Diffraction‐limited Polarimetry from the Infrared Imaging Magnetograph at Big Bear Solar Observatory

Abstract

ABSTRACTThe Infrared Imaging Magnetograph (IRIM) system developed by Big Bear Solar Observatory (BBSO) has been put into preliminary operation. It is one of the first imaging spectropolarimeters working at 1565 nm and is used for the observations of the Sun at its opacity minimum, exposing the deepest photospheric layers. The tandem system, which includes a 4.2 nm interference filter, a unique 0.25 nm birefringent Lyot filter, and a Fabry‐Pérot etalon, is capable of providing a bandpass as low as 0.01 nm in a telecentric configuration. A fixed quarter‐wave plate and a nematic liquid crystal variable retarder are employed for analyzing the circular polarization of the Zeeman components. The longitudinal magnetic field is measured for the highly Zeeman‐sensitive Fe i line at 1564.85 nm (Landé factor g = 3). The polarimetric data were taken through a field of view of ∼145′′ × 145′′ and were recorded by a 1024 × 1024 pixel, 14 bit HgCdTe CMOS focal plane array camera. Benefiting from the correlation tracking system and a newly developed adaptive optics system, the first imaging polarimetric observations at 1565 nm were made at the diffraction limit on 2005 July 1 using BBSO’s 65 cm telescope. After comparing the magnetograms from IRIM with those taken by the Michelson Doppler Imager on board SOHO, it was found that all the magnetic features matched very well in both sets of magnetograms. In addition, Stokes V profiles obtained from the Fabry‐Pérot etalon scan data provide access to both the true magnetic field strength and the filling factor of the small‐scale magnetic flux elements. In this paper, we present the design, fabrication, and calibration of IRIM, as well as the results of the first scientific observations.