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Astron. Astrophys. 354, L71-L74 (2000)

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2. Instrument and calibration

SUMER is a stigmatic normal-incidence telescope-spectrograph operating in the wavelength range from 465 Å to 1610 Å using two orders of diffraction in superposition. In their common wavelength range between 465 Å and 1480 Å, one of two identical detectors (detector A and B) can be used for observations. Each photocathode of the detectors has two areas (bare microchannel plate and potassium bromide coating) with different spectral responses which can be used to separate the first and second order contributions. The telescope mirror can be stepped in multiples of single steps of 0.38" to move the solar image across the spectrometer entrance slit (1" corresponds to [FORMULA]715 km on the Sun). The slit with angular dimensions of [FORMULA] is imaged by the spectrograph on the detectors with a resolution element of about 1" per pixel in spatial direction and 44 mÅ per pixel in the spectral regime.

The instrument has been calibrated with both detectors before flight using a transfer standard source which itself had been calibrated against the primary radiometric source standard BESSY I (Hollandt et al. 1996). This laboratory calibration established the responsivity of the instrument at a set of wavelengths provided by the transfer source. By the fact that none of the observed lines presented in this communication showed any decline of intensity, it could be verified that the instrument responsivity was stable during the mission time (Schühle et al. 1998) until the SOHO accident when the nominal spacecraft attitude was lost for three months. Particularly, the use and comparison of the two detectors, when only one detector had been used during half a year, indicated that there was no systematic effect of any one detector. After the recovery of SOHO, a change of responsivity of SUMER was found. From continued measurements, the trends before and after the accident have been compared, and a loss factor could be derived for those wavelengths measured.

The relative uncertainty levels of the radiometric calibration were 15% (1[FORMULA]) for the A detector (Wilhelm et al. 1997) and 20% (1[FORMULA]) for the B detector (Schühle et al. 1999) in the range from 540 Å to 1250 Å and 30% for longer wavelengths.

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000
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