HAO 2011 Profiles In Science: Dr. Roberto Casini

Contact:

Dr. Roberto Casini is a Scientist III Section Head in the High Altitude Observatory of the National Center for Atmospheric Research. He specializes in spectro-polarimetric diagnostics of electric and magnetic fields in the solar chromosphere and corona. He is involved with the development and maintenance of numerical codes for the forward modeling and inversion of scattering polarization in spectral lines, in the presence of electric and magnetic fields. Scattering polarization with PRD in complex atoms. Lead scientist of the ProMag instrument.

Profile:

Roberto is a leading authority in the fundamental theory underlying the formation of polarized radiation emanating from the solar atmosphere. Roberto has focused his efforts in toward interpretation and instrumentation development. By defining and improving methods to help interpretation of observed solar polarization spectra, and to the development of instrumentation needed to advance the observational understanding of magnetic fields in the solar atmosphere. Roberto's research focus contributes fundamentally to the growing need for diagnostics of solar magnetic fields higher in the solar atmosphere, and also reflects an effort to apply the theory he has developed (and continues to develop) to further our understanding of the solar atmosphere. Roberto is the Instrument Scientist and Principal Investigator on the HAO ProMag instrument. He is also the PI for the Visible Spectro-Polarimeter (ViSP) being developed for the Advanced Technology Solar Telescope (ATST). In addition, Roberto sits on the Science Working Groups for both ATST and a possible future multinational space mission, Solar-C.

The demands of polarimetric science with the ATST/ViSP on the photon flux budget of the instrument have motivated a detailed study of the polarized efficiency of diffraction gratings. The Chandezon method for the diffraction efficiency was successfully implemented into a software package that was ultimately used to design custom gratings to address specific science cases for the ViSP, as well as to test the performance of the stock gratings that have been selected for first light. Another study of relevance for ATST polarimetric science was the investigation of the effects of atmospheric seeing on polarization signal cross-talk (Casini, de Wijn, and Judge 2011). The results of this work are used to determine the minimum camera frame-rate and polarization modulation frequency needed to meet the desired goal for polarimetric accuracy.

Roberto continues development of computer tools for the design of broadband, optimally efficient, polarization modulators (Tomczyk et al. 2010) and of superachromatic waveplates for polarization calibration purposes. By addeing capabilities to create multi-crystalline compounds the routines improve the efficiency response across the spectral range. With these tools we have created a set of possible designs for the polarization optics that will be implemented at the ATST. These solutions are currently undergoing feasibility checks with potential vendors.

Publications

(1)
Casini, Roberto; Alfred G. de Wijn; and Philip G. Judge. 2011: Analysis of Seeing-Induced Polarization Cross-Talk and Modulation Scheme Performance. Applied Optics, submitted.

Abstract: Polarimetric measurements of the sun made with ground-based instruments suffer from cross-talk induced by atmospheric seeing. We analyze the generation this cross-talk, and its effects on the noise statistics of spectropolarimetric measurements for both single-beam and dual-beam instruments, from a statistical point of view. We investigate the time evolution of seeing-induced correlations between different states of one modulation cycle, and compare the response to these correlations of two popular polarization modulation schemes in a dual-beam system.

(2)
Tomczyk, Steven; Roberto Casini; Alfred G. de Wijn; and Peter G. Nelson. 2011: Wavelength-diverse polarization modulators for Stokes polarimetry. Applied Optics, 49, issue 18, 3580.

Abtract: Information about the three-dimensional structure of solar magnetic fields is encoded in the polarized spectra of solar radiation by a host of physical processes. To extract this information, solar spectra must be obtained in a variety of magnetically sensitive spectral lines at high spatial, spectral, and temporal resolution with high precision. The need to observe many different spectral lines drives the development of Stokes polarimeters with a high degree of wavelength diversity. We developed a new way of designing polarization modulators that operate over a wide wavelength range with near optimal polarimetric efficiency. They are directly applicable to the next generation of multi-line Stokes polarimeters. These modulators are not achromatic in the usual sense because their polarimetric properties vary with wavelength, but they do so in an optimal way. We refer to these modulators as polychromatic. In this paper, we presented the theory behind polychromatic modulators, illustrated the concept with design examples, and presented the performance properties of a prototype polychromatic modulator.