Prabal Saxena

Simulating the WFIRST coronagraph integral field spectrograph

A primary goal of direct imaging techniques is to spectrally characterize the atmospheres of planets around other stars at extremely high contrast levels. To achieve this goal, coronagraphic instruments have favored integral field spectrographs (IFS) as the science cameras to disperse the entire search area at once and obtain spectra at each location, since the planet position is not known a priori. These spectrographs are useful against confusion from speckles and background objects, and can also help in the speckle subtraction and wavefront control stages of the coronagraphic observation. We present a software package, the Coronagraph and Rapid Imaging Spectrograph in Python (crispy) to simulate the IFS of the WFIRST Coronagraph Instrument (CGI). The software propagates input science cubes using spatially and spectrally resolved coronagraphic focal plane cubes, transforms them into IFS detector maps and ultimately reconstructs the spatio-spectral input scene as a 3D datacube. Simulated IFS cubes can be used to test data extraction techniques, refine sensitivity analyses and carry out design trade studies of the flight CGI-IFS instrument. crispy is a publicly available Python package and can be adapted to other IFS designs.

Current science requirements and planned implementation for the WFIRST-CGI Integral Field Spectrograph (IFS)

One of the key science goals of the Coronograph Instrument (CGI) on the WFIRST mission is to spectrally characterize the atmospheres of planets around other stars at extremely high contrast levels. To achieve this goal, the CGI instrument will include a integral field spectrograph (IFS) as one of the two science cameras. We present the current science requirements that pertain to the IFS design, describe how our design implementation flows from these requirements, and outline our current instrument design.

WFIRST CGI integral field spectrograph performance and post-processing in the OS6 observing scenario

The WFIRST coronagraph instrument (CGI) will have an integral field spectrograph (IFS) backend to disperse the entire field of view at once and obtain spatially-resolved, low-resolution spectra of the speckles and science scene. The IFS will be key to understanding the spectral nature of the speckles, obtain science spectra of planets and disks, and will be used for broadband wavefront control. In order to characterize, predict, and optimize the performance of the instrument, we present a detailed model of the IFS in the context of the new OS6 observing scenario. The simulation includes spatial, spectral, and temporal variations of the speckle field on the IFS detector plane, which allows us to explore several post-processing methods and assess what gains can be expected. The simulator includes the latest models of the detector behavior when operating in photon-counting mode.

Wavefront control methods for high-contrast integral field spectroscopy

Direct Imaging of exoplanets using a coronagraph has become a major field of research both on the ground and in space. Key to the science of direct imaging is the spectroscopic capabilities of the instrument, our ability to fit spectra, and understanding the composition of the observed planets. Direct imaging instruments generally use an integral field spectrograph (IFS), which encodes the spectrum into a two-dimensional image on the detector. This results in more efficient detection and characterization of targets, and the spectral information is critical to achieving detection limits below the speckle floor of the imager. The most mature application of these techniques is at more modest contrast ratios on ground-based telescopes, achieving approximately 5-6 orders of magnitude suppression. In space, where we are attempting to detect Earth-analogs, the contrast requirements are more severe and the IFS must be incorporated into the wavefront control loop to reach 1e-10 detection limits required for Earth-like planet detection. We present the objectives and application of IFS imagery for both a speckle control loop and post-processing of images. Results, tested methodologies, and the future work using the Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS) and the Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies (PISCES) at the JPL High Contrast Imaging Testbed are presented.

Commissioning and performance results of the PISCES instrument

Direct imaging of exoplanets has become a priority in the field of exoplanet discovery and characterization due to its ability to directly obtain evidence about a planet’s atmosphere and some bulk properties. Features such as atmospheric composition, structure and clouds are just some of the planetary properties obtainable from directly imaged spectra. However, detecting and observing spectra of exoplanets using direct imaging is challenging due to the combination of extreme star to planet contrast ratios and the relatively small apparent physical separation between a host star and an orbiting planet. Detection of Earth-sized planets in reflected visible light requires contrast ratios of 1010, while even detection of Jupiter-sized planets and large young self-luminous planets requires contrast ratios of 108 and 106, respectively. Consequently, direct detection of exoplanets requires observing strategies which push the boundaries of high contrast imaging. The use of coronagraphy to occult a host star has been combined with adaptive optics (AO) technology to yield a particularly promising means of potentially achieving the required contrast ratios in regions close-in enough to the host star. Ground based adaptive optics systems such as The Gemini Planet Imager (GPI)1 and Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE)2 instrument have been able to achieve contrast ratios nearing 107 using post-processing techniques3, 4 and have yielded a number of direct detections of young self-luminous planets. Advancing these technologies onto a space based platform immune to the difficulties posed by the effects of Earth’s atmosphere is the next step in accessing even larger contrast ratios.

Flight Integral Field Spectrograph (IFS) optical design for WFIRST coronagraphic exoplanet demonstration (Conference Presentation)

Based on the experience from Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies (PISCES) for WFIRST, we have moved to the flight instrument design phase. The flight instrument is similar to PISCES, but there are important changes to its design as our requirements have evolved. Beginning with the science and system requirements, we discuss a number of critical trade-offs. Most significantly there are trades in the type of IFS, lenslet array shape and layout, detector sampling, and accommodating the larger Field Of View (FOV) and wider wavelength band for a potential Starshade. Finally, the traditional geometric optical design is also investigated and traded. We compare a reflective versus refractive design, and the telecentricity of the relay. The relay before the lenslet array controls the chief angle distribution on the lenslet array. Our previous paper1 has addressed how the relay design combined with lenslet array/pinhole mask can further suppress the residual star light and increase the contrast. Highlighting all of these design trades, we present the phase A IFS optical design for the WFIRST coronagraph instrument.

A model of the primordial lunar atmosphere

Abstract We create the first quantitative model for the early lunar atmosphere, coupled with a magma ocean crystallization model. Immediately after formation, the moons surface was subject to a radiative environment that included contributions from the early Sun, a post-impact Earth that radiated like a mid-type M dwarf star, and a cooling global magma ocean. This radiative environment resulted in a largely Earth-side atmosphere on the Moon, ranging from ∼104 to ∼102 pascals, composed of heavy volatiles (Na and SiO). This atmosphere persisted through lid formation and was additionally characterized by supersonic winds that transported significant quantities of moderate volatiles and likely generated magma ocean waves. The existence of this atmosphere may have influenced the distribution of some moderate volatiles and created temperature asymmetries which influenced ocean flow and cooling. Such asymmetries may characterize young, tidally locked rocky bodies with global magma oceans and subject to intense irradiation.