Dealing with spikes

External (or internal) interference (RFI) is a common occurrence with the longer wavelengths in use at the JCMT; that is, particularly in spectra taken with receiver A2. Usually this is offset from the centre of the band and is of no major concern. It seems to be worse for position-switching and may disappear entirely in the azimuth beamswitched mode. Sometimes, however it can be a real nuisance. An example is given in Figure .

Figure: A position-switched DAS spectrum of G34.3 in the area of 239 GHz showing `ringing' caused by a strong internal interference spike. Note that it affects only the subband in which it occurs. The offset from zero is the result of a combination of the source continuum emission and a difference in the airmass between the signal and reference positions. The line features are due to CH$_3$CCH and CH$_3$CN.

Such interference spikes may be very effectively removed by applying Hanning smoothing to the spectrum:

$>\!>$ hann

The result is shown in Figure . A multitude of weak line features, as expected, are now revealed. The spike itself may be removed by using the spike-removal command:

$>\!>$ rem-spike

Figure: The result of applying Hanning smoothing to the spectrum shown in Figure . Note that the `ringing' completely disappears, revealing previously obscured line features. The spectrum is itself somewhat smoothed in the process.