Photometric Zero-points

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In the UV there is no standard photometric system. We have implemented several different magnitude systems that will allow the users to compare their data with other instruments.

[edit] Method 2 (General method. Flux based on Vega flux scale)

Zero-points (by definition the magnitude of an object producing one count per second), have been derived by using for each filter the equation:

$Zeropoint_{\rm Vega}=2.5 log_{10}(CPS_{\rm Vega})+m_{\rm Vega} \,.$

Vega zero-points have been defined in such a way that Vega has $m_{\rm Vega} = 0.025 \,mag$ in all TAUVEX filters (following the convention by XMM-Newton). The instrumental magnitude has to be calculated using the formula

$mag=-2.5log_{10}(CPS)+Zeropoint_{\rm Vega}\,,$

where the zero-points for each filter are in the following table:


Filter	Zero-point_Vega
NBF3	13.58
SF1	13.6
SF2	14.78
SF3	14.45
BBF	16.45

The prevalent flux-based systems at UV and visible wavelengths are the AB system and the STMAG system. Both define an equivalent flux density for a source, corresponding to the flux density of a source of predefined spectral shape that would produce the observed count rate, and convert this equivalent flux to a magnitude. The conversion is chosen so that the magnitude in V corresponds roughly to that in the Johnson system.

[edit] Method 3 (AB magnitude system)

In AB system the flux density is expressed per unit frequency and the reference spectrum is flat in $F ν$ . The AB system is defined as:

$m_{\rm AB}^{\rm Filter}=Zeropoint^{\rm Filter}_{\rm AB} - 2.5 log_{10}(CPS^{\rm Filter})\,,$

where the Zero-points for each filter come from the original definition of AB magnitude as:

$Zeropoint^{\rm Filter}_{\rm AB} = -48.60 - 2.5 log_{10}(1/n_{\rm phot})\,,$

with $n phot$ being the count rate measured in a filter for a constant incoming flux, $F_{\nu}=1\,$ erg/sec/cm²/hz.


Filter	Zero-point_AB
NBF3	15.4
SF1	15.75
SF2	16.67
SF3	16.2
BBF	18.2

[edit] Method 4 (ST magnitude system)

In the STmag system, the flux density is expressed per unit wavelength, and the reference spectrum is flat in Fλ. Another way to express these zero-points is to say that an object with $F_{\lambda}=3.63 \times 10^{-9}\,$ erg/sec/cm²/A will have magnitude STmag=0 in every filter.