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<title>Munipack ‒ Photometric Systems Definition</title>
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<section>
<h1 class="noindent">Photometric Systems Definition</h1>
<p class="abstract">
Definition of a FITS table providing parameters of photometric systems.
</p>
<p>
One uses selected photometric systems from
<a href="http://ulisse.pd.astro.it/Astro/ADPS/Systems/index.html">The Asiago
Database on Photometric Systems</a> as the reference.
</p>
<p>
This table specification has been developed for internal use in Munipack
and backward compatibility is not guarantied.
</p>
<h2>File Structure</h2>
<p>
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.
</p>
<div class="table">
<table>
<caption>Photometric system FITS file structure</caption>
<tr><th>HDU</th><th>EXTNAME</th><th>Description</th></tr>
<tr><td>0</td><td></td><td>Dummy<sup><a href="#dummy">[†]</a></sup></td></tr>
<tr><td>1</td><td>identifier</td><td>Short identification of system</td></tr>
<tr><td>...</td><td>...</td><td>...</td></tr>
</table>
</div>
<div class="notes">
<p id="dummy">
<sup><a href="#dummy">[†]</a></sup>
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.
</p>
</div>
<h2>Photometric System HDU</h2>
<p>
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.
</p>
<p>
All quantities must be expressed strictly in SI.
</p>
<div class="table">
<table>
<caption>Photometric System HDU Table</caption>
<tr><th>Column</th><th>Type</th><th>Description</th><th>units</th></tr>
<tr><td>FILTER</td><td>12A</td><td>Filter designation<sup><a href="#filter">[1]</a></sup></td><td></td></tr>
<tr><td>LAM_EFF</td><td>E</td><td><i>λ</i><sub>eff</sub> effective wavelength<sup><a href="#leff">[2]</a></sup></td><td>m</td></tr>
<tr><td>LAM_FWHM</td><td>E</td><td><i>Δλ</i> passband FWHM<sup><a href="#lfwhm">[3]</a></sup></td><td>m</td></tr>
<tr><td>NU_EFF</td><td>E</td><td><i>ν</i><sub>eff</sub> effective frequency<sup><a href="#neff">[4]</a></sup></td><td>Hz</td></tr>
<tr><td>NU_FWHM</td><td>E</td><td><i>Δν</i> passband FWHM<sup><a href="#nfwhm">[5]</a></sup></td><td>Hz</td></tr>
<tr><td>FNU_REF</td><td>E</td><td><i>f<sub>ν</sub></i> spectral density flux per frequency unit for zero magnitude star<sup><a href="#flx">[6]</a></sup></td><td>W/m2/Hz</td></tr>
<tr><td>FLAM_REF</td><td>E</td><td><i>f<sub>λ</sub></i> spectral density flux per wavelength unit for zero magnitude star<sup><a href="#flx">[6]</a></sup></td><td>W/m2/nm</td></tr>
</table>
</div>
<div class="notes">
<p id="filter">
<sup><a href="#filter">[1]</a></sup>
A unique filter identifier in the given system. The string will be
matched against to FILTER keyword in headers of calibrated frames.
</p>
<p id="leff">
<sup><a href="#leff">[2]</a></sup>
Effective wavelength is computed as a mean wavelength of the given
filter transmissivity.
</p>
<p id="lfwhm">
<sup><a href="#lfwhm">[3]</a></sup>
Passband full width at half of maximum (FWHM) given in wavelengths.
</p>
<p id="neff">
<sup><a href="#neff">[4]</a></sup>
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 <i>ν</i>=c/<i>λ</i>.
</p>
<p id="nfwhm">
<sup><a href="#nfwhm">[5]</a></sup>
Passband FWHM in frequency units. When not defined,
the formula <i>Δν</i>=(c/<i>λ</i><sup>2</sup>)<i>Δλ</i> is used to derive it.
</p>
<p id="flx">
<sup><a href="#flx">[6]</a></sup>
Spectral fluxes densities are rarely given with a required precision.
The formula of reciprocity between fluxes per wavelength and frequency
in Hertz is <i>f<sub>ν</sub></i> = (<i>λ</i><sup>2</sup>/c) <i>f<sub>λ</sub></i>
(by a convention, use 10<sup>-9</sup>m=1nm multiplicand to specify it in nanometers).
</p>
</div>
<h2>Modification of Photometric System Table</h2>
<p>
The system table is created from a text file which can be found at source
tree as <samp>muniphot/photosystems.lst</samp>.
The text file can be modified as needed,saved to my_photosystems.lst,
and new FITS table will be created as
</p>
<pre>
$ munipack fits --restore my_photosystems.lst,my_photosystems.fits
</pre>
<p>
The modified file can be used as
</p>
<pre>
$ munipack phfotran --phsystab SOMEWHERE/my_photosystems.fits ...
$ munipack phcal --phsystab SOMEWHERE/my_photosystems.fits ...
</pre>
<p>
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.
</p>
<h2>See Also</h2>
<p>
<a href="man_phcal.html">Photometric Calibration</a>,
<a href="man_phfotran.html">Photometric System Transformation</a>,
<a href="http://ulisse.pd.astro.it/Astro/ADPS/Systems/index.html">The Asiago Database on Photometric Systems</a>,
<a href="http://www.astro.umd.edu/~ssm/ASTR620/mags.html">Astronomical Magnitude Systems</a>
</p>
</section>
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