# Photometric Systems Definition

Definition of a FITS table providing parameters of photometric systems.

One uses selected photometric systems from The Asiago Database on Photometric Systems as the reference.

This table specification has been developed for internal use in Munipack and backward compatibility is not guarantied.

## File Structure

The photometric system definition file is a FITS binary table. Every particular photometric system occupies a single FITS extension. The extension name is a short identification of the corresponding system. Bands (filters) of the system are stored as rows of the table and contains full photometric description of the filter.

Photometric system FITS file structure
HDU | EXTNAME | Description
--- | --- | ---
0 |  | Dummy[†]
1 | identifier | Short identification of system
... | ... | ...

[†] The dummy section contains keyword AUTHOR identifying who compiled the table (by default set to 'Munipack'), HDUNAME describing the file which is set mandatory to 'PHOTSYS' and the link to this page as a comment. Please, if your are modifying the photometric table, change AUTHOR field and leave untouched HDUNAME, because one is the main identifier of this table format.

## Photometric System HDU

Every photometric system occupies a single extension. The system is identified by EXTNAME keyword. The reference to a definition of the system is in REFID keyword. Other keywords defines the table structure. Comments can contain additional specifications.

All quantities must be expressed strictly in SI.

Photometric System HDU Table
Column | Type | Description | units
--- | --- | --- | ---
FILTER | 12A | Filter designation[1] |
LAM_EFF | E | λeff effective wavelength[2] | m
LAM_FWHM | E | Δλ passband FWHM[3] | m
NU_EFF | E | νeff effective frequency[4] | Hz
NU_FWHM | E | Δν passband FWHM[5] | Hz
FNU_REF | E | fν spectral density flux per frequency unit for zero magnitude star[6] | W/m2/Hz
FLAM_REF | E | fλ spectral density flux per wavelength unit for zero magnitude star[6] | W/m2/nm

[1] A unique filter identifier in the given system. The string will be matched against to FILTER keyword in headers of calibrated frames.

[2] Effective wavelength is computed as a mean wavelength of the given filter transmissivity.

[3] Passband full width at half of maximum (FWHM) given in wavelengths.

[4] Effective frequency is computed as a mean frequency. Usually, the system definitions in optical bands are established by effective wavelengths and the quantity is computed as ν=c/λ.

[5] Passband FWHM in frequency units. When not defined, the formula Δν=(c/λ2)Δλ is used to derive it.

[6] Spectral fluxes densities are rarely given with a required precision. The formula of reciprocity between fluxes per wavelength and frequency in Hertz is fν = (λ2/c) fλ (by a convention, use 10-9m=1nm multiplicand to specify it in nanometers).

## Modification of Photometric System Table

The system table is created from a text file which can be found at source tree as `muniphot/photosystems.lst`. The text file can be modified as needed,saved to my_photosystems.lst, and new FITS table will be created as

    $ munipack fits --restore my_photosystems.lst,my_photosystems.fits

The modified file can be used as

    $ munipack phfotran --phsystab SOMEWHERE/my_photosystems.fits ...
    $ munipack phcal --phsystab SOMEWHERE/my_photosystems.fits ...

Of course, many alternative ways can be used to create the table. It is only important respect the structure of the file, the main identifiers and data types.

## See Also

Photometric Calibration, Photometric System Transformation, The Asiago Database on Photometric Systems, Astronomical Magnitude Systems