Astronomy Principles and Practice Fourth Edition.pdf

364 Radio telescopes
Figure 21.12. The polar diagram of a two-beam radio interferometer.
Fringe
pattern
Power
P S
P B
Time
Figure 21.13. The output of a simple two-antenna interferometer illustrating the effects of the background signals
P B and P S .
as it does on the area of the sky which is in the beam. The combination of these two sources within the
signal is displayed in figure 21.13 with an interferometric pattern superimposed. The presence of the
signals from backgrounds raises problems as follows.
(1) The final output needs correction by subtraction of the background contributions.
(2) As a result of variations in the gain of the receiver, the strengths of both P B and P S will appear to
drift.
(3) The background signals are themselves noisy and cause fluctuations on the output signal.
The problems associated with (3) are fundamental and unavoidable but can be reduced by using
long time constants within the recording circuitry to smooth out their effects. The problem under (2)
can be reduced by having high stabilization of the electronic components. However, by the principle of
phase-switching the effects of both (1) and (2) are removed. This technique involves the introduction
of a phase delay, equivalent to half a wave path length, which is switched in and out of one of the arms
of the interferometer. A schematic diagram of the modified interferometer is shown in figure 21.14.
The phase-sensitive detector measures two levels, one corresponding to the normal situation without
the phase delay, the other to that with the phase delay added, the latter effectively shifting the pattern
by half a fringe. The switching frequency is set to be much faster than the frequency associated with
the fringe record.