The New Solar Telescope in Big Bear: Polarimetry II

IRIM (Infrared Imaging Magnetograph) is one of the first imag- ing solar spectro-polarimeters working in the near infrared (NIR). IRIM is be- ing installed and commissioned in the Coude Lab of the 1.6-meter New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO). This innovative sys- tem, which includes a 2.5 nm interference filter, a unique 0.25 nm birefringent Lyot filter, and a Fabry-Perot etalon, is capable of providing a bandpass as low as 0.01 nm over a field-of-view of 50 '' in a telecentric configuration. An NIR waveplate rotates ahead of M3 in the NST as the polarimeter modulator, and ahead of it locates a calibration unit to reduce polarization cross-talk induced by subsequent oblique mirrors. Dual-beam differential polarimetry is employed to minimize seeing-induced spurious polarization. Based on the unique advantages in IR window, the very capable NST with adaptive optics, IRIM will provide unprecedented solar spectro-polarimetry with high Zeeman sensitivity (10 −3 Ic), high spatial resolution (0:2 '' ), and high cadence (15 s). In this paper, we discuss the design, fabrication, and calibration of IRIM, as well as the results of the first light observations.