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Invited Article: Millimeter-wave bolometer array receiver for the Atacama pathfinder experiment Sunyaev-Zel'dovich (APEX-SZ) instrument
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Image of FIG. 1.
FIG. 1.

The 12 m APEX telescope on Llano de Chajnantor at 5107 m altitude.

Image of FIG. 2.
FIG. 2.

The APEX-SZ reimaging optics design. Each bundle of rays represents the beam from a single detector.

Image of FIG. 3.
FIG. 3.

Calculated atmospheric transmittance at APEX for 2 mm (solid line) and 1 mm (thin solid line) PWV (precipitable water vapor) at 55° elevation plotted with the measured APEX-SZ observation band (for type-2 detectors) (dashed line). The atmospheric transmittance data are from the APEX transmittance calculator based on the ATM model (Ref. 36).

Image of FIG. 4.
FIG. 4.

Schematic diagram showing optical components inside the APEX-SZ cryostat. Photons enter the cryostat via the Zotefoam window and pass through low-pass filters at 60 K and 4 K, two HDPE lenses, and a band-defining filter at 280 mK. Finally, smooth-walled conical horns couple photons to the bolometers.

Image of FIG. 5.
FIG. 5.

Schematic diagram of the APEX-SZ integrating cavity geometry for a single bolometer. The bolometers are lithographed on a silicon wafer and centered behind an array of cylindrical waveguides that couple to the smooth-wall horns. The horn/waveguide array is positioned 400 μm from the wafer, and there is a 4 mm diameter, 250 μm deep relief at the end of the waveguide which reduces radiation loss along the wafer. The bolometer absorber is suspended on a silicon nitride membrane above a 20 μm vacuum gap etched in the silicon. The wafer is mounted on invar. In type-1 detectors, the invar mount serves as a 3λ/4 backshort. In type-2 detectors, two thin wafers are joined with a metal film between them to serve as a λ/4 backshort. The wafer and bolometer thicknesses are exaggerated for clarity.

Image of FIG. 6.
FIG. 6.

Results from HFSS simulation of the integrating cavity. For unit power input in the horn-coupled waveguide, the plot shows power absorbed by the spiderweb (solid lines) and power radiated radially in the wafer and air gap (dashed lines) as a function of frequency. The λ/4 backshort geometry (crosses) has better performance than the 3λ/4 backshort geometry (circles).

Image of FIG. 7.
FIG. 7.

(a) Three spiderweb bolometers on a 55-element sub-array. (b) A close-up of the center of the spiderweb shows the elements of the bolometer. The dark areas along the absorber legs and inside the gold ring are regions in which the silicon has been etched away from the spiderweb.

Image of FIG. 8.
FIG. 8.

(a) A fabricated bolometer sub-array in its holder. Each 55-element sub-array is mounted on an invar triangle and wirebonded to a multiplexer circuit board. Multiplexer circuit elements (tuned capacitors and inductors, described in Sec. V) are housed under the aluminum guard. A single micro-D connector provides the connection to the SQUIDs. (b) The TES bolometer array. Six identical sub-arrays are assembled into single planar array which has 330 bolometers and is 133 mm in diameter. Each bolometer has two leads which run from the spiderweb to bonding pads at the edge of each triangle. These are visible as light traces between the bolometers.

Image of FIG. 9.
FIG. 9.

Schematic diagram showing a single module of the frequency multiplexed SQUID readout system. Sinusoidal bias voltages at different frequencies are summed and applied to a group of bolometers via a single line. LC filters in series with the bolometers isolate an individual bias for each bolometer. The modulated signals from the bolometers are summed at the input of a SQUID array.

Image of FIG. 10.
FIG. 10.

