Continuum observers will choose the observing frequency at a given resolution by balancing astronomical criteria against instrumental performance. A high frequency may be necessary for polarimetry because Faraday effects decrease with increasing frequency (degrees of linear polarization are generally higher at higher frequencies and electric vectors lie closer to their intrinsic position angles). Polarimetric data are also less corrupted by the Earth's ionosphere at high frequencies. Studies of Faraday effects (e.g., depolarization) in continuum sources may demand low observing frequencies, however. The spectral index of the continuum also influences the choice of frequency--optically thick thermal emission may be easier to detect at high frequencies whereas transparent synchrotron sources will be easier to detect at low frequencies.
If a continuum project has frequency agility, the choice of
also involves the receiver sensitivity and
antenna performance as a function of frequency. High frequency
observations are often limited by worsening receiver noise
performance, antenna efficiency and pointing accuracy
(as a fraction of the primary beam size) with increasing frequency.
Low frequency observations are limited by
interference at many radio observatories.