Operator`s manual

Haystack Observatory - - Small Radio Telescope
Measure Aperture Efficiency
Introduction:
Aperture efficiency η, is the ratio of the effective aperture of a radio telescope
divided by the true aperture. True aperture A, is defined as the collecting area of
the telescope surface. Effective aperture A
e,
can be defined as the losses from a
perfect reflector due to such things as blockage of the surface by the feed and
feed supports, over/under-illumination of the surface by the feed and other
factors such as surface irregularities. Typically, total losses amount to 35%-50%
of the theoretical limit. That is, 50%-65% of the power from the observed source
reaches the receiver.
η = Ae/A
For the purposes of the use of the SRT, the aperture efficiency (η) is expressed
by the following equation:
η = 2k T
A
/FA
where, T
A
= the antenna temperature in degrees K, k = Boltzman’s constant (1.38
x 10
-23
w Hz
-1
K
-1
), F = the radio source flux density in Janskys (10
-26
w m
-2
) and
A = the area of the reflector in m
2
Measurement of the aperture efficiency of the SRT can be carried out on several
sources: Cygnus-X, Cas-A, the Moon and even the Sun. The procedure below
will use Cas-A.
Procedure:
The user should first move the telescope to an area of the sky near the source to
be observed. The calibration should typically be done at the same elevation as the
source but offset in azimuth by at least two beamwidths (~14 degrees for a 2.1m
dish). Since this is a "continuum" measurement, the frequency setting for the
receiver should be away from the 1420.4 MHz hydrogen line.
The observing mode will be beamswitching. Beamswitching involves alternate
on-source/off-source observations with each off-source observation alternating in
a plus/minus azimuth direction. A typical command file for a five-minute,
beamswitched run on CasA might look like this (Analog receiver commands in
blue):