Numerical optimization of integrating cavities for diffraction-limited millimeter-wave bolometer arrays

Far-infrared to millimeter-wave bolometers designed to make astronomical observations are typically encased in integrating cavities at the termination of feedhorns or Winston cones. This photometer combination maximizes absorption of radiation, enables the absorber area to be minimized, and controls the directivity of absorption, thereby reducing susceptibility to stray light. In the next decade, arrays of hundreds of silicon nitride micromesh bolometers with planar architectures will be used in ground-based, suborbital, and orbital platforms for astronomy. The optimization of integrating cavity designs is required for achieving the highest possible sensitivity for these arrays. We report numerical simulations of the electromagnetic fields in integrating cavities with an infinite plane-parallel geometry formed by a solid reflecting backshort and the back surface of a feedhorn array block. Performance of this architecture for the bolometer array camera (Bolocam) for cosmology at a frequency of 214 GHz is investigated. We explore the sensitivity of absorption efficiency to absorber impedance and backshort location and the magnitude of leakage from cavities. The simulations are compared with experimental data from a room-temperature scale model and with the performance of Bolocam at a temperature of 300 mK. The main results of the simulations for Bolocam-type cavities are that (1) monochromatic absorptions as high as 95% are achievable with <1% cross talk between neighboring cavities, (2) the optimum absorber impedances are 400 Ω/sq, but with a broad maximum from ~150 to ~700 Ω/sq, and (3) maximum absorption is achieved with absorber diameters ≥1.5λ. Good general agreement between the simulations and the experiments was found

A comparison of radome- and astrodome-enclosed large radio telescopes at millimeter wavelengths: The Large Millimeter Telescope

We present a systematic comparison of the main figures of merit for an open-air radio telescope and two different types of enclosed antennas: (1) an ordinary radome, with a metal space frame providing the required mechanical rigidity and a dielectric membrane, and (2) an “astrodome,” i.e., a corotating rigid dome with a large window covered by a tensile membrane structure. The analysis is limited to submillimeter and millimeter wavelengths and large (≳30 m) antenna/enclosure systems, where the window tensile structure is very unlikely to be removable and is supported by either a metal space frame or cable networks. As compared with previous studies of this type, here we concentrate on the specific effects that these large metallic support structures have on sensitive astronomical observations. In particular, we critically discuss how the wind-induced random motions of the metal space frame can limit the sensitivity of continuum observations, as a result of fluctuating shadowing and spillover effects combined with various beam-chopping techniques. Using the Large Millimeter Telescope as a benchmark, we provide baselines for future projects where a similar comparison is needed

Prototype design of a dielectrically embedded mesh lens

Here we present a prototype design for a dielectrically embedded mesh lens consisting of stacked layers of printed circuit board (PCB) material and embedded copper elements. The dielectrically embedded mesh lens consists of layers of dielectric which contain subwavelength- dimension metal elements laid out in a grid fashion, and is both flat and lightweight. It has been demonstrated that the sizes of these metal elements can be varied according to their position in the apparatus, using models based on transmission line theory, to create a lens which focuses a plane wave at millimeter wavelength to a Gaussian beam with very low transmission loss, even without the use of antireflective coating. We present the phase design for our lens which was designed, using transmission line theory and electromagnetic modelling software, to operate at 20GHz. We further present an\ud analysis of the transmission line components which will make up the lens

A millimeter-wave kinetic inductance detector camera for long-range imaging through optical obscurants

Millimeter-wave imaging provides a promising option for long-range target detection through optical obscurants such as fog, which often occur in marine environments. Given this motivation, we are currently developing a 150 GHz polarization-sensitive imager using a relatively new type of superconducting pair-breaking detector, the kinetic inductance detector (KID). This imager will be paired with a 1.5 m telescope to obtain an angular resolution of 0.09° over a 3.5° field of view using 3,840 KIDs. We have fully characterized a prototype KID array, which shows excellent performance with noise strongly limited by the irreducible fluctuations from the ambient temperature background. Full-scale KID arrays are now being fabricated and characterized for a planned demonstration in a maritime environment later this year

