364 research outputs found
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 high signal to noise ratio map of the Sunyaev-Zel'dovich increment at 1.1 mm wavelength in Abell 1835
We present an analysis of an 8 arcminute diameter map of the area around the
galaxy cluster Abell 1835 from jiggle map observations at a wavelength of 1.1
mm using the Bolometric Camera (Bolocam) mounted on the Caltech Submillimeter
Observatory (CSO). The data is well described by a model including an extended
Sunyaev-Zel'dovich (SZ) signal from the cluster gas plus emission from two
bright background submm galaxies magnified by the gravitational lensing of the
cluster. The best-fit values for the central Compton value for the cluster and
the fluxes of the two main point sources in the field: SMM J140104+0252, and
SMM J14009+0252 are found to be ,
6.5 mJy and 11.3 mJy, where the first error
represents the statistical measurement error and the second error represents
the estimated systematic error in the result. This measurement assumes the
presence of dust emission from the cluster's central cD galaxy of
mJy, based on higher frequency observations of Abell 1835. The cluster image
represents one of the highest-significance SZ detections of a cluster in the
positive region of the thermal SZ spectrum to date. The inferred central
intensity is compared to other SZ measurements of Abell 1835 and this
collection of results is used to obtain values for and the cluster peculiar velocity km/s.Comment: 9 pages, 5 figure
Polypropylene Embedded Metal-Mesh Broadband Achromatic Half Wave Plate for Millimeter Wavelengths
We describe a novel multi-layered metal mesh achromatic half wave plate for
use in astronomical polarimetric instruments. The half wave plate is designed
to operate across the frequency range from 125-250 GHz. The wave plate is
manufactured from 12-layers of thin film metallic inductive and capacitive
grids patterned onto polypropylene sheets, which are then bonded together using
a hot pressing technique. Transmission line modelling and 3-D electromagnetic
simulations are used to optimize the parameters of the metal-mesh patterns and
to evaluate their optical properties. A prototype half wave plate has been
fabricated and its performance characterized in a polarizing Fourier transform
spectrometer. The device performance is consistent with the modelling although
the measured differential phase shift for two orthogonal polarizations is lower
than expected. This difference is likely to result from imperfect patterning of
individual layers and misalignment of the grids during manufacture.Comment: 14 pages, 13 Figures, 1 Tabl
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