An important property of a bolometer receiver is its spectral response. Knowing this function and the atmospheric transmission, the effective observing frequency for radio sources of given emissivity index can be calculated (see NIC-manual 1.4-07, Appendix C, ``Effective observing frequency''). Furthermore this information is essential in order to estimate the line contribution to the bolometer signal.
The spectral response of a bolometer receiver is a function of many parameters. Unfortunately some of them are difficult to determine and a direct measurement of the complete bolometer system is very desirable.
This can be done by using the receiver to take a spectrum of a source of known emissivity (a black body) with a spectrometer of known efficiency (a Martin-Puplett-interferometer). The spectral response is then obtained by dividing the measured spectrum by the source spectrum and the spectrometer's efficiency curve. A relative spectral calibration of this kind is usually done before a new bolometer system is shipped to the telescope.
Figure 1 shows the spectral sensitivity of the central
channel of the 37 element 1.2 mm bolometer array used during the
observing campaign in winter 98/99 (''MAMBO I''). The variation of
the shape of the spectral sensitivities of the different channels is
typically below 5% so that this curve gives a good estimate for all
channels of this bolometer array. According to ongoing improvements
of the bolometers the spectral response of a receiver depends on the
year. A compilation of spectral responses of the MPIfR
bolometer systems used at the 30 m MRT and the 10 m HHT can be found