Because the index and data records in a BSC file represent interleaved
integrations of individual channels, if the user wishes to extract a
single channel from the index structure they must use SEL_BSC:
IDL> sel_bsc,bsc_index,bsc_data,new_index,new_data,wave,flux,eflux,chan=chan
The selected index and data structures are returned in new_index and
new_data.
The wavelength, flux, and uncertainties in the flux are returned in the arrays
wave, flux and eflux. The following keywords are also accepted: TSTART, TEND.