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Gain elevation curves show the point source sensitivity or aperture efficiency of the telescope versus elevation. [[http://www.iram.es/IRAMES/mainWiki/AstronomerOfDutyChecklist#Gain_elevation_correction|The newest information as of April-2012 on current gain elevation corrections has been compiled by J.Penalver here]]. Older fits to the observations of August 2007, indicate a maximum gain at 49.2deg, as the following image shows. {{attachment:gain-el-aug07.png}} |
Gain elevation curves show the point source sensitivity or aperture efficiency of the telescope versus elevation. [[http://www.iram.es/IRAMES/mainWiki/AstronomerOfDutyChecklist#Gain_elevation_correction|The newest information as of April-2012 on current gain elevation corrections has been compiled by J.Penalver here]]. |
Contents
Telescope efficiencies and beam widths
Update on telescope efficiencies and errorbeam parameters
The following table is taken from the report on "Improvement of the IRAM 30m telescope pattern" of 26-August 2013 which is available here. It gives the strengths (in percent) of the main beam, the three errorbeams, and the forward efficiency, as a function of observing frequency.
Freq
Beff
P1'
P2'
P3'
SumP
Feff
eta_fss
86
81
0
7
6
12
95
2
115
78
1
8
6
14
94
2
145
74
2
9
6
17
93
2
210
63
4
11
9
24
94
6
230
59
4
11
11
25
92
8
280
49
3
11
15
29
87
9
340
35
2
11
14
28
81
19
345
34
2
11
14
28
80
18
See the report for explanations on the errorbeam parameters P1', P2', P3', SumP. The half power beamwidths (HPBW) of the main beam are described to a good accuracy by HPBW(arcsec) = 2460/Freq(GHz). For the aperture efficiencies and point source sensitivities see previous observations listed below.
Parameters describing the beam of the 30m telescope: HPBW, Feff, Beff, Aeff, point source sensitivity, Errorbeams
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.
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 report by Kramer, Penalver, Greve of August 2013 is available online here. 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. The newest information as of April-2012 on current gain elevation corrections has been compiled by J.Penalver here.
Past efficiencies
Efficiencies measured with EMIR in 2009
Freq
HPBW
Feff
Beff
Aeff
S/TA*
Date
Comments
GHz
arcsec
%
%
%
Jy/K
86
29
95
81
63
5.9
4.4.2009
115
78
predicted
(1)
145
16
93
74
57
6.4
4.4.2009
210
11
94
63
49
7.5
29.3.2009
230
10.7
92
58
46
14.11.2012
(2)
260
9
88
53
41
8.4
29.3.2009
330
340
7.5
81
35
29
10.9
24.11.2009
Comments: (1) The beam efficiency at 115GHz is predicted from the measured beam efficiency at adjacent frequencies, using the Ruze formula. <CK, 15-Jan-2013> See also the EMIR Users Manual and the EMIR Commissioning Report. (2) See report by JP.
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
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.