Filters and Photometric systems

The hyperz package provides also the response functions of the filters, contained in the file ./ZPHOT/filters/FILTER.RES. This file is an extension of the file provided in the GISSEL library by Bruzual & Charlot (1993) and at this stage it includes $193$ filters. You can add your own filters appending their transmission functions at the end of the file according to the following format: the first line must contain the number of points forming the transmission function (compulsory, no more than $10000$ points are accepted) and the name of the filter (optional). The subsequent lines must contain in the first column the number of the point, in column two the wavelength in Å and in column three the value of the transmission. Take care that if your response function contains more than $200$ points, the program will proceed with a rebinning to include only $200$ points, because otherwise the integration would be too CPU time consuming. Notice that the points must be ordered by increasing wavelengths. Moreover, do not care about the normalization of the transmission functions, because this operation will be performed internally in hyperz. In Figure 8 we illustrate the transmission functions of the most used filters, corresponding to the records # $12$, $91$,$92$, $93$, $94$, $162$, $76$, $75$, $77$, $131$, $122$, $123$, $124$,$121$.

Two reminder files are included in the package, even if they are not used during the computation: ./ZPHOT/filters/filters.log contains the list of the filters contained in ./ZPHOT/filters/FILTER.RES, their record number and the number of points composing their transmission function; ./ZPHOT/filters/filters.dat contains a description of the filters by means of their $\lambda_{\rm eff}$, their surface $S$, their width, computed as described in Section 3.2, and the conversion between Vega and AB magnitudes..

Figure:Normalized filter's transmission functions of some of the most used filters.

The program hyperz works with ``standard'' Vega magnitudes, as well as with AB magnitudes. The first photometric system is calibrated on the Vega Spectral Energy Distribution shown in Figure 9 and taken from the Bruzual & Charlot (1993) library. In the formulae we have $$m_{\rm Vega} = -2.5 \left[ \log \int R(\lambda) f(\lambda)\, ......g \int R(\lambda) f_{\rm Vega}(\lambda)\, d\lambda \right] \: ,$$

where $f(\lambda)$ is in unit of ${\rm erg \,s^{-1} \,cm^{-2}\,\AA^{-1}}$. AB magnitudes are defined in such a way that the zero point is set equal for all filters and are directly related to the flux $f(\nu)$: $m_{\rm AB} = -2.5 \log f(\nu) -48.60$, where $f(\nu)$ is given in ${\rm erg\,s^{-1}\,cm^{-2}\,Hz^{-1}}$, and thus AB magnitudes can be monochromatic. An object having a flat spectrum will have equal magnitudes in all the filters. The conversion between Vega and AB magnitudes for a given filter is computed using the equation:

$${\rm conv_{AB}} = m_{\rm AB}-m_{\rm Vega} = m_{AB}{\rm (Vega)......t R(\nu) f_{\rm Vega}(\nu)\,d\nu}{\intR(\nu)\,d\nu} -48.60 \:.$$

Figure:Spectral Energy Distribution of $\alpha$Lyrae (Vega). Data from GISSEL library.

2000-12-10