The frequency switching technique is of great potential interest to the millimeter spectroscopist, since it combines important advantages over other observing modes. It is very efficient, since 100% of the observing time is spent on source, it can be used for very extended sources (no emission-- free reference field needed), and it provides better cancellation of atmospheric emission fluctuations due to potentially higher switching speeds.
Despite these assets frequency switching (FSw) was not used much at the 30m telescope in the past, primarily because FSw spectra tended to have poor spectroscopic baselines. During 1994, motivated in part by the arrival of a new generation of SIS receivers, we have therefore made several test runs with the aim to bring the FSw equipment and its control software up to date, to find out what the limits are for the various FSw parameters, and last, but not least, investigate the quality of the spectroscopic baseline for FSw observations.
As the test results were quite encouraging, frequency switching will be offered now on a trial basis. During this initial trial period several restrictions will apply, the most important ones concern the availability of receivers (only the 3mm SIS and/or the 1mm SIS receiver No.1) and backends (only the 100kHz filter bank and the autocorrelator). Other limitations concerning the switching rates and frequency throw are described below. A Technical Report on the FSw test results, including recommended observing procedures, technical implementation, and related items will be available soon. The trial period will hopefully tell us how strong the demand is for frequency switching, and what the remaining problems are. So, please, FSwitchers, report your findings, good and bad, to the undersigned.