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Flux Calibration Files

Flux Calibration Files

There are two types of flux calibration files used by the task STANDARD.

Standard Star Calibration Files
This type of file is any file that does not contain the parameter "type" with a value of "blackbody". The only other parameter used by this type of calibration file is the "units" parameter for the wavelength units. If the units are not specified then the wavelengths default to Angstroms. All older calibration files will have no parameter information so they are interpreted as standard star calibration files with wavelengths in Angstroms.

The calibration files consist of lines with wavelengths, calibration magnitudes, and bandpass widths. The magnitudes are m_AB defined as

m_AB(star) = -2.5 * log10 (f_nu) - 48.60

where f_nu is in erg/cm^2/s/Hz. The m_AB calibration magnitudes are converted to absolute flux per unit frequency using the parameter fnuzero of the STANDARD task. This conversion is defined by

Fnu = fnuzero * 10. ** (-0.4 * magnitude)

Thus, fnuzero is the flux at m_AB of zero. The flux units are determined by this number. The default value was chosen such that Vega at 5556 Angstroms has a magnitude of 0.048 and a flux of 3.52e-20 ergs/cm2/s/Hz.

Blackbody Calibration Files
This type of file has the comment parameter "type" with a value of "blackbody". It must also include the "band" and "weff" comment parameters. If no "units" comment parameter is given then the default units are Angstroms.

The rest of the file consists of lines with wavelengths, m_AB of a zero magnitude star (in that band magnitude system), and the bandpass widths. The m_AB are defined as described previously. Normally all the m_AB values will be the same though it is possible to adjust them to produce a departure from a pure blackbody flux distribution.

The actual m_AB calibration magnitudes for the star are obtained by the relation

m_AB(star) = mag + m_AB(m=0) - 2.5 * log10 (B(weff,teff)/B(w,teff))

where m is the magnitude of the star in the calibration band, m_AB(m=0) is the calibration value in the calibration file representing the magnitude of a m=0 star (basically the m_AB of Vega), weff is the effective wavelength for the calibration file, and teff is the effective temperature of the star. The function B(w,T) is the blackbody function in f_nu that provides the shape of the calibration. Note how the normalization is such that at weff the last term is zero and m_AB(star) = m + m_AB(m=0).

The m_AB(star) computed using the calibration values and the blackbody function are then in the same units and form as for the standard star files. The conversion to f_nu and the remaining processing proceeds in the same way as for standard star calibration data.

The parameters \Imag and teff are specified by the user when running the STANDARD task.

Note that it is important that the flux values are in magnitudes. If the fluxes are not given in magnitudes, as occurs with some files imported from other sources, the STANDARD task will behave incorrectly.

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