We report technology development of millimeter/submillimeter polarization modulators that operate by introducing a variable, controlled phase delay between two orthogonal polarization states. The variable-delay polarization modulator (VPM) operates via the introduction of a variable phase delay between two linear orthogonal polarization states, resulting in a variable mapping of a single linear polarization into a combination of that Stokes parameter and circular (Stokes V) polarization. Characterization of a prototype VPM is presented at 350 and 3000 microns. We also describe a modulator in which a variable phase delay is introduced between right- and left- circular polarization states. In this architecture, linear polarization is fully modulated. Each of these devices consists of a polarization diplexer parallel to and in front of a movable mirror. Modulation involves sub-wavelength translations of the mirror that change the magnitude of the phase delay.
MMT-POL is an adaptive optics optimized imaging polarimeter designed for use at the 6.5m MMT. By taking full
advantage of the adaptive optics secondary mirror of the MMT, this polarimeter offers diffraction-limited polarimetry with very low instrumental polarization and minimal thermal background. MMT-POL permits observations as diverse as protoplanetary discs, comets, red giant winds, (super)novae and ejecta, galaxies, and AGN. We report on the initial on-sky commissioning results of the instrument including a description of the instrument.
MMT-POL is an adaptive optics optimized imaging polarimeter for use at the 6.5m MMT. By taking full advantage of
the adaptive optics secondary mirror of the MMT, this polarimeter will offer diffraction-limited polarimetry with very
low instrumental polarization. This instrument will permit observations as diverse as protoplanetary discs, comets, red
giant winds, galaxies and AGN. We report on progress toward regular operation of MMT-POL, including early
laboratory calibration and optimization. Characterization of the 1-5μm Virgo array and supporting electronics is
included, as are tests of the polarimetry optics at the heart of this instrument.
We describe our ongoing project to build a far-infrared polarimeter for the HAWC instrument on SOFIA. Far-IR
polarimetry reveals unique information about magnetic fields in dusty molecular clouds and is an important
tool for understanding star formation and cloud evolution. SOFIA provides flexible access to the infrared as
well as good sensitivity to and angular resolution of continuum emission from molecular clouds. We are making
progress toward outfitting HAWC, a first-generation SOFIA camera, with a four-band polarimeter covering 50 to
220 microns wavelength. We have chosen a conservative design which uses quartz half-wave plates continuously
rotating at ~0.5 Hz, ball bearing suspensions, fixed wire-grid polarizers, and cryogenic motors. Design challenges
are to fit the polarimeter into a volume that did not originally envision one, to minimize the heating of the
cryogenic optics, and to produce negligible interference in the detector system. Here we describe the performance
of the polarimeter measured at cryogenic temperature as well as the basic method we intend for data analysis.
We are on track for delivering this instrument early in the operating lifetime of SOFIA.
We describe the design and construction of a Variable-delay Polarization Modulator (VPM) that has been built and integrated into the Hertz ground-based, submillimeter polarimeter at the SMTO on Mt. Graham in Arizona. VPMs allow polarization modulation by controlling the phase difference between two linear, orthogonal polarizations. This is accomplished by utilizing a grid-mirror pair with a controlled separation. The size of the gap between the mirror and the polarizing grid determines the amount of the phase difference. This gap must be parallel to better than 1% of the wavelength. The necessity of controlling the phase of the radiation across this device drives the two novel features of the VPM. First, a novel, kinematic, flexure is employed that passively maintains the parallelism of the mirror and the grid to 1.5 μm over a 150 mm diameter, with a 400 μm throw. A single piezoceramic actuator is used to modulate the gap, and a capacitive sensor provides position feedback for closed-loop control. Second, the VPM uses a grid flattener that highly constrains the planarity of the polarizing grid. In doing so, the phase error across the device is minimized. Engineering results from the deployment of this device in the Hertz instrument April 2006 at the Submillimeter Telescope Observatory (SMTO) in Arizona are presented.
Precise astronomical polarization measurements generally require the use of polarization modulation. We have developed a new modulator, the Variable-delay Polarization Modulator (VPM) which uses two modified Martin- Puplett interferometers to induce a physical path length difference between polarization components. This highly durable and efficient device can easily be adapted to a wide range of wavelengths and temperatures, making it well suited for air- and space-borne facilities. This paper discusses the basic modulator design and a comparison to the half-wave plate, as well as details of VPM tests conducted at the Submillimeter Telescope Observatory (SMTO).
The Submillimeter High Angular Resolution Camera II (SHARC-II) is a 32 x 12 pixel submillimeter camera that is used with the ten-meter diameter Caltech Submillimeter Observatory (CSO) on Mauna Kea. SHARC-II can be operated at either 350 or 450 microns. We are developing an optics module that we will install at a position between the SHARC-II camera and the focus of the CSO's secondary mirror. With our module installed, SHARC-II will be converted into a sensitive imaging polarimeter. The basic idea is that the module will split the incident beam coming from the secondary into two orthogonally polarized beams which are then re-imaged onto opposite ends of the “long and skinny” SHARC-II bolometer array. When this removable polarimetry module is in use, SHARC-II becomes a dual-polarization 12 x 12 pixel polarimeter. (The central 12 x 8 pixels of the SHARC-II array will remain unused.) Sky noise is a significant source of error for submillimeter continuum observations. Because our polarimetry module will allow simultaneous observation of two orthogonal polarization components, we will be able to eliminate or greatly reduce this source of error. Our optical design will include a rotating half-wave plate as well as a cold load to terminate the unused polarization components.