Nova Del 2013

Absolute flux calibration using photometric method


Nova Del Spectrum

SC 25 cm F/D = 10
LISA R = 1100

The intensity scale is relative : 1 for range [6050,6100]

The purpose is to obtain an abolute flux calibration in erg/cm2/s


The method is to use a V band photometry to calibrate the spectrum

i.e. computing the absolute value of the continuum at lambda = 5500 angströms


Photometry in V

A photometry with Johnson V filter is obained just before the spectrum

Scope : 66ED
CDD : starlight SXV-H9
Exposure : 9 x 5 seconds
Bias, Dark substracted, flat divided

Photometric reduction
Software : MIRA Pro AP

Reference star : 000-BLC-955 V = 8.004
Check star : 000-BLC-940 V = 7.878

Result : Nova Del = 6.80
Check = 7.88

V = 6.80 +/- 0.02

UT 2013-08-30T19:37:59




Using a Johnson V transmission curve, the spectrum is convolved

Multiplication in ISIS :
Tools>Spectra Processing 1>Profiles arithmetic

The profile shows the (relative) luminosity throw the V filter.

The integrated flux between 4400 and 6600 A is proportional to V magnitude (but also to its value in erg/cm2/s in the Vega System)



Johnson V transmission curve fit


What we need is the value of the absolute intensity of a point of the continuum (i.e. 5500 angströms)

We have then to substract the contribution of the emissions lines to the integrated luminosity, according to the method described by Skopal (2003, 2007) in his studies of symbiotics stars (other stars, but same issue with intense emission lines).

This can be done, using "Continuum" in 5. Profile window of ISIS

A smooth continuum is computed (in red)

Perhaps the method should be improved

Using VSpec, we compute :

the integrated (relative) flux of the spectrum between 4400 and 6700 (Fs)
the integrated (relative) flux of the continuum between 4400 and 6700 (Fc)

Numerical values :

Fs = 211 177
Fc = 174 678

The absolute flux calibrated spectrum will have to be multiplied by a factor Fc/Fs

Fc/Fs = 0.827 (0.83)





The spectrum is calibated in absolute flux with V = 6.80

ISIS>Tools>Spectra processing 3

And corrected for emission lines flux (multiplication by factor 0.83)

ISIS>Tools>Spectra processing



Last but not least, the spectrum must be corrected for interstellar extinction

with E(B-V) = 0.18 (Mubari & al., ATel#5297)

Final calibrated and dereddened spectrum

A measure of H alpha gives 9.9 10E-9 erg/cm2/s