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30-m Antenna Efficiencies

Efficiency measurements have been carried out with the 30m antenna during two technical time sessions in March 2005. The four cryostats A, B, C and D have been used at frequencies of 86, 145, 210 and 260 GHz. At each frequency we used two different receivers with orthogonal polarization. The astronomical sources used have been Mars (diameter $5.8\hbox{$^{\prime\prime}$}$, Sun distance $63^\circ$) and Uranus (diameter $3.3\hbox{$^{\prime\prime}$}$, Sun distance $32^\circ$) at elevations between $23^\circ$ and $45^\circ$. The atmospheric conditions were good with a precipitable water vapour of $\approx 4.3$ mm during the first session and $\approx
1.2$ mm during the second session. The antenna forward efficiency $F_{eff}$ has been determined by means of 10 skydips spaced at different times during our test measurements and using our standard dual load calibration system. The parameters: beam width HPBW, aperture efficiency $\eta_A$ and beam efficiency $B_{eff}$ have been determined with 37 continuum cross scans of 8 subscans in beam switching mode, each subscan crosses the source with a length of $110\hbox{$^{\prime\prime}$}$ and a duration of 30 seconds. The Gaussian fit parameters to the cross scans (i.e. maximum intensity, half width) were then used together with the physical parameters of the planets to determine the deconvolved beam size and the aperture and beam efficiencies using the methods described in Kramer, C. 1997, "Calibration of spectral line data at the IRAM 30m radio telescope", IRAM Report. The results of the measurements are compiled in Tab. 3.

Table 3: Measurements of the 30-m telescope characteristics in March 2005
Freq.(GHz) $F_{eff}$ HPBW( $\hbox{$^{\prime\prime}$}$) $\eta_A$ $B_{eff}$
86 0.950 $\pm$ 0.015 28.3 $\pm$ 0.4 0.60 $\pm$ 0.02 0.76 $\pm$ 0.04
145 0.945 $\pm$ 0.021 17.0 $\pm$ 0.2 0.51 $\pm$ 0.02 0.65 $\pm$ 0.02
210 0.915 $\pm$ 0.019 11.8 $\pm$ 0.4 0.45 $\pm$ 0.06 0.57 $\pm$ 0.04
260 0.877 $\pm$ 0.020 9.5 $\pm$ 0.4 0.37 $\pm$ 0.04 0.46 $\pm$ 0.04

The main contribution to the errors of the previous results comes by the fact that two independent receivers have been used at each frequency and it seems that the illumination, the optic or both are slightly different. With the $\eta_A$ values at several frequencies we can apply a linear least-square fit to the "Ruze" formula $\eta_a = \eta_{ill} \times exp
( (4 \pi \sigma_s /\lambda)^2)$ where $\lambda$ is the wavelength at which the observations have been made, $\eta _{ill}$ the illumination efficiency and $\sigma _s$ the total r.m.s. of antenna roughness; and we find that $\eta_{ill} = 0.63 \pm 0.02$ and $\sigma_s
= 67.4 \pm 7.6 \mu$m. A historical overview of the 30m $\eta _{ill}$ and $\sigma _s$ is given in Tab. 4.

Table 4: Historical review of the 30-m $\eta _{ill}$ and $\sigma _s$.
Date Publisher $\eta _{ill}$ $\sigma_s (\mu \rm {m})$  
up to 1992 collected by H. P. Reuter 0.59 92  
August 1994 C. Kramer & W. Wild 0.63 87  
July 1997 The 30m Manual, V. 2.0 0.63 80  
July 1999 holography and panels adjustment  
January 2000 U. Lisenfeld & R. Mauersberger 0.66 72  
September 2002 installed temperature control of the counterweights  
March 2005 actual measurements 0.63 67  

The total r.m.s. of antenna roughness $\sigma _s$ has improved in the course of the years due to several modifications of the antenna, which are resumed below. In July 1999 the last holography measurements and panel adjustments were carried out. Results of surface inhomogeneities given by the holography were nominally slightly better than the value shown in the previous table because those results only considered the roughness due to the main reflector, while the value of $\sigma _s$ given above also includes in the budget the roughness due to the subreflector, Nasmyth mirrors and receiver optics. In September 2002 the antenna temperature control was modified to include the counterweights. Previously, only the main reflector support structure was controlled in temperature. The main goal was to remove the structural stress that produced the astigmatism, but also the antenna pointing and antenna efficiencies were improved. The actual results show an excellent performance of the 30m surface, which is better than ever. Various improvements also lead to a much better pointing stability. While prior to 1992 $\eta_A$ at 350 GHz was just 0.1, actual conditions predict an $\eta_A$ of 0.23. We conclude that, technically, the 30m telescope seems now suitable for observations at frequencies above 300 GHz, which might be an option e.g. for zero spacing for the Plateau de Bure interferometer.


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Next: Proposals for IRAM Telescopes Up: IRAM Newsletter 63 (August 2005) Previous: CLOUDSAT - consequences for