The Atacama Cosmology Telescope (ACT) is a custom six-meter telescope that is located in northern Chile.
It has the following properties (Swetz et al. 2010):
PHYSICAL PROPERTIES OF THE TELESCOPE AND OPTICS
Telescope Properties
  
Location
Telescope height
12 m
Altitude
5190 m
Ground screen height
13 m
Longitude
67° 47' 15"W
Total mass
52 t
Latitude
22° 57' 31"S
Moving Structure mass
40 t
Optics
f-numbera
2.5
Azimuth range
±220°
FOV
1 deg2
Max. az speed
2°/s
Primary reflector Dia
6m
Max. az acc.
10°/s2
No. primary panels
71
Elev. range
30.°5 - 60°
Secondary reflector Dia
2m
Max. elev speed
0.2°/s
No. secondary panels
11
    

aAt telescope Gregorian focus

ACT observes simultaneously in three frequency bands centered on 148 GHz, 218 GHz, and 277 GHz. These are often called AR1, AR2, and AR3 respectively. Celestial radiation in each band is imaged onto 32 X 32 arrays of transition-edge sensor (TES) bolometers. The arrays have the following properties:

 

DETECTOR ARRAY PHYSICAL AND OPTICAL PROPERTIES

Detector Physical Properties

 
Horizontal
Vertical
Pixel size
1.050 mm
1.050 mm
Pixel spacinga
1.050±0.002 mm
1.22±0.02 mm
Array configuration
32
32
Array sizeb
33.6±0.1 mm
39.0±0.1 mm
Optical Properties: 148 and 218 GHz Arrays
 
Horizontal
Vertical
Effective focal lengthc
5.17±0.12 m
5.03±0.13 m
Primary illuminationc
5.60±0.21 m
5.59±0.23 m
Focal ratioc
0.93±0.02
0.90±0.02
Detector spacing
41±4″
50±4″
Optical Properties: 277 GHz Array
Horizontal
Vertical
Effective focal lengthc
5.41±0.11 m
5.06±0.12 m
Primary illuminationc
5.48±0.19 m
5.50±0.20 m
Focal ratioc
0.98±0.04
0.91±0.04
Detector spacing
40±4″
49±4″
aHorizontal error estimated from fabrication tolerances. Vertical error estimated from machining tolerances.
bErrors are estimated from machining and assembly tolerances.
cThe number varies across the array and the "±" indicates the maximum and minimum of the range.

 

PROPERTIES OF THE MBAC FILTERS
Array
148(GHz)
218(GHz)
277(GHz)
Comp. Bandcentera
146.9
217.3
273.5
Max. Transmissiona
0.74
0.72
0.69
Bandcenterb 149.2±3.5 219.7±3.5 277.4±3.5
Bandwidthb 18.4 17.0 20.9
Noise Bandwithb 27.6 25.9 34.8
Band FWHMb 27.0 22.2 30.3

Effective Bandcentersc

Synchrotronc 147.6 217.6 274.8
Free-freec 147.9 218.0 275.4
Rayleigh-Jeansc 149.0 219.1 276.7
Dusty Sourcec 149.7 219.6 277.4
CMBc 148.4 218.3 274.7
SZ Effectc 146.9 220.2 277.2

Conversion factors

δTCMB ⁄ δTRJ 1.71±0.04 3.02±0.10 5.44±0.20
δW ⁄ δTRJ (pW ⁄K) 0.569±0.055 0.483±0.055 0.699±0.055
Γ (μK ⁄Jy) 6826±350 5824±240 4373±550
aBased on a composite of room temperature measurements. The source spectrum has been divided out. The effect of the lenses and coupling to the detectors is not included.
bMeasurements were made with all filters installed in MBAC along with the lenses and the ACT detectors. This measurement includes neutral density filters in each band and the coupling of the FTS to MBAC. The neutral density filters are not present when observing the sky. Three measures of the width of the band are given. The uncertainties are obtained from a combination of the rms of the measurements and an estimate of the systematic er- ror. The source spectrum has been divided out of the measured response.
cEffective band centers for synchrotron emission (α = -0.7), free-free emission (α = -0.1), Rayleigh-Jeans emission (α = 2.0), dusty source emission (α = 3.5). These values are based on the average of the computed composite of all filters and the measured response in MBAC. The uncertainty on all values is 3.5 GHz. For example, for the response to the CMB we use 148 GHz, 218 GHz in AR1 and AR2. Since the CMB will be diącult to detect in AR3 we use the RJ band center of 277 GHz.

 

DETECTOR PARAMETERS

Array

G

(pW/K)

Tc

(mK)

Dark Noise

(W ⁄ √Hz)

τ

(ms)
148 GHz 80±20 510±20 (6±2)x10-17 1.9±0.2
218 GHz 120±30 510±20 (5±1)x10-17 2.5±0.4
277 GHz 300±60 500±30 (7±1)x10-17 1.6±0.3

 

SUMMARY OF BEAM PARAMETERS
  148GHz 218GHz 277GHz
Solid Anglea (nsr) 217.7±3.6 117.6±2.8 97±12
Major FWHMa (′) 1.401±0.003 1.012±0.001 0.891±0.04
Minor FWHMa (′) 1.336±0.001 0.991±0.001 0.858±0.005
Axis Anglea (°) 66±1 45±2 66±10
aThe error bars are effective 1- errors. The relatively large size of the
277 GHz error bar is due to the systematic errors from atmospheric
modeling and is expected to improve.

 

 
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