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Telescope efficiencies and beam widths
Below you find the forward and beam efficiencies upto 280 GHz measured in March 2005 ([http://www.iram.fr/IRAMFR/ARN/aug05/node6.html IRAM Newsletter 8/05]). Values above 300 GHz are predictions based on the observations compiled in the IRAM Annual Report 2007 attachment:IRAM_2007.pdf ).
freq
HPBW
Beff
Aeff
S/TA*
Feff
Comments
(GHz)
(arcsec)
(%)
(%)
(Jy/K)
(%)
(1)
(2)
(3)
(4)
(5)
(6)
72 (extrapolated)
34
79
6.0
95
77 (extrapolated)
32
79
6.0
95
86
29
78
6.0
95
110
22
75
6.3
95
145
17
69
6.7
93
170
14.5
65
7.1
93
210
12
57
7.9
91
235
10.5
52
8.7
91
260
9.5
46
9.5
88
279
9
42
10.4
88
The half power beam width, HPBW, can be well fitted by: HPBW/arcsec = 2460/freq/GHz.
Main beam efficiency Beff. The data can be well fit by a Ruze function Beff = 1.2 Aeff' exp[-(4pi R sigma/ lambda)^2] with sigma being the rms value of the surface errors of the main dish, R the reduction factor for a steep main reflector, Aeff' is the aperture efficieny of the perfect telescope and lambda the wavelength in mm. The data can be fitted by R*sigma = 0.07 and epsilon = 0.69.
Aperture efficiency, Aeff. The aperture efficiency of the 30m telescope can be obtained via pointings on point-like celestial calibrators like Uranus or Mars. In the approximation that the beam is Gaussian and the edge taper is -14dB, it can also be derived from the beam efficiency using Aeff=Beff*0.80 (attachment:spatial_response_framework_v1.8.pdf)
Point source sensitivity S/TA*. S/TA* is 3.906*Feff/Aeff Jy/K for the 30m (see attachment:cali_rep.pdf).
Gain-elevation curves. The most recent curves are given in the IRAM Annual Report 2007
Forward efficiency Feff. The values for Feff are valid after the 12th of December 2000 when a new reflecting ring was put around the secondary mirror. Forward efficiencies are derived from skydips.
Error beams. A part of the power pattern is distributed in three error beams (see the analysis of attachment:greve_1998.pdf). The size of the described Gaussians is unchanged, however the main beam efficiencies have been improved in the meanwhile, lowering the strengths of the error beams. A new paper is in preparation. Astronomers should take the contribution of the error beam into account when converting antenna temperatures to brightness temperatures, especially when mapping extended sources.
Historic values: [http://www.iram.es/IRAMES/telescope/telescopeSummary/beam_effis.html Plot of efficiencies against frequency, measured in 2000], [http://www.iram.es/IRAMES/telescope/telescopeSummary/effi_history.html Compilation of efficiencies obtained in the past till 2001].
New values measured in August 2007:
Freq
HPBW
Feff
Beff
Aeff
S/TA*
Comments
GHz
arcsec
%
%
%
Jy/K
86
28.5
98
78
64
5.9
145
16.9
95
64
53
6.9
210
11.3
94
62
51
7.2
260
9.0
90
53
44
8.0
345
7.0
87
39
32
10.6
estimated
Half power beam width HPBW. The HPBW can be well fitted by: HPBW/arcsec=2406/Freq/GHz or HPBW/rad=1.166 W/D, with the wavelength W and the telescope diameter D.
Forward efficiency Feff. The values for Feff were updated after the 12th of December 2000 when a new reflecting ring was put around the secondary mirror. Forward efficiencies are derived from skydips. Values in the table are from measurements in August 2007.
Main beam efficiency Beff. Beff is the ratio of main beam solid angle over the entire antenna pattern solid angle. It is best derived from a source which has a diameter comparable to the size of the main beam. It can be calculated from the peak antenna temperature TA*, the HPBW, the source diameter, and source brightness temperature Tb (see Eq. 18 of attachment:cali_rep.pdf). For a source which fills the main beam, Beff=TA* Feff/Jnu(Tb), where Jnu(Tb) is the Rayleigh Jeans brightness temperature at frequency nu. Assuming a pure Gaussian beam, Beff=1.21Aeff (CHECK: Reference).
Aperture Efficiency Aeff. Aeff can be obtained via pointings on point-like celestial calibrators with a well known flux, like Uranus or Mars, when it is small. Aeff can be computed from 3.906 K TA* Feff / Ssou, where K is the correction factor that considers the coupling of the disk size of the planet to the HPBW, TA* is the peak antenna temperature, and Ssou is the intrinsic flux density of the planet. (see Eq.16 in attachment:cali_rep.pdf or attachment:spatial_response_framework_v1.8.pdf)
Point source sensitivity S/TA*. S/TA* is expressed as 3.906 Feff/Aeff in Jy/K (see Eq.17 in attachment:cali_rep.pdf)
Error beams. A part of the power pattern is distributed in three error beams (see the analysis of attachment:greve_1998.pdf). The size of the described Gaussians is unchanged, however the main beam efficiencies have been improved since 1998, lowering the strengths of the error beams. A new paper is in preparation. Astronomers should take the contribution of the error beam into account when converting antenna temperatures to brightness temperatures, especially when mapping extended sources.
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