The files in the modewave relation are ascii text files containing a list of wavelength values appropriate for each HST instrument mode and are used by the pysynphot method countrate() to compute simulated spectra. The wavelength values in each table cover the range of sensitivity for a given instrument mode, and have increments from one value to the next that closely match the actual dispersion per detector element (pixel, diode, etc.) for that instrument mode. Having an accurate representation of the increment per pixel is necessary in order to get an accurate estimate of the number of counts that will be detected.
Modewave files are named according to the instrument mode. For example, for an observation using the FOS blue-side with the G130H grating, countrate() will use the "fos_blue_g130h.dat" table in to get an accurate list of the wavelength range covered by the 512 diodes in the FOS detector.
This data is in general only useful for spectroscopic instruments, but files are included for imaging instruments (NICMOS, WF/PC, WFPC2, FOC) for completeness.
The data can be found in the modewave directory.