# Filters and Filter Data¶

SLUG comes with a fairly extensive list of filters, adapted from the list maintained by Charlie Conroy as part of fsps. However, users may wish to add additional filters, and so the format of the filter list is documented here for convenience.

Filter data is stored in two ASCII text files, FILTER_LIST and allfilters.dat, which are stored in the lib/filters directory. The FILTER_LIST file is an index listing the available filters. In consists of five whitespace-separated columns. The first column is just an numerical index. The second is the name of the filter; this is the name that should be entered in the phot_bands keyword (see Photometric Filter Keywords) to request photometry in that filter. The third and fourth columns the value of $$\beta$$ and $$\lambda_c$$ (the central wavelength) for that filter – see Spectra and Photometry for definitions. Anything after the fourth column is regarded as a comment, and can be used freely for a description of that filter.

The allfilters.dat file contains the filter responses. The file contains a series of entires for different filters, each delineated by a header line that begins with #. The order in which filters appear in this file matches that in which they appear in the FILTER_LIST. After the header line, are a series of lines each containing two numbers. The first is the wavelength in Angstrom, and the second is the filter response function at that wavelength.