For several reasons some continuum observations may benefit from using the digital correlator in spectral line mode. At wavelengths of 6 cm or longer, bandwidth smearing can become important. Depending on how large a field of view is required one can either reduce the total bandwidth from 50 MHz per IF to 25 or 12.5 MHz and observe in continuum mode or one can select a bandwidth of, e.g., 25 MHz and observe in spectral line mode, using 3.125 MHz wide channels. The OSS provides more information on how to calculate bandwidth smearing.
A second situation in which one might prefer spectral line mode is when trying to achieve very high dynamic range maps (larger than about 5,000:1 at P-band and of order 100,000:1 at other wavelengths). The closure errors are significantly reduced, and can be calibrated out, by observing in spectral line mode (see e.g., the discussion in Chapter 16 of SIRA)
Because of interference, at wavelengths of 20 cm or longer spectral line mode is often preferred over continuum mode as narrow interference spikes can be excised. One strategy which has been successful at P-band, where interference is mainly found at multiples of 12.5, 5 and 1 MHz, is to observe in 4IF mode, selecting a 3.125 MHz wide total bandwidth for each IF pair and centering the AC IF at 327.5 MHz and the BD IF at 332.5 MHz, i.e., between the strongest interference spikes. In the calibration stage, the continuum channel (channel 0) is discarded and a new continuum is created by selecting only those channels which are interference-free.
It should be noted, however, that there is a price to pay for these
strategies. First, the amount of data is increased by an order of
magnitude. Second, in a standard continuum observation the two IF
pairs (AC and BD) are put at different central frequencies so as to
cover a total of 100 MHz. In spectral line mode the widest band which
can be usefully employed per IF is 25 MHz. After discarding the
``edge'' channels in the calibration stage, choosing the band centers
again in such a way that they cover different frequencies, a total
bandwidth of about 35 MHz can be synthesized. This corresponds to a
loss in sensitivity of a factor of
.
Moreover, when using this set-up the cross polarization terms (i.e. LR and RL), which provide information on the linear polarization, are lost. When full polarization information is essential one would have to choose the PA (or PB) spectral line mode which employs one IF pair (either AC or BD) and provides all four Stokes parameters. In that case the effective bandwidth is down to about 18 MHz which, in order to achieve the same signal to noise as in a standard continuum run, would require a six-fold increase in observing time.