The final step

To anticipate for a moment, the final step in the flux calibration process is carried out by the command `spflux'. `spflux' multiplies an observed spectrum by a calibration spectrum, to create a flux-calibrated spectrum. Each element of the calibration spectrum contains a value which is effectively the instrumental response of the detector at a given wavelength, the response being in units of `Flux density units per count per second per Angstrom'. The `Flux density units' may be any linear units, e.g. mJy. `spflux' assumes that the spectrum to be calibrated is still in counts, and for each element calculates the wavelength range covered by that element, and then combines that with the counts in that element, the value of the calibration spectrum at that element, and the exposure time for the spectrum, to generate a flux density for the central wavelength of the element. The result is, of course, a spectrum in `Flux density units'-whatever they happen to be.

`spflux' is straightforward enough; the problem, as with similar functions, is to generate the flux calibration spectrum.