4.2 Spectral Processor

The Spectral Processor has two banks, A and B, which may be used individually, but are usually combined. The bank label is stored in the Primary Header keyword INSTRUME, and may take the values SPA, SPB or SPAB. Depending upon the choice of banks, the Spectral Processor may have 1, 2, 4 or 8 channels. This number is stored in the STATE binary table keyword RCVRS, and the actual values are stored in the RECEIVER binary table column labelled RCVRID.

Using the Spectral Processor values INSTRUME and RCVRID, find the IF Manager ROW with corresponding values for SpectralProcessor BANK and CHANNEL. This selects the appropriate row in the IF Manager. Extract the appropriate values of SFF_sideband, SFF_multipler and SFF_offset from this row.

Extract the appropriate value of LO1 from the LO1 FITS file.

The SpectralProcessor may perform further IF conversions, but all of these are included in the IF Manager SFF values. Each IF CHANNEL is divided into N resolution elements (backend channels). I cannot find this value stored in the FITS file; it may be calculated by dividing BANDWD by FREQRES, or implicitly from the size of the DATA array.

For a DATA array with N frequency elements, counting from 1, the value of ``center_IF'' in the IF FITS file corresponds to the centre of the channel given by (N/2)+1. This corresponds to F_backendfor that channel. The backend channel spacing is given by the SpectralProcessor FITS file FREQRES quantity in the RECEIVER binary table. This allows the center IF frequency of the remaining channels to be calculated. Using each of these values for F_backend, the value of LO1 and the sky frequency formula allows the topocentric sky frequency for each backend channel to be calculated.