Contents
Telescope efficiencies and beam widths
Efficiencies measured with EMIR
Freq
HPBW
Feff
Beff
Aeff
S/TA*
Date
Comments
GHz
arcsec
%
%
%
Jy/K
86
29
95
81
63
5.9
4.4.2009
115
145
16
93
74
57
6.4
4.4.2009
210
11
94
63
49
7.5
29.3.2009
230
260
9
88
53
41
8.4
29.3.2009
330
340
7.5
81
35
29
10.9
24.11.2009
see also the EMIR Users Manual and the EMIR Commissioning Report
Efficiencies measured with ABCD receivers in 8/07 (and 6/08)
Freq
HPBW
Feff
Beff
Aeff
S/TA*
Comments
GHz
arcsec
%
%
%
Jy/K
72
33.4
98
79
65
5.9
estimated
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
- Measurements were conducted during night time when effects of anomalous refraction and any panel buckling are strongly reduced. In addition, the panel backstructure is heated in a random fashion since 8/05, improving on its thermal balance. (JP, CT, CK 2/09). ABCD receivers were used to observe Uranus and Mars, while small. Receivers were tuned to single sideband. Planetary brightness temperatures Tb from ASTRO/GILDAS:
- Mars: 215K constant with frequency
Uranus: 139K at 86GHz, 116K at 145GHz, 102K at 210GHz, 94.5K at 260GHz, 85.6K at 345GHz following Griffin & Orton 1993
Half power beam width HPBW. The observed HPBWs can be well fitted by HPBW/arcsec=2460/Freq/GHz or HPBW/rad=1.166 Lambda/D, with the wavelength Lambda 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 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. Here, we assumed a pure Gaussian beam, HPBW=1.166Lambda/D, and derived the beam efficiency from Beff=1.21 Aeff (cf. Eq. 5.33 in Baars 2007 "The Parabolical Reflector Antenna in Radio Astronomy and Communication").
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 cali_rep.pdf or 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 cali_rep.pdf)
Error beams. A part of the power pattern is distributed in three error beams (see the analysis of 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.
Gain elevation curves
Gain elevation curves show the point source sensitivity or aperture efficiency of the telescope versus elevation. Fits to the observations of August 2007, indicate a maximum gain at 49.2deg, as the following image shows.
Efficiencies before 2007
Efficiencies measured on August, 31, 2007, Annual Report 2007,page 15ff
Efficiencies of 3/2005, see also the IRAM Newsletter 8/05.