Abstract
The diffuse interstellar medium is studied primarily via radio spectroscopic techniques as far as the colder atomic and low-density molecular structures are concerned. In this chapter, we discuss some of the practical considerations involved in making and analyzing these observations. Some of the topics discussed here should be useful in preparing observing proposals or reading observational papers on this medium. Topics covered include basic radio spectroscopic and mapping techniques, Stokes parameters, antenna temperature conventions, radio frequency interference, virial analysis, principal component analysis, and statistical analysis of the velocity fluctuations.
My dear Kepler, what would you say of the learned here, who, replete with the pertinacity of the asp, have steadfastly refused to cast a glance through the telescope? What shall we make of this? Shall we laugh, or shall we cry? — Galileo Galilei
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Notes
- 1.
A few radio telescopes (e.g., the Five hundred meter Aperture Spherical Telescope (FAST) and the Arecibo 305-m radio telescope) use spherical collecting surfaces.
- 2.
The flux density is the integral of the intensity over a source: S = ∫ ∫ I ν (θ, ϕ) dΩ, where θ and ϕ are sky coordinates and Ω is the solid angle subtended by the telescope beam (equivalent to its pdf ).
- 3.
A jansky is a flux unit of the spectral flux density, or spectral irradiance. It has a value of 1 × 10−23 erg [s cm2 Hz]−1. The power is assumed to be completely contained within the psf and therefore the flux and not the surface brightness is being measured.
- 4.
This includes one of the authors of this book who, as a graduate student, mistook the telluric line for a high-latitude interstellar cloud which found its way into the literature as MBM 10 (Magnani et al. 1985).
- 5.
- 6.
Most radio observatories will have the power pattern of their antennas available by way of technical memoranda or internal reports. Another correction that is applied to 21 cm data is the confusion effect produced by rotation of the beam on the sky produced by the motion of an alt-azimuth telescope.
- 7.
A discussion of the various issues involved can be found in Spectrum Management for Science in the 21st Century, Publication of the National Research Council of the National Academies, The National Academies Press (2010).
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Magnani, L., Shore, S.N. (2017). Observing the Diffuse ISM: Making Sense of the Radio Observations. In: A Dirty Window. Astrophysics and Space Science Library, vol 442. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-54350-4_4
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