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.

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. A fit of the Ruze formula to these main beam efficiencies results in a beam efficiency in the low frequency limit of Beff0 = 0.863 and a total surface rms of 66 micrometer: beff-jp-02nov2016.png. This includes the contribution of all mirrors between primary and the EMIR horns.

Lower frequencies have been made available in December 2015, with the upgrade of the E090 band. Beam properties are described in the commissioning report here.

Parameters describing the beam of the 30m telescope: HPBW, Feff, Beff, Aeff, point source sensitivity, Errorbeams

  1. 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.

  2. 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.

  3. 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. When using p.59, Eq.4.13, Eq.5.33 of Baars (2007, The Paraboloidal Reflector Antenna in Radio Astronomy and Communication), we find that the observed relation, HPBW=2460/nu with HPBW in arcsec and nu in GHz, is consistent with an edge taper of -16.2dB, HPBW=1.19 lambda/D in radian, and Beff=1.27Aeff (cf. te.greg, CK, 8-May-2015). Previously, we had sometimes assumed, HPBW=1.17Lambda/D, corresponding to HPBW=2405/nu, and to an canonical edge taper of -13dB, leading to Beff=1.21 Aeff.

  4. 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)

  5. Point source sensitivity S/TA*. S/TA* is expressed as 3.906 Feff/Aeff in Jy/K (see Eq.17 in cali_rep.pdf)

  6. 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

The point source sensitivity or aperture efficiency drops for large and for low elevations above/below the optimum of about ~50deg. This effect becomes more pronounced with increasing frequency. At 210GHz, the aperture efficiency drops to 80% of its optimum value at an elevation of 20deg or 80deg (cf. Report of April-2012 by J.Penalver where we also offer a CLASS script to observers to correct for the gain elevation curve).

Gain variation with wobbler throw

The point source sensitivity or aperture efficiency also drops with increasing wobbler throw. This effect becomes more pronounced with increasing frequency. At about 230GHz, the point source sensitivity drops to 80% of its optimum value for a wobbler throw of +-2' (cf. Figure 6 in Greve et al. 1996).

Past efficiencies

Efficiencies measured with EMIR in 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)

Efficiencies before 2007

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Iram30mEfficiencies (last edited 2016-11-03 18:07:57 by CarstenKramer)