ISIS - flux calibration using visual magnitude

[Robin Leadbeater]

Hello Christian,

I would like to use the flux calibration feature in ISIS using V mag but I need to know more about how it works and what assumptions are made.

It appears to use the flux measured at one wavelength in the spectrum (5556A) but V magnitude depends on the integrated flux over the passband of the V filter . What happens if 5556A lies on an emission or absorption line?

Thanks
Robin

[Christian Buil]

For erg/cm2/s/A conversion I compute:

C1 = 10.0 ^ ( -0.4 * V_magnitude - 8.449)
COEF = C1 / MEASURED_SIGNAL

MEASURED_SIGNAL is the signal measured in your spectrum in numerical count near 5556 A wavelength
(more precisely I compute the mean signal in a range of +/-2 spectral sample around
the nearest 5556 A pixel sample).

All points of your spectrum ar multiplied by COEF

S_ASBOLUTE = S_OBERSERVED x COEF

For W/m2/A the conversion equation is:

C2 = 10.0 ^ ( -0.4 * V_magnitude - 11.449)
COEF = C2 / MEASURED_SIGNAL

Of course the presence of line in the +/-2 pixel sample range
can be a problem. My recommandation is to interpollate
the spectrum near 5550 A for erase accidental lines before
flux conversion.

Christian

[Robin Leadbeater]

Thanks Christian,

Do you have a reference I can quote for the Vmag to flux at 5556A conversion factor?

Cheers
Robin

[Martin Dubs]

Hi Robin,

a good introduction to spectrophotometry you find here:
http://www.astro.virginia.edu/class/oco ... 14-f03.pdf
It looks however that it is based on a different photometric system (Bessell vs. Johnson). This also shows some of the complexity of these standards.

Regards, Martin