./guide
spurinterp.pro

Last modification date:

Wed Sep 28 13:27:09 2016

spurinterp

procedure spurinterp[, buffer]

Replace data values at the known locations of VEGAS ADC spurs with the average of the two adjacent channel values.

VEGAS produces spurs (spikes) at channels corresponding to integer multiples of the ADC sampler frequency divided by 64. The normal behavior of sdfits is to flag these channels when the data are filled (use listflags to see the list of flags for the currently opened input data set). When flagged data is retrieved by GBTIDL, the flagged channels are replaced with NaNs and will not contribute to most operations. If the skipflag option is used to retrieve the data then flagging is not applied and the original data values will appear at all channels.

It is sometimes useful to replace the spur data values (either NaNs if the flags have been used or the original data values if the flags are ignored) with interpolated values. This routine replaces those values with the average of the two channels on either side of the spur (or the adjacent channel if the spur is at the end of the spectrum).

A spur is also typically present at the center channel (NCHAN/2 when counting from 0). That spur does not arise in the ADC in VEGAS and does not move as the spectral window is tuned across the ADC bandpass. This routine does not interpolate across that channel. Normal sdfits usage does that interpolation unless the "-nointerp" option is used.

If the buffer is not specified then buffer 0 is used.

If the display is not already frozen and buffer 0 is used then the show command is used to redisplay the data.

This spur locations are determined using the VSPDELT, VSPRPIX, and VSPRVAL header values. For data filled using older versions of sdfits, these values are not present in the SDFITS table and will appear as NaN in GBTIDL. In that case, no spur interpolation is possible and a warning message will be printed. The data should be refilled by the most recent version of sdfits to make use of this procedure.

Uses

dcspurinterp

Version

$Id$

Parameters
buffer in, optional integer (def. 0)	Input data buffer (0 through 15).