Radio Spectrometer

in radio astronomy, an instrument for studying the spectrum of cosmic radio emissions received by a radio telescope. It is used principally to study radio spectral lines formed in the interstellar medium.

A radio spectrometer consists of a highly sensitive superheterodyne receiver, a spectrum analyzer, and a recording device. In the receiver, high-frequency electric oscillations caused by the radiation under study are amplified and converted to frequencies with which the analyzer can operate. Most widespread are multichannel analyzers with narrow-band filters tuned to various frequencies in the part of the spectrum being studied. At the output of each filter, a signal is registered that is proportional to the power of the electric oscillations passing through the filter. Radio spectrometers are usually provided with devices for eliminating the influence of continuous spectra from cosmic radio sources and for eliminating the effects of the receiver’s intrinsic noise.

The fundamental parameters of a radio spectrometer are the frequency-resolution Δf_r and the sensitivity. In spectrometers equipped with filter analyzers, Δf_r is determined by the frequency bandwidth of the narrow-band filters; depending on the width of the narrowest details of the spectrum under study, values of Δf_r range from 10² to 10⁶ hertz. The sensitivity is defined by the formula , where ΔP is the minimum measurable power encompassed in the band Δf_r, α is a coefficient of the order of unity, P is the sum intensity of the receiver noise and the received radiation, and T is the integration time of the output signals expressed in seconds. In order to increase the sensitivity of a radio spectrometer, it is necessary to use low-noise quantum or parametric amplifiers and long integration times (up to several hours) for the output signals.