Re: PNV J20233073+2046041 mag 6.8
Posted: Sun Sep 29, 2013 8:43 am
Hello Francois,
if you are interested in the continuum and the continuum is well defined (this is not obvious in the case of Nova Del 2013) then your method of determination of the weighted flux of the full spectrum and the spectrum with emission lines removed, calculating a magnitude correction factor and applying the flux equation to the continuum spectrum should give a correct result. It gives the flux of the pseudo continuum and if e.g. the pseudo continuum at 550 nm agrees with the true spectrum at 550 nm you get the correct result by applying the flux conversion in ISIS. However, in my opinion it is unnecessary complicated. The method I propose is based on the definition of magnitude, a weighted (with the response function, in particular Bessell_V) integral of the stellar flux, which includes emission lines. It can be applied without any assumptions on what is the continuum. The method even works if no continuum is present or for stars which do not have a well defined continuum, such as M stars. A small error is introduced in my method because the measured magnitudes are additionally weighted by atmospheric extinction, which reduces the blue intensities somewhat, and the calculated flux is based on the extraterrestric spectrum. The uncertainty is increased by not knowing the used filter and CCD efficiency curve for the magnitude measurement. This difficulty is avoided by Christians direct flux measurement by comparison with a reference star at the same air mass, without using a possibly biased magnitude measurement.
I have studied the papers by Skopal. Their goal is somewhat different. They are interested in color indices U - B and B - V for the continuum, in order to classify the stars. For this it is necessary to remove the emission lines. The equation for the magnitude correction is the same as I use for the flux calculation. A similar situation would arise if you are interested in the spectrum of a star inside a gaseous nebula. Depending on the slit area on the sky you measure a smaller or larger contribution from the nebula in addition to the star, so if you are interested in the properties of the star you would subtract the contribution of the nebula first.
I am not sure if this clarifies the issue somewhat.
Regards, Martin
if you are interested in the continuum and the continuum is well defined (this is not obvious in the case of Nova Del 2013) then your method of determination of the weighted flux of the full spectrum and the spectrum with emission lines removed, calculating a magnitude correction factor and applying the flux equation to the continuum spectrum should give a correct result. It gives the flux of the pseudo continuum and if e.g. the pseudo continuum at 550 nm agrees with the true spectrum at 550 nm you get the correct result by applying the flux conversion in ISIS. However, in my opinion it is unnecessary complicated. The method I propose is based on the definition of magnitude, a weighted (with the response function, in particular Bessell_V) integral of the stellar flux, which includes emission lines. It can be applied without any assumptions on what is the continuum. The method even works if no continuum is present or for stars which do not have a well defined continuum, such as M stars. A small error is introduced in my method because the measured magnitudes are additionally weighted by atmospheric extinction, which reduces the blue intensities somewhat, and the calculated flux is based on the extraterrestric spectrum. The uncertainty is increased by not knowing the used filter and CCD efficiency curve for the magnitude measurement. This difficulty is avoided by Christians direct flux measurement by comparison with a reference star at the same air mass, without using a possibly biased magnitude measurement.
I have studied the papers by Skopal. Their goal is somewhat different. They are interested in color indices U - B and B - V for the continuum, in order to classify the stars. For this it is necessary to remove the emission lines. The equation for the magnitude correction is the same as I use for the flux calculation. A similar situation would arise if you are interested in the spectrum of a star inside a gaseous nebula. Depending on the slit area on the sky you measure a smaller or larger contribution from the nebula in addition to the star, so if you are interested in the properties of the star you would subtract the contribution of the nebula first.
I am not sure if this clarifies the issue somewhat.
Regards, Martin