Radiometric calibration of the telescope and ultraviolet spectrometer SUMER on SOHO

Abstract

The prelaunch spectral-sensitivity calibration of the solar spectrometer SUMER (Solar Ultraviolet Measurements of Emitted Radiation) is described. SUMER is part of the payload of the Solar and Heliospheric Observatory (SOHO), which begins its scientific mission in 1996. The instrument consists of a telescope and a spectrometer capable of taking spatially 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 determine plasma temperatures and densities in the solar atmosphere, the instrument needs an absolute spectral-sensitivity calibration. Here we describe the prelaunch calibration of the full instrument, which utilizes a radiometric transfer-standard source. The transfer standard was based on a high-current hollow-cathode discharge source. It had been calibrated in the laboratory for vacuum UV radiometry of the Physikalisch-Technische Bundesanstalt by use of the calculable spectral photon flux of the Berlin electron storage ring for synchrotron radiation (BESSY)-a primary radiometric source standard.

The solar hydrogen Lymanαto Lymanβline ratio

“…-first, we used the well-documented B-detector ground calibration performed before launch (Hollandt et al 1996) and verified at the beginning of the mission (Schühle et al 2000) (with the keywords/BARE and /BEFORE in the radiometry procedure); -in data acquired after the 1998 SoHO "vacation", we noted that SUMER experienced a continuous decline in sensitivity with time. To take this loss into account, the calibrations performed with the α Leo star (Lemaire 2002) were used and extrapolated to both 2008 and 2009 to determine the sensitivity loss factor f 3 (set1: 1.80; set2: 1.85; set3: 1.90); -the final correction applied was that for the door partial aperture ( f 1 ).…”

The SUMER spectral atlas of solar-disk features

“…Approximately 50 spectral pixels at the extreme ends of the detectors are covered by a mesh providing a 1:10 attentuation for H i Lyα observations. During the radiometric laboratory calibration (Hollandt et al 1996), the responsivity of SUMER was determined for both detectors with a transfer standard light source. In-flight calibration refinements indicate that this calibration was valid and stable until the SOHO accident in June 1998.…”

“…The calibration of the spectral response of the SUMER instrument is based on a comparison with a radiometric transfer standard source, which had been calibrated against the Berlin Electron-Storage ring for SYnchrotron radiation (BESSY I) as primary radiometric standard (Hollandt et al 1996). The transfer source provided 16 emission lines with known photon fluxes of rare gases in the SUMER wavelength range.…”

“…Given the radiometric calibration of SUMER before and during the mission (Hollandt et al 1996;Wilhelm et al 1997a;Schühle et al 1998Schühle et al , 2000, we are able to present the absolute spectral radiances of different solar features in the wavelength range from 660Å to 1485Å in the first order of diffraction using the detector "B". For the sake of completeness, we have added the spectral range from 1485Å to 1609Å taken from a "A"-detector spectrum of the quiet Sun.…”

The solar disk spectrum between 660 and 1175 Å (first order) obtained by SUMER on SOHO

“…During the laboratory calibration (Hollandt et al 1996) the SUMER instrument with both detectors was radiometrically calibrated against a secondary standard light source. In-flight calibration measurements with detector A indicate that this calibration is still valid within ±15% (1 σ) (Wilhelm et al 1997b).…”