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Update in progress..... BackgroundIn a heterodyne receiver incoming signals from the sky are combined ("mixed") with a pure local oscillator (LO) signal, the frequency of which is adjusted for the relative motion of the astronomical target and the receiver. The mixer output consists of two sideband responses separated from the LO by plus and minus the intermediate frequency - presently 4 GHz, and each (in A3) about 2 GHz wide. Thus the "signal" sideband is centered at the rest frequency, and the "image" sideband is 8 GHz either below or above the signal sideband, depending on which sideband (upper or lower respectively) is chosen as the the one containing the signal. For example: observations of CO 2-1 at 230.538 GHz which place the line in the upper sideband will be accompanied by signals received in the lower sideband, centered on 222.538 GHz.Receivers B3 and W have an on-board interferometer allowing the image sideband to be rejected in "single-sideband" (SSB) mode or retained in "double-sideband" (DSB) operation. Note that the rejection is not perfect; in B3 it is at best about 13dB, and in W it has not been well determined. SSB operation results in better calibration in general. Receiver A3, however, does not have sideband rejection and thus is a DSB instrument by default. This has some special observational consequences, not only because the signals from both sidebands are observed simultaneously, but particularly for line calibration. The latter point is addressed here. Consequences of DSB operationA perfect receiver would offer equal gain to lines observed in both sidebands; i.e. the sideband ratio would be 1.0. It is not easy to measure the sideband ratio, however. If lines of equal intensity were available approximately every 8 GHz apart across the RF spectrum, say, via an RF signal generator, then these lines received in both the USB and LSB would be of equal strength if the receiver gains were equal for both sidebands, and the sideband ratio would be unity. However, we do not have access to such a signal generator (although, see below), and in its absence the software assumes the sideband ratio to be unity. Note that measuring a given line alternately in the USB and LSB does not yield the sideband ratio.Thus for any receiver operating in double-sideband mode, the strength of a given line observed in, say, the upper sideband (USB) is not likely to be exactly the same when observed in the lower sideband (LSB), and may be quite different. Observers should be aware of this effect and be sure to observe a source for which the same line has been observed as a "standard" wherever possible for comparison purposes. For A3 the presence of the pronounced "hump" in the receiver temperture between about already indicates that one needs to be especially careful in this region, and, because of the DSB performance of A3, be aware that having either sideband within the region is going to change the apparent line strength. Examples to illustrate this effectcan be had by looking at spectra of, say, NGC2071IR, in CO 2-1, CS 5-4, and HCN 3-2. In each case two spectra which place the line in the USB and LSB are shown superposed in green and red respectively. The CO spectrum is closely similar whether observed in LSB or USB, but CS should be observed in the USB, and HCN in the LSB, to avoid the calibration difficulties associated with the "hump" and the assumption of equal sideband gain. Further examples can be found in the "representative spectra" for A3. Similar databases exist for B3 and W. In A3 this problem was studied quite extensively during commissioning in December 1998, partly as an outcome of observations of SO and SO2 at the time, and these early results are reported here.
![[A3: HC3N linestrengths in CRL2688 and Trec]](/JACpublic/JCMT/Heterodyne_observing/Instrument_homes/RxA3i/hc3n_a3.gif)
Thanks to Stephane Claude, HIA, Victoria (and now at IRAM), for providing the above information. 
<Please address any comments, suggestions or requests to Henry Matthews at the Joint Astronomy Centre. Updated: 31 May 2002 |
