By M.L. Meeks
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Extra resources for Astrophysics,Part C, Radio Observations (Methods of Experimental Physics)
This technique, with a receiver designed by Williams (see Weaver et ~ 7 1 . ' ~ )In comparison is made to the spectrum viewed through two windows symmetrically displaced in frequency on either side of the line frequency. A switching pattern provides equal time on each position (line, window A , line, window B ) , with the sequence repeated at the switching rate. The frequency switching takes place close to the front end of the receiver, hence the bandpass of the following components acts as a constant multiplier.
However, despite the availability of this and other computing techniques, published data are rarely corrected. If the observed width of a spectral line is indeed of the order of the bandwidth, it is generally agreed preferable to reobserve the line using a finer resolution than to apply a large correction term. 6. The Use of Switching Techniques in Spectroscopy In spectral-line measurements some form of switching or differencing is invariably used to obtain adequate gain stability and an appropriately flat baseline.
Dickel, Zcarus 6, 417 (1967)l. 11 12 4. SINGLE-ANTENNA OBSERVATIONS record shown is the difference output from the switched radiometer and represents the signal received in the feed at the indicated position angle P A minus that in the feed at P A 90". 5% at a position angle of 180" is detectable on the records. The instrumental effect is apparently caused by distortions of the dish surface which slightly alter the amount of power reflected at different position angles. That there is not a different response in the two feed systems can be verified by making switched-polarization observations at two indicated position angles separated by 90%.
Astrophysics,Part C, Radio Observations (Methods of Experimental Physics) by M.L. Meeks