RADIOMETRIC CALIBRATION OF THE TELESCOPE AND ULTRAVIOLET SPECTROMETERSUMER ON SOHO

The prelaunch spectral-sensitivity calibration of the solar spectromet er SUMER (Solar Ultraviolet Measurements of Emitted Radiation) is desc ribed. SUMER is part of the payload of the Solar and Heliospheric Obse rvatory (SOHO), which begins its scientific mission in 1996. The instr ument consists of a telescope and a spectrometer capable of taking spa tially and spectrally highly resolved images of the Sun in a spectral range from 50 to 161 nm. The pointing capabilities, the dynamic range, and the sensitivity of the instrument allow measurements both on the solar disk and above the limb as great as two solar radii. To determin e plasma temperatures and densities in the solar atmosphere, the instr ument needs an absolute spectral-sensitivity calibration. Here we desc ribe the prelaunch calibration of the full instrument, which utilizes a radiometric transfer-standard source. The transfer standard was base d on a high-current hollow-cathode discharge source. It had been calib rated in the laboratory for vacuum UV radiometry of the Physikalisch-T echnische Bundesanstalt by use of the calculable spectral photon flux of the Berlin electron storage ring for synchrotron radiation (BESSY)- a primary radiometric source standard. (C) 1996 Optical Society of Ame rica