The telescope efficiencies (main beam and aperture efficiency) are given in Appendix A of ``The 30m Manual'' and the IRAM newsletter No. 18, (November 1994). A one-page summary of the telescope system is on the web (http://www.iram.es/Telescope/systsumm.ps).
At 1.3 mm (and a fortiori at shorter wavelengths) a large
fraction of the power pattern is distributed in an error
beam which can be approximated by two Gaussians of FWHP 
and 800'' (see [16, 1] for details).
Astronomers should take into account this error beam when converting
antenna temperatures into brightness temperatures. The effect is
particularly important at 0.8 mm, where the power in the
first error beam of FWHP 
is twice larger than in
the main beam.
The aperture efficiency depends somewhat on the elevation, particularly at shorter wavelengths. This gain/elevation effect is evaluated in [15].