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Astron. Astrophys. 362, 737-745 (2000)

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2. Observational data

SUMER (Wilhelm et al. 1995) observes in the wavelength range from 465 to 1610 Å, depending on the spectral order and the choice of detector. Its spectral resolution is 44 mÅ/pixel in first order and 22 mÅ/pixel in second order and its spatial resolution is 1". The CDS (Harrison et al. 1995) normal incidence spectrometer (NIS) observes in the two bands from 310 to 380 Å and from 517 to 633 Å. The spectral pixel size of the CDS NIS ranges from 0.070 Å at 310 Å to 0.118 Å at 630 Å. The effective pixel size of CDS is 4" in horizontal (cross slit) direction and 1.68" vertical (along slit), although the actual spatial resolution is lower (Thompson 1998; Haugan 1999). Since the beginning of the SOHO mission an intercalibration programme has been carried out which includes simultaneous observations of CDS and SUMER of common targets near solar disk centre devoid of any notable activity (Pauluhn et al. 1999). The spectral lines recorded simultaneously during these measurements were HeI at 584 Å and the two MgX lines at 609 Å and 624 Å. SUMER additionally observed in NeVIII 770 Å and since September 1996 also in NV 1238 Å. Our sample of lines with good statistics thus covers formation temperatures ranging from [FORMULA] K to [FORMULA] K, i.e. the chromosphere, transition region and corona.

Each measurement consisted of a raster scan over an extended area with a spectrum recorded at every spatial position. The instruments scanned an area of 60"[FORMULA]240" (CDS), and 60"[FORMULA]300" (SUMER, prior to November 1996). After SUMER's scanning mechanism was disabled, the SUMER spectrometer slit was kept fixed, and the scanning was produced by solar rotation during the exposure time, limiting the raster size to about 4" in the cross-slit direction.

The SUMER data were corrected for the flatfield, for the geometric image distortion, and the electronic dead-time of the detector. The CDS data were corrected for burn-in and the flatfield. After the instrumental corrections and the radiometric calibration, the solar radiances were determined by integration over the line profiles, which were derived by least-squares fits of single or multi-Gaussian functions and a linear background. For more information on the data and the reduction we refer to Pauluhn et al. (1999).

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© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000