SOURCE STANDARDS FOR THE RADIOMETRIC CALIBRATION OF ASTRONOMICAL TELESCOPES IN THE VUV SPECTRAL RANGE

On the basis of a high-current hollow-cathode discharge we have develo ped two transfer source standards suitable for the radiometric calibra tion of vacuum ultraviolet (VUV) telescopes. The source standards are transportable and (in their current design) produce a collimated beam of 5 and 10 mm diameter. By irradiating the entrance aperture of the t elescope with this beam, the overall spectral response of the instrume nt can be determined and spectral responsivity variations over the ent rance aperture can be directly evaluated. The transfer standards descr ibed in this paper have been calibrated in the radiometry laboratory o f the Physikalisch-Technische Bundesanstalt (PTB) by use of the calcul able spectral photon flux of the Berlin electron storage ring for sync hrotron radiation BESSY a primary radiometric VUV source standard. The output of the source standards has been determined in 57 emission lin es covering the wavelength range 15 to 150 nm. The photon flux in thes e emission lines ranges from 10(4) to 10(9) s(-1) and the overall unce rtainty of the photon flux in any given line is found to be not more t han 8% (1 sigma value).