scattered solar radiation is essentially zero for almost all aerosol and cloud cases. Required conditions for non-zero circular polarization include multiple-scattering and large scatterer size relative to wavelength. The single-scattering of incident solar radiation can produce linearly polarized light but not circularly polarized light. A second scattering event can transform some of the linearly polarized light into circularly polarized light. Additional scattering events can both create and destroy circular polarization via the transformation process with linear polarization. The peak in circular polarization ratio magnitude occurs at the optical depth for which the multiplescattering processes have maximized its creation-to-destruction rate. Provided multiple-scattering has occurred, circular polarization can only exist for scatterers of large size relative to the wavelength. For aerosols, this implies desert dust or oceanic aerosols and short wavelength observations (i.e., less than 0.5μm). All cloud particles are considered large as they are roughly an order of magnitude larger than aerosols.
This paper presents a conceptual approach toward the remote sensing of cirrus cloud particle size and optical depth using the degree of polarization and polarized reflectance associated with the first three Stokes parameters I, Q, and U for the 0.865 and 2.25 μm wavelengths. A vector line-by-line equivalent radiative transfer program including the full Stokes parameters based on the adding method was developed. The retrieval algorithm employs the steepest descent method in the form of a series of numerical iteration procedures to search for the simulated polarization parameters that best match the measured polarization parameters. Sensitivity studies were performed to investigate the behavior of phase matrix elements as functions of scattering angles for three ice crystal size-shape combinations. Overall, each phase matrix element shows some sensitivity toward ice crystal shape, size, and suface roughness due to the various optical effects. Synthetic retrievals reveal that the retrieval algorithm itself is highly accurate, while polarimetric and radiometric error sources cause very small retrieval errors. Finally, an illustrative example of applying the retrieval algorithm to airborne POLDER data during EUCREX is presented.
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