The calibration temperature Tcal is provided by the system group. Please note, that sometimes it is technically difficult to determine the true value of Tcal (especially for receivers in the secondary focus), hence a regular check is not always possible. If there is some doubt about the used value for Tcal, this is mentioned. Be aware, that for most receivers there is a possibility to change the strength of the calibration signal (between two or more values). The system temperature in zenith Tsys is derived by a least-square-fit of the observed system temperature against airmass (A = 1/sin Elv) using the relation Tsys(A) = T0 + Tatm · (1-exp(-tau·A)). T0 is the extrapolated noise temperature for airmass=0, it is the sum of the receiver temperature, and contributions of the antenna, the ground, etc.; tau is the atmospheric opacity. Note, that unfortunate weather conditions could increase Tsys significantly, especially for frequencies >10GHz. To determine the sensitivity R ("Kelvins per Janksy") the known flux density of suitable primary calibrators (like 3C286, NGC7027, see e.g. Baars et al. (1977), A&A 61, p.99) is compared to the increase of antenna temperature caused by those sources. Usually this was done after a correction for elevation- and time-dependend effects and - for frequencies > 12GHz - for the atmospheric opacity. The aperture efficiency AE (for an illumination of 100m) is given by R/2.844 (2.844 K/Jy = (geometric aperture)/(2·k) would be the highest reachable sensitivity for an ideal 100m dish). The SEFD (system equivalent flux density) is derived by SEFD = Tsys/S. It is independent of the true value of Tcal! The main beam efficiency MBE and TMB/S are derived using the relations TMB/S = (wavelength2[cm])/(2.65 · FWHM2[arcmin]) and MBE = TA/TMB. The elevation dependence of the sensitivity is described in terms of a parabola which is normalized to one. Hence, the true sensitivity is equal to the peak sensitivity mutiplied with the gain curve. Be aware, that for Elevations below 20deg, the influence of the surrounding hills could lead to a significant deviation from the parabola. More details about antenna calibration could be found e.g. in Rohlfs & Wilson "Tools of Radio Astronomy".
If you have questions or comments, please contact Alex Kraus (akraus_at_mpifr-bonn.mpg.de).
