Source standards for the radiometric calibration of astronomical telescopes in the VUV spectral range.

Abstract

On the basis of a high-current hollow-cathode discharge we have developed two transfer source standards suitable for the radiometric calibration of vacuum ultraviolet (VUV) telescopes. The source standards are transportable and (in their current design) produce a collimated beam of 5 and 10mm diameter. By irradiating the entrance aperture of the telescope with this beam, the overall spectral response of the instrument can be determined and spectral responsivity variations over the entrance aperture can be directly evaluated. The transfer standards described in this paper have been calibrated in the radiometry laboratory of the Physikalisch-Technische Bundesanstalt (PTB) by use of the calculable spectral photon flux of the Berlin electron storage ring for synchrotron radiation BESSY - a primary radiometric VUV source standard. The output of the source standards has been determined in 57 emission lines covering the wavelength range 15 to 150nm. The photon flux in these emission lines ranges from 10^4^ to 10^9^s^-1^ and the overall uncertainty of the photon flux in any given line is found to be not more than 8% (1 sigma value).