(a) Two SQUID cards with 4 K to 280 mK wiring and a Cryoperm sheath. Traces on flexible kapton ribbon, soldered directly to the SQUID card, are in series with woven NbTi cable. In APEX-SZ we use 510 and 360 mm lengths of flexible-ribbon cable. The NbTi cable has a copper block epoxied at its midpoint for heatsinking at 370 mK. The male micro-D connector at the end of the cable mates with the female connector on the multiplexer board shown in Fig. 8. (b) A detailed image of the SQUID card with magnetic shielding. The SQUID card has eight SQUID arrays, each of which is mounted on a square pad of niobium foil. The card slides into the Cryoperm sheath shown above the card, leaving the gold contacts exposed for inserting into a Peripheral Component Interconnect (PCI) connector. The left side of the card has eight bias chip resistors. The upper edge of the photograph shows part of a SQUID card before the SQUID arrays have been attached. (c) A close-up image of the 100-element series array SQUID chip.

Image of FIG. 11.
FIG. 11.

Drawings showing two orthogonal sections of the APEX-SZ cryostat. Many components are labeled. The cryostat is shown looking upward in the orientation used in the laboratory for work on the focal plane. The optical section (upper half) can be removed for access to the focal plane and readout components. The aluminized mylar sheet (red) acts an RF shield and separates the optical and readout sections of the cryostat. Optical components are shown in light blue, 280 mK components in orange, 380 mK in yellow, and the 4 K SQUID card housing and readout wiring module in green. When mounted in APEX, the cryostat is downward-looking, with the window pointed at a mirror on the floor of the cabin.

Image of FIG. 12.
FIG. 12.

Elliptical Gaussian beam fits of 177 optically live channels. Ellipses represent best fit 2D Gaussian FWHM of individual channel maps; pluses mark the center of each beam. The circle in the lower left corner is 60′′. The ellipticity and orientation of the beams change slightly across the array. The median beamwidth is 58 ± 6′′ with a median axial ratio of 1.17.

Image of FIG. 13.
FIG. 13.

The lines show the mean Gaussian beam area (black) and axial ratio (blue) versus radial offset from the focal plane center. Error bars show the standard deviation of each value for the pixels in each radial bin. The beam area and axial ratio increase by 17% from the center to edge of the array, but the variation in beam area increases by a factor of 5.

Image of FIG. 14.
FIG. 14.

Comparison of measured versus predicted average beam pattern of type-1 and type-2 detectors. The individual channel maps from a planet observation are coadded and radially averaged to derive an average beam profile for the type-1 (black solid line) and type-2 (red solid line) detectors. The average predicted beam, also radially averaged (dashed lines), shows near sidelobes due to truncation at the Lyot stop. The measured beam central lobes are well fit by a 59′′ Gaussian (dotted line). The near sidelobe level −14 dB (−15 dB) of the type-1 (type-2) profile is consistent with optical crosstalk of 2.5% (<1%) between adjacent bolometers.

Image of FIG. 15.
FIG. 15.

Bandpass for a perfect cavity (dotted line) and that of type-1 (black) and type-2 (blue) cavity geometries. The bandpass of the perfect cavity is calculated from the mesh filter × waveguide transmittance. The type-1 and type-2 detectors have curves for the FTS measured bandpass (solid lines with uncertainties) and the calculated mesh filter × waveguide × cavity transmittance (dashed lines). Estimated statistical measurement uncertainties are shown; these are not uniform for the measured type-2 spectrum due to post-measurement corrections for differences between the filters in the test setup and those in the field. In addition, there are systematic uncertainties due to the optical coupling geometry between the detectors and the FTS which may cause ripples in spectrum. The FTS measured results are normalized to the peak calculated transmittance of the corresponding detector type. The absorbed power of each configuration is proportional to the area under the curve. The measured type-1 bandpass is much narrower than expected, while the measured and calculated bandpass of the type-2 configuration are more similar.

Image of FIG. 16.
FIG. 16.

Skydip scans of varying opacity. We fit each scan according to Eq. (10) to find τ, assuming an atmospheric temperature of 273 K. The curves shown here have, from top to bottom, τ = 0.072, 0.046, 0.035, 0.025, 0.017.

