fnuzero of standard task

Guest wrote on Apr 03, 2007

Dear IRAF team,

When I read the chapter STANDARD STAR CALIBRATION FILES at
the IRAF Help page for "standard"(V2.12.1):
http://iraf.noao.edu/scripts/irafhelp?standard

I wondar why the definition of fnuzero is different
from that of the above equation.
Is "Fnu" not the same as "f_nu"?
Or is "magnitude" in
Fnu = fnuzero * 10. ** (-0.4*magnitude)
not the same as "m_AB(star)"?
Or anything else??

Thanks in advance,
Kohki Konishi @ Univ. of Tokyo
Post generated using Mail2Forum (http://www.mail2forum.com)

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Francisco Valdes wrote on Apr 03, 2007

Hello,

In the documentation Fnu and f_nu are the same thing and magnitude and m_AB are the same thing. So the equations are

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

f_nu = fnuzero * 10. ** (-0.4 * m_AB)

I recognize that the description was not as precise as it should be and I have made changes. Adding to this confusion the statement that with the default fnuzero Vega has an AB magnitude of 0.048 is wrong. The value is 0.0336. I have changed the help page to reflect this. The point of the choice of fnuzero is to produce a value of 3.52e-20 for Vega at 5556A which was what versions of STANDARD did 20 years ago. This is not saying that this is the current best calibration of Vega. The reason fnuzero is a parameter is because users can change it to meet current absolute flux standards using the same historical calibration files.

Please read the help again at

http://iraf.noao.edu/scripts/irafhelp?standard

to see the changes.

Yours,
Frank Valdes

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Last post on Apr 03, 2007