Using a Standard Star

The extracted orders, together with the dispersion relation, define the observed flux as a function of the wavelength for each order:

$$F = F_m(\lambda)$$ (7.4)

This flux has to be corrected for two effects in order to get absolute fluxes: first, for the echelle blaze effect, and second, for the chromatic response of the instrument. For a given configuration, the blaze  effect is a function of the position in the order, while the instrument response is, essentially, a function of the wavelength.

The solution adopted in the reduction, using the standard star , is to correct for both the blaze effect and the instrument response simultaneously. This is done by comparing a standard star, observed with the same configuration as the object, to a table of absolute fluxes. The standard star is reduced exactly as the object and then correction factors are calculated by comparing the flux values in the table to the observed counts sampled at the same wavelength intervals as the fluxes in the table. The resulting response is normalised to an exposure time of one second. There is no effect due to differences between the flatfield of the object and the one corresponding to the standard star given that the flatfields are normalized (see Section 7.7).

If, as usually is the case, OBJ and STD were observed through different airmasses, the spectra have to be corrected for extinction using command EXTINCTION/SPECTRUM. More information about this command is available in Vol.2, Chapter L (Spectral Data). The command RESPONSE/ECHELLE is used to compute the instrument response as described here. Southern spectrophotometric standards have been published by Hamuy and al. (1992, 1994).