Arcminute Cosmology Bolometer Array Receiver

ACBAR detector array
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Instrument Description


Telescope and Site

VIPER VIPER
VIPER VIPER

These figures show the Viper telescope at South Pole Station. Viper, which sits within the shown conical ground shield, is an off-axis gregorian reflector with a 2 m primary mirror and an additional 0.5 m radius of skirt. It has a chopping tertiary at an image of the primary in order to sweep the beams on the sky.


The next figure shows the measured variance in atmospheric opacity at Mauna Kea (Hawaii), Chajnantor (Chile), and the South Pole. The Viper telescope is located at the South Pole, which has a pressure elevation of ~11,000’ and is arguably the best site on the planet for millimeter wave astrophysics. Winter observations with the ACBAR instrument will be detector noise dominated.


ACBAR Cryostat


This photo is the fully assembled ACBAR instrument being tested in Berkeley. The detectors are deep inside the blue liquid helium cryostat. The electronics which amplify the signals from the detectors and control and monitor cryogenics are housed in the two rugged aluminum boxes. The ACBAR liquid helium cryostat maintains the receiver package at a temperature of 4 K for four days between cryogen fills. A liquid He3 sorption refrigerator cools the detectors to 240 mK.


The next photo is the fully-assembled ACBAR receiver mounted on the Viper telescope at the South Pole in December 2000.


Micromesh Bolometers


The bolometers used in ACBAR are SiN micromesh spiders with NTD germanium thermistors. These bolometers have a low cross section to cosmic rays and are very fast due to reduced heat capacity. They are similar to the detectors baselined for the Planck HFI. They were built in the Micro Lab at JPL.


16 Element 250mK Array

focal plane, above focal plane
focal plane "money shot"

The ACBAR bolometer array consists of four rows with four frequencies in each row. The feeds have been designed to give matched 4’ beam sizes at all frequencies. The array is swept across 3° of sky at 1 Hz giving complete frequency coverage on each strip of sky. The first picture shows the beam forming optics, filters and detectors of the ACBAR array. The following pair of pictures show the individual feed horns.

feed horns both focal planes

 

The following two images are the beam maps using Mars and Venus as targets.

mars venus

The left image is a plot of the beam pattern of each of the 16 pixels of the ACBAR receiver measured by mapping Mars. The receiver frequencies corresponding to each column are (from left to right): 345 GHz, 150 GHz, 219 GHz, and 274 GHz. The measured FWHM of the beam agrees well within predictions, with beams ranging from 3.5’ to 4.5’ in the 150, 274 and 219 channels. The right image is the same thing but using Venus instead of Mars.


Measured Performance of ACBAR

Frequency Band (GHz) 150 219 274
Bandwidth (GHz) 29 30 50
Number of Detectors 8 4 4
Optimal Efficiency (%) 40 32 31
Beam Size (arcminute FWHM) 4.8 3.9 3.9
Detector NEP (10-17 W Hz-1/2) 8.2 7.4 16.2
NETRJ (μK s1/2) 180 200 270
NETCMB (μK s1/2) 300 590 1470
NEy (10-4 s1/2) 1.1 N/A 5.5
NEFD (mJ s1/2) 110 140 290

Lastly, here's a bit of graffiti found on the dewar...
Akbar on ACBAR
 
Copyright Holzapfel Group, 2002 Page last modified December 16, 2002