M.A. Holdaway 
February 20, 1996
Keywords: phase calibration, submillimeter phase calibration, masers, simultaneous dual band observing
We investigate how well calibrating 690 GHz observations with SiO masers at 86 GHz will work, and take a look at how many SiO masers are bright enough to act as phase calibrators at 690 GHz. Although some SiO masers are fantastically bright, they have a bandwidth of only about 10 km/s. Masers brighter than 20 Jy will work 80% of the time for simultaneous 86 GHz and 690 GHz observations, or 40% of the time for time shared multiband observations. Masers fainter than 4 Jy will seldom be very useful for either technique because the time required to detect them with sufficient SNR is greater than the atmospheric stability time scale. We estimate there are about 100 SiO masers with peak fluxes greater than 4 Jy. With an 86 GHz beam width of 75", simultaneous dual band calibration will be relevant to about 0.04 square degrees. And finally, since SiO masers are usually associated with evolved stars, most types of MMA observations would not be aided by calibrating on an SiO maser in the beam.
Because of the very limited usefulness of simultaneous dual band calibration, and because the Chilean atmosphere plus fast switching or radiometric phase correction will permit observing at 690 GHz for a substantial fraction of the time, we argue against incorporating simultaneous dual band observations with the MMA.
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