Notes on Radio Astronomy Techniques
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						DTE. 1999-07-27.


   The emission being detected falls roughly into 2 different
categories: (1) broadband continuum, and (2) spectral line.

   Usually (but not, e.g., pulsars) the emission and its distribution
in space and frequency can be considered constant.

   The strength of signal received by a radio telescope is a tiny
fraction (e.g. 1 part in 100,000 or less) of the internally generated
noise in the receiver, or the background noise received by the telescope.
Sensitivity is achieved by AVERAGING the signal.

    dT = K. Tsys/sqrt(B.t)

K depends on observing technique, but is typically 2.0.

  For continuum measurements, use as wide a bandwidth as possible.
  For spectral line measurements, the bandwidth is decided by the
anticipated Doppler shift.  From few kHz to tens of MHz usually.

   Some switching technique is needed, to remove instrumental drifts,
and, at high frequencies, varying emission from clouds etc.

  Switching techniques used: 
Dicke switching
Beam switching
Frequency switching
Position switching
On-the-fly observing
  etc.

RESOLUTION PROBLEM

   Resolution:  beam=1.2*lambda/D

Single dish telescopes from 1 degree to one 200th degree (very roughly).
BUT interferometers can extend D.  Can be connected, or use high-stability
recording (VLBI) techniques.

One interferometer extracts one component of spatial frequency.
Fourier synthesis of an image.
(What is spatial frequency?)
Earth rotation synthesis.
Instruments like the VLA.
Think of a single dish as a multi-element cross-correlator.  The 
interference of different terms gives the Fourier components, = resolution.