SMOOTH Type: Adverb (REAL(3)) Use: SMOOTH specifies the type of spectral smoothing to be applied to a uv database or other spectra. The default is not to apply any smoothing. The elements of SMOOTH are as follows: SMOOTH(1) = type of smoothing to apply: 0 => no smoothing 1 => Hanning applied before bandpass calibration 2 => Gaussian applied before bandpass calibration 3 => Boxcar applied before bandpass calibration 4 => Sinc (i.e. sin(x)/x) applied before bandpass cal. 5 => Hanning applied after bandpass calibration 6 => Gaussian applied after bandpass calibration 7 => Boxcar applied after bandpass calibration 8 => Sinc (i.e. sin(x)/x) applied after bandpass cal. SMOOTH(2) = the "diameter" of the function, i.e. width between first nulls of Hanning triangle and sinc function, FWHM of Gaussian, width of Boxcar. Defaults (if < 0.1) are 4, 2, 2 and 3 channels for SMOOTH(1) = 1 - 4. SMOOTH(3) = the diameter over which the convolving function has value - in channels. Defaults: 1, 3, 1, 4 times SMOOTH(2) used when input SMOOTH(3) < net SMOOTH(2). When applied to uv data, it is important to decide whether the smoothing should be applied before or after application of the bandpass calibration. If SMOOTH was used in BPASS (or CPASS), then the same SMOOTH must be used with all the data (and SMOOTH(1) must be 1 through 4). If SMOOTH was not used in BPASS, then SMOOTH(1) must be 5 - 8 for correct results. Null Value: 0 A null value of SMOOTH(1) means no smoothing is desired; the useage of null values for other parameters depend on the application. The usual defaults are listed above. Discussion: Frequency smoothing of UV data interacts with data flagging in ways that are not entirely intuitive. In fact, it is strongly recommended that any data smoothing to be applied before the bandpass be done, once and for all (with e.g. SPLAT), before any channel-dependent flagging is done. Data with RFI frequently requires a Hanning smooth up front to reduce ringing. Do this with SPLAT once and for all eraly in your data reduction. Post bandpass smoothing also interacts with flags and the computatin of Stokes parameters. A SMOOTH of, e.g. 6,0,0 functions as follows: 1. Smooth each polarization separately ignoring flagged channels and maintaining the flagging. 2. Combine polarizations to make the Stokes parameters. This can be particularly odd with Stokes I - which is taken to be (RR+LL)/2 unless one of RR and LL is flagged, in which case the other is taken. Thus smooth values of RR and LL are averaged but isolated channels of only RR or LL also appear. These isolated channels will have smoothed values appropriate to RR or to LL but not to the average unless RR and LL are nearly identical. Thus single channels will stick out from the otherwise smooth spectrum. Even with Stokes F (formal I), Q, U, and V, odd results may arise. Flagging of one of the two observed polarizations for these parameters leads to flagging of the result at the particular flagged channel. But, if one of e.g. RL and LR is not flagged, then those unflagged values will affect the adjacent channels in the smoothing that preceeds the Stokes computation. Thus, at a good channel, the results will be different from the results if all polarizations rather than just 1 are flagged at the flagged channels. POSSM has a special option where the spectra are averaged with no smoothing and then smoothed just before displaying them. In this way, most of the complications described above are avoided.