Abstract
Aims. Radio observing efficiency can be improved by calibrating and reducing the observations in total power mode rather than in frequency, beam, or position-switching modes. Methods. We selected a sample of spectra obtained from the Institut de Radio-Astronomie Millimétrique (IRAM) 30-m telescope and the Green Bank Telescope (GBT) to test the feasibility of the method. Given that modern front-end amplifiers for the GBT and direct Local Oscillator injection for the 30 m telescope provide smooth pass bands that are a few tens of megahertz in width, the spectra from standard observations can be cleaned (baseline removal) separately and then co-added directly when the lines are narrow enough (a few km s−1), instead of performing the traditional ON minus OFF data reduction. This technique works for frequency-switched observations as well as for position- and beam-switched observations when the ON and OFF data are saved separately. Results. The method works best when the lines are narrow enough and not too numerous so that a secure baseline removal can be achieved. A signal-to-noise ratio improvement of a factor of √2 is found in most cases, consistent with theoretical expectations. Conclusions. By keeping the traditional observing mode, the fallback solution of the standard reduction technique is still available in cases of suboptimal baseline behavior, sky instability, or wide lines, and to confirm the line intensities. These techniques of total-power-mode reduction can be applied to any radio telescope with stable baselines as long as they record and deliver the ONs and OFFs separately, as is the case for the GBT.
Highlights
For a long time, heterodyne receivers in the millimeter domain were made of front-end mixers with optical injection of the Local Oscillator followed by intermediate-frequency amplifiers and production of spectra via filter banks for which the homogeneity of the gain was not secured from channel to channel
The calibration of mm-wave radiotelescopes has been discussed in Kutner & Ulich (1981) and a technical report from the Institut de Radio-Astronomie Millimétrique (IRAM) 30-m presents the details of the calibration procedure2 for that telescope which is representative of mm-wave radiotelescope calibrations
Though the IRAM 30-m telescope is still equipped with frontend mixers and the antenna is on-axis, the replacement of the optical LO injection by line injection in the new EMIR receivers has improved the baseline performance of the observations, and even frequencyswitched observations are only subject to relatively limited baseline ripples
Summary
Heterodyne receivers in the millimeter (mm) domain were made of front-end mixers with optical injection of the Local Oscillator followed by intermediate-frequency amplifiers and production of spectra via filter banks for which the homogeneity of the gain was not secured from channel to channel. With the advent of high-frequency amplifiers, drop of LO optical diplexer injection, Fourier-transform spectrometers, and the benefit of an off-axis telescope such as the Green Bank Observatory 100-m Telescope (GBT), the quality and stability of the bandpass has considerably improved, meaning that total power (staring in a fixed direction at fixed frequency) mode observations can be reconsidered. This would avoid the q√uadratic addition of noise fluctuations and save the 2 factor discussed above.
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