Prototype design of a dielectrically embedded mesh lens

Here we present a prototype design for a dielectrically embedded mesh lens consisting of stacked layers of printed circuit board (PCB) material and embedded copper elements. The dielectrically embedded mesh lens consists of layers of dielectric which contain subwavelength- dimension metal elements laid out in a grid fashion, and is both flat and lightweight. It has been demonstrated that the sizes of these metal elements can be varied according to their position in the apparatus, using models based on transmission line theory, to create a lens which focuses a plane wave at millimeter wavelength to a Gaussian beam with very low transmission loss, even without the use of antireflective coating. We present the phase design for our lens which was designed, using transmission line theory and electromagnetic modelling software, to operate at 20GHz. We further present an analysis of the transmission line components which will make up the lens

BFORE: The B-mode Foreground Experiment

The B-mode Foreground Experiment (BFORE) is a proposed NASA balloon project designed to make optimal use of the sub-orbital platform by concentrating on three dust foreground bands (270, 350, and 600 GHz) that complement ground-based cosmic microwave background (CMB) programs. BFORE will survey ~1/4 of the sky with 1.7 - 3.7 arcminute resolution, enabling precise characterization of the Galactic dust that now limits constraints on inflation from CMB B-mode polarization measurements. In addition, BFORE's combination of frequency coverage, large survey area, and angular resolution enables science far beyond the critical goal of measuring foregrounds. BFORE will constrain the velocities of thousands of galaxy clusters, provide a new window on the cosmic infrared background, and probe magnetic fields in the interstellar medium. We review the BFORE science case, timeline, and instrument design, which is based on a compact off-axis telescope coupled to >10,000 superconducting detectors.Comment: 7 pages, 4 figures, conference proceedings published in Journal of Low Temperature Physic

A Fluctuation Analysis of the Bolocam 1.1mm Lockman Hole Survey

We perform a fluctuation analysis of the 1.1mm Bolocam Lockman Hole Survey, which covers 324 square arcmin to a very uniform point source-filtered RMS noise level of 1.4 mJy/beam. The fluctuation analysis has the significant advantage of utilizing all of the available data. We constrain the number counts in the 1-10 mJy range, and derive significantly tighter constraints than in previous work: the power-law index is 2.7 (+0.18, -0.15), while the amplitude is equal to 1595 (+85,-238) sources per mJy per square degree, or N(>1 mJy) = 940 (+50,-140) sources/square degree (95% confidence). Our results agree extremely well with those derived from the extracted source number counts by Laurent et al (2005). Our derived normalization is about 2.5 times smaller than determined by MAMBO at 1.2mm by Greve et al (2004). However, the uncertainty in the normalization for both data sets is dominated by the systematic (i.e., absolute flux calibration) rather than statistical errors; within these uncertainties, our results are in agreement. We estimate that about 7% of the 1.1mm background has been resolved at 1 mJy.Comment: To appear in the Astrophysical Journal; 22 pages, 9 figure

Current status of Bolocam: a large-format millimeter-wave bolometer camera

We describe the design and performance of Bolocam, a 144-element, bolometric, millimeter-wave camera. Bolocam is currently in its commissioning stage at the Caltech Submillimeter Observatory. We compare the instrument performance measured at the telescope with a detailed sensitivity model, discuss the factors limiting the current sensitivity, and describe our plans for future improvements intended to increase the mapping speed

A Bright Submillimeter Source in the Bullet Cluster (1E0657--56) Field Detected with BLAST

We present the 250, 350, and 500 micron detection of bright submillimeter emission in the direction of the Bullet Cluster measured by the Balloon-borne Large Aperture Submillimeter Telescope (BLAST). The 500 micron centroid is coincident with an AzTEC 1.1 mm point-source detection at a position close to the peak lensing magnification produced by the cluster. However, the 250 micron and 350 micron centroids are elongated and shifted toward the south with a differential shift between bands that cannot be explained by pointing uncertainties. We therefore conclude that the BLAST detection is likely contaminated by emission from foreground galaxies associated with the Bullet Cluster. The submillimeter redshift estimate based on 250-1100 micron photometry at the position of the AzTEC source is z_phot = 2.9 (+0.6 -0.3), consistent with the infrared color redshift estimation of the most likely IRAC counterpart. These flux densities indicate an apparent far-infrared luminosity of L_FIR = 2E13 Lsun. When the amplification due to the gravitational lensing of the cluster is removed, the intrinsic far-infrared luminosity of the source is found to be L_FIR <= 10^12 Lsun, consistent with typical luminous infrared galaxies.Comment: Accepted for publication in the Astrophysical Journal. Maps are available at http://blastexperiment.info