Vega spectrum originally downloaded from:

ftp://ftp.stsci.edu/cdbs/current_calspec/alpha_lyr_stis_005.fits

See:

http://www.stsci.edu/hst/observatory/cdbs/calspec.html

For details, see Bohlin & Gilliland, 2004, AJ, 127, 3508
http://adsabs.harvard.edu/abs/2004AJ....127.3508B

This was then converted to units of nu - Fnu, instead of lambda - Flambda

Observed solar spectrum comes from here:

ftp://ftp.stsci.edu/cdbs/current_calspec/sun_reference_stis_001.fits

and theoretical spectrum comes from here:

ftp://ftp.stsci.edu/cdbs/grid/k93models/standards/sun_kurucz93.fits

The theoretical spectrum is scaled to match the observed spectrum from 1.5 - 2.5 microns, and then it is used where the observed spectrum ends.

Here are the notes on the theoretical spectrum:

Vega and the Sun are the primary basic standards for absolute 
spectrophotometric calibrations at optical and infrared wavelengths,
respectively. As part of Kurucz'93 atlas, the model spectra for Vega and
the Sun are also provided. The physical parameters used by Kurucz in the modeling of the Sun and Vega can be found in the Table. A second model 
spectrum using the ATLAS12 code with the same parameters has recently been computed by Castelli & Kurucz (1994, AA 281, 817). 


            PARAMETERS OF KURUCZ MODELS FOR THE SUN AND VEGA


 STAR    FILENAME      log_Z         T_eff        log_g           V_{Johnson}
   
 SUN       sun         +0.0           5777        +4.44              -26.75 
 VEGA    vega_k93      -0.5           9550        +3.95              +0.035 
 VEGA    vega_c95      -0.5           9550        +3.95              +0.035 



VEGA_K93.tab and VEGA_C95.tab:

Kurucz (1993, CD-ROM 13) model spectrum (vega_k93.tab) covers the full ultraviolet to infrared range from 0.05 to 10 microns with a non-uniform wavelength spacing ranging from 1 nm in the ultraviolet to 10-20 nm in the near-infrared, and 40 nm in the infrared (beyond ~ 3 microns). On the other hand, the present version of the ATLAS12 based model spectrum (vega_c95.tab; Castelli's spectrum hereafter) covers the 0.09 to 2.6 microns range with a uniform resolution of 0.1 nm. 
 
The ATLAS9 and ATLAS12 based model spectra are converted to absolute flux 
units by normalizing to the average flux of the Hayes (1985, IAU Symp. 111) spectrum in the Johnson V filter as represented by Buser & Kurucz (1978, AA
70, 555) bandpass, i.e. to 3.55E-9 erg s**{-1} cm**{-2} Å**{-1}. Vega has a visual magnitude V_{Johnson}= +0.035, in the Johnson system (see Colina & 
Bohlin 1994, AJ 108, 1931 for details).