Image of FIG. 17.
FIG. 17.

Histogram of individual channel sensitivities in the array. Sensitivities are measured in the signal band of each detector type after analysis pipeline atmospheric removal and data cuts. NET values are plotted for the 135 type-1 channels (dark gray) which remain after all data cuts during an observation of the Bullet cluster. Also included are 20 channels from a type-2 sub-array (blue) measured in a separate observation. The type-2 histogram does not obscure any type-1 values.

Image of FIG. 18.
FIG. 18.

The circular drift scan pattern used for mapping individual clusters. The telescope scans in 12′ diameter circles with a 5 s period (black line). The circle center is stationary in AZ/EL, but tracks in RA/DEC. After performing 20 circles, the telescope is reset for a subsequent circular drift scan. Also shown are the FOV of the bolometer array at the initial pointing (blue dashed line) and the source position (×).

Image of FIG. 19.
FIG. 19.

Measured power spectral density (PSD) for a type-1 bolometer channel during a single scan made in weather typical of the August 2007 observing run. The dashed line shows the noise spectrum as measured. Its 1/f knee is 10 Hz. The solid line shows the same data after atmospheric noise removal by the analysis pipeline. The 1/f knee of the data after atmospheric removal is at 1.4 Hz, below the signal band.

Image of FIG. 20.
FIG. 20.

SZE map of the Bullet cluster based on 6.5 h of observation. Contours are spaces at 100μK. The map has 85′′ resolution with 55 μK rms noise.


Generic image for table
Table I.

APEX-SZ specifications and measured performance. The number of detectors typically used in data analysis are listed as “live channels” (see Sec. VII A). Bolometer performance values are listed for both type-1 and type-2 detectors (Sec. VII). The NET and NEy are listed for a single detector and refers to the median performance of the array (Sec. VII H).

Generic image for table
Table II.

APEX-SZ tertiary optics parameters. The optical elements are described in the text and in Fig. 2. The Gaussian beam width is the radius at which the power in the fundamental Gaussian mode for a single pixel's beam drops to e −2 of its peak power. The fractional spillover is fractional power vignetted by the optical element for a pixel on the edge of the focal plane array.

Generic image for table
Table III.

Typical TES Bolometer Parameters. The terms are defined in the text of Sec. IV.

Generic image for table
Table IV.

Cooling power provided by the pulse-tube cooler and the three-stage helium sorption refrigerator at different temperature stages of the APEX-SZ cryostat.

Generic image for table
Table V.

Power dissipation on mK stages.

Generic image for table
Table VI.

Cavity efficiency and effective bandwidth calculated for predicted and measured band pass spectra for the two detector types.

Generic image for table
Table VII.

Optical loading and loss for individual optical elements. T e is the physical temperature of each element, ε is the emissivity, L s is the spillover/scattering loss of each element, T s is the temperature of the spillover/scattered radiation absorber, η e is the estimated transmittance/efficiency of each element, and P opt is the contribution to total background optical power absorbed by the bolometer. Results are tabulated for type-1 bolometers. The total load corresponds to a RJ loading temperature of 44 K.

Generic image for table
Table VIII.

Summary of contributions to NEP. Median values of the array are used to calculate the expected noise contributions from readout, detectors and optical loading. The photon bunching noise is listed as an upper limit corresponding to ξ = 1.

Generic image for table
Table IX.

Observed median noise and sensitivity per channel. NEP is the measured noise equivalent power in the detector. NEFD is the noise equivalent flux density, measuring the sky-signal sensitivity. The NET values are the noise equivalent temperature referred to a source at the RJ limit and the CMB temperature. NEy is the noise equivalent y, the dimensionless Comptonization parameter.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Invited Article: Millimeter-wave bolometer array receiver for the Atacama pathfinder experiment Sunyaev-Zel'dovich (APEX-SZ) instrument