The multi-colour classification performed in the COMBO-17 and MANOS surveys produces estimates for the redshift and spectral type of galaxies and QSOs which are best fits to the measured multi-band SED. Given the spectral coverage from U- to I-band, it is possible to estimate restframe luminosities within this wavelength range directly from the measured photometry. If a restframe band falls outside the U-to-I range because of the redshift, then estimating the luminosity involves an estimated extrapolation of the SED beyond the range probed by the photometry. This is called K-correction.
The determination of luminosities involves two parts, a cosmological part unique to the redshift value, and a spectrophotometric part unique to the object. The former consists of calculating a distance modulus based on a set of cosmological parameters. In COMBO-17, we have so far published values for (Omega-m,Omega-l,H0)=(0.3,0.7,100).
Changing the Hubble constant means only shifting the zeropoint of the distance modulus scale, e.g. H0=70 means that all distance moduli have to be increased by -0.775 mag. In contrast, changing the cosmological parameters means that the distance moduli change in a non-trivial redshift-dependent way. The prgs for restframe calculation in COMBO-17 also calculate luminosity distances and luminosities for (Omega-m,Omega-l,H0)=(1.0,0.0,100) and (Omega-m,Omega-l,H0)=(0.2,0.0,100).
The spectrophotometric part unique to every object places the best-fitting template into the observed SED by linking it to the measured broadband nearest to the given redshifted restframe band in question (with a bias towards the red, where we expect higher and more reliably measured flux from the object). Here, a K correction table is used which in design looks entirely like a colour library and contains all galaxy (or QSO) templates for the necessary range of redshifts as one object per row with the colour indices in the columns. However, the colour indices in the K correction table are not *observed-frame*-minus-*observed-frame* like in the colour library used for the classification; instead they are *restframe*-minus-*observed-frame*, linking various interesting restframe bands (Jonhson, Bessell, SDSS, ...) to the various observed-frame ones that appear in the survey data (WFI-UBVRI).
The practical procedure to calculate restframe luminosities is then as follows: