Complex field measurements are increasingly becoming the standard for state-of-the-art astronomical instrumentation. Complex field measurements have been used to characterize a suite of ground, airborne, and space-based heterodyne receiver missions,1-6 and a description of how to acquire coherent field maps for direct detector arrays was demonstrated in Davis et. al. 20177. This technique has the ability to determine both amplitude and phase radiation patterns from individual pixels on an array for direct comparison to optical simulations. Phase information helps to better characterize the optical performance of the array (as compared to total power radiation patterns) by constraining the fit in an additional plane.4 This is a powerful technique to diagnose optical alignment errors through the optical system, as a complex field scan in an arbitrary plane can be propagated either forwards or backwards through optical elements to arbitrary planes along the principal axis. Complex radiation patterns have the advantage that the effects of optical standing waves and alignment errors between the scan system and the instrument can be corrected and removed during post processing.
Here we discuss the mathematical framework used in an analysis pipeline developed to process complex field radiation pattern measurements. This routine determines and compensates misalignments of the instrument and scanning system. We begin with an overview of Gaussian beam formalism and how it relates to complex field pattern measurements. Next we discuss a scan strategy using an offset in z along the optical axis that allows first-order optical standing waves between the scanned source and optical system to be removed in post-processing. Also discussed is a method by which the co- and cross-polarization fields can be extracted individually for each pixel by rotating the two orthogonal measurement planes until the signal is the co-polarization map is maximized (and the signal in the cross-polarization field is minimized). We detail a minimization function that can fit measurement data to an arbitrary beam shape model. We conclude by discussing the angular plane wave spectral (APWS) method for beam propagation, including the near-field to far-field transformation.
Here we summarize the initial results from a complex field radiation pattern measurement of a kinetic inductance
detector instrument. These detectors are phase insensitive and have thus been limited to scalar, or amplitude-only, beam
measurements. Vector beam scans, of both amplitude and phase, double the information received in comparison to scalar
beam scans. Scalar beam measurements require multiple scans at varying distances along the optical path of the receiver
to fully constrain the divergence angle of the optical system and locate the primary focus. Vector scans provide this
information with a single scan, reducing the total measurement time required for new systems and also limiting the
influence of system instabilities. The vector scan can be taken at any point along the optical axis of the system including
the near-field, which makes beam measurements possible for large systems at high frequencies where these
measurements may be inconceivable to be tested in-situ. Therefore, the methodology presented here should enable
common heterodyne analysis for direct detector instruments. In principle, this coherent measurement strategy allows
phase dependent analysis to be performed on any direct-detect receiver instrument.
Polarized thermal emission from interstellar dust grains can be used to map magnetic fields in star forming molecular clouds and the diffuse interstellar medium (ISM). The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLASTPol) flew from Antarctica in 2010 and 2012 and produced degree-scale polarization maps of several nearby molecular clouds with arcminute resolution. The success of BLASTPol has motivated a next-generation instrument, BLAST-TNG, which will use more than 3000 linear polarization- sensitive microwave kinetic inductance detectors (MKIDs) combined with a 2.5 m diameter carbon fiber primary mirror to make diffraction-limited observations at 250, 350, and 500 µm. With 16 times the mapping speed of BLASTPol, sub-arcminute resolution, and a longer flight time, BLAST-TNG will be able to examine nearby molecular clouds and the diffuse galactic dust polarization spectrum in unprecedented detail. The 250 μm detec- tor array has been integrated into the new cryogenic receiver, and is undergoing testing to establish the optical and polarization characteristics of the instrument. BLAST-TNG will demonstrate the effectiveness of kilo-pixel MKID arrays for applications in submillimeter astronomy. BLAST-TNG is scheduled to fly from Antarctica in December 2017 for 28 days and will be the first balloon-borne telescope to offer a quarter of the flight for “shared risk” observing by the community.
Here we present the methodology and initial results for a new near-field antenna radiation measurement system for submillimeter receivers. The system is based on a 4-port vector network analyzer with two synthesized sources. This method improves on similar systems employing this technique with the use of the network analyzer, which reduces the cost and complexity of the system. Furthermore, a single set of test equipment can analyze multiple receivers with different central frequencies; the frequency range of the system is limited by the output range of the network analyzer and/or the power output of the source signal. The amplitude and phase stability of the system in one configuration at 350 GHz was measured and found to be accurate enough to permit near field antenna measurements. The proper characterization of phase drifts across multiple test configurations demonstrates system reliability. These initial results will determine parameters necessary for implementing a near-field radiation pattern measurement of a Schottky diode receiver operating between 340-360 GHz.