Radiometric calibration of the vacuum-ultraviolet spectrograph SUMER on the SOHO spacecraft with the B detector

The Solar Ultraviolet Measurement of Emitted Radiation (SUMER) vacuum-ultra violet spectrograph was calibrated in the laboratory before the integration of the instrument on the Solar and Heliospheric Observatory (SOHO) spacecr aft in 1995. During the scientific operation of the SOHO it has been possib le to track the radiometric calibration of the SUMER spectrograph since Mar ch 1996 by a strategy that employs various methods to update the calibratio n status and improve the coverage of the spectral calibration curve. The re sults for the A Detector were published previously [Appl. Opt. 36, 6416 (19 97)]. During three years of operation in space, the B detector was used for two and one-half years. We describe the characteristics of the B detector and present results of the tracking and refinement of the spectral calibrat ion curves with it. Observations of the spectra of the stars alpha and rho Leonis permit an extrapolation of the calibration curves in the range from 125 to 149.0 nm. Using a solar coranal spectrum observed above the salar di sk, we can extrapolate the calibration curves by measuring emission line pa irs with well-known intensity ratios. The sensitivity ratio of the two phot ocathode areas can be obtained by registration of many emission lines in th e entire spectral range on both KBr-coated and bare parts of the detector's active surface. The results are found to be consistent with the published calibration performed in the laboratory in the wavelength range from 53 to 124 nm. We can extrapolate the calibration outside this range to 147 nm wit h a relative uncertainty of +/-30% (1 sigma) for wavelengths longer than 12 5 nm and to 46.5 nm with 50% uncertainty for the short-wavelength range bel ow 53 nm. (C) 2000 Optical Society of America OCIS codes: 120.5630, 120.620 0,300.6190, 300.6540.