The purpose of this routine is to provide photometric measurements of IS0
SWS and LWS spectra or other spectra introduced into the ISAP environment.
The routine will either integrate or average a source energy distribution
(SED) over a specified range, and will do so while weighting the
distribution with a "filter" function. With the appropriate filter
function, the user may make flux comparisons with measurements from
ISOPHOT, IRAS, KAO instruments, ground-based telescope instruments, or
theoretical predictions.

Prior to averaging (to produce a flux density) or integrating (to produce a
flux) one of the two functions, SED or filter, is
resampled at the resolution of the other. The lower resolution function (as
determined by comparing sampling intervals) is resampled at the sampling of
the higher resolution function.

Modes:

Averaging: First the filter function is normalized. All points on
the filter function are multiplied by a scaler so that their sum will be 1.
Then the SED function and the filter function are multiplied pairwise and
summed yielding the average flux density. Valid units for the output
flux density are [Jy], [W/cm2/um], and [W/m2/um]. No extrapolation of the filter or
SED function is made; outside their range they are set to 0.
Integrating: Here the filter function is used as a weighting
function and the SED is integrated over the range of the filter function.
No normalization of the filter function is performed. As an example - if
one wanted to calculate the integrated flux from 60 to 100 um of an SED,
one could set up (instructions below) a filter as two points (60um, 1) and
(100um, 1). This filter function would be resampled by the photometry routine
at all the sample points of the SED function, each time with the weight set
(by interpolation) to 1. The routines then integrates by multiplying the
SED flux density values by the wavelength interval at each point and by the
filter function (1) at each point, and thus integrating to determine the
flux. If one instead wanted to predict the
flux that would be observed with a photometer of known spectral transmission,
with transmission ranging between 0 and 1, the user would supply the
transmission function as the filter function.
Average: nufnu: This mode has two implementations, one for the 4
IRAS filters and one for all non-IRAS filters. We first discribe the
implementation for non-IRAS filters. The filter values are taken as
transmission values and, as in the integration mode above, the in-band flux
is computed. An effective central wavelength and width for the filter is
determined. Finally, under the assumption that source SED shape is of the
form (nu x f_nu) = constant, or equivalently (lambda x f_lambda) =
constant, a flux density at the center wavelength is computed. (This
assumption is made for convenience - it allows for invariance of reported
flux densities when filters are expressed as a function of frequency or
wavelength. The details
of the determination of the filter width and central wavelength may be
found in the - detailed photometry
notes . For IRAS filters,
the same steps as above are conducted, but in units of frequency so that the
established filter widths: (13.48, 5.16, 2.58, 1.00) x 10^12 Hz, and
center wavelengths: (12, 25, 60, 100um) may be adhered to.
Units:

If the Integration mode is used, the valid units are the flux units:
[W/m2], [W/cm2]. In the average and nufnu modes, the valid units are the
flux density units: [Jy], [W/m2/um], and [W/cm2/um]. For the average
and nufnu modes, Magnitudes are an alternative output unit, provided a
filter's "zero-point" flux density is included in the filter_zero_point.dat
file in the filter directory, in Jy. Recall,
mag =2.5 * LOG (zero_point/flux_density.
For more details on any aspect of the photometry
processing see - detailed photometry
notes

User Supplied Filters:

The ISAP startup script sets the ISAP filter path,
e.g. .../isap/data/filters/. In this area, the user may set up personal
ASCII filter files, with the data (wavelength, transmission) starting on
the 6th line of the file.