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Astron. Astrophys. 337, 287-293 (1998)

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2. Observations

2.1. The data

SUMER is a normal incidence spectrograph operating over the wavelength range 400 Å to 1610 Å. The off-axis parabola mirror is movable in two dimensions around the focal point allowing pointing of the instrument independent of the spacecraft pointing. Four slits are available: [FORMULA], [FORMULA], [FORMULA], and [FORMULA] arc sec2. For the data obtained here we used the [FORMULA] arc sec2 slit. Both [FORMULA] and [FORMULA] orders are superimposed on the detector, with the dispersion in wavelength varying from 45 mÅ/pixel ([FORMULA] order) to 22.5 mÅ/pixel ([FORMULA] order) at 800 Å to 41.8 mÅ/pixel and 20.9 mÅ/pixel at 1600 Å. The detector (see Siegmund et al. 1994) has 1024 spectral pixels and 360 spatial pixels. The central area is coated with KBr which increases the quantum efficiency by an order of magnitude in the range 900 Å to 1500 Å.

The observations reported here were obtained on [FORMULA] and [FORMULA] Aug 1996 using exposure times of 300 s with detector A. After each exposure, the slit was moved by 1.1 arc sec, accumulating an image covering an area of [FORMULA] arc sec2. At each of the three solar locations (disk center, 800 arc sec from the center and the limb), data was obtained for three spectral settings, 1041 Å, 1245 Å, and 1352 Å each covering 40 Å. The objective of the observing programme was to obtain a good mean quiet Sun spectrum. In deriving the mean spectrum, the maximum number of individual scans was thirty, however some of these were unusable. Below we give more details:

  • DISK CENTER For the files with reference wavelength = 1041 Å we took the mean of 27 spectral scans with XCEN/YCEN between -15.75/0.0 and 14.62/0.0 (i.e., in arc sec from disk center where positive X is West and positive Y in North). For the files with reference wavelength = 1245 Å we took the mean of 19 spectral scans with XCEN/YCEN between 13.87/0.0 and 37.62/0.0, and finally, for the files with reference wavelength = 1352Å we took the mean of 17 spectral scans with XCEN/YCEN between 47.75/0.0 and 65.25/0.0.

  • 800 arc sec from disk center For the files with reference wavelength = 1041 Å we took the mean of 30 spectral scans with XCEN/YCEN between 785.37/0.0 and 815.81/0.0.

  • LIMB-A For the files with reference wavelength = 1041 Å we took the mean of 15 spectral scans with XCEN/YCEN between 936.25/0.0 and 956.56/0.0. For the files with reference wavelength = 1245 Å we took the mean of 12 spectral scans with XCEN/YCEN between 946.44/0.0 and 956.56/0.0, and for the files with reference wavelength = 1352 Å the mean of 12 spectral scans with XCEN/YCEN between 945.69/0.0 and 955.81/0.0.

  • LIMB-B For files with reference wavelength = 1041 Å we took the mean of 6 spectral scans with XCEN/YCEN between 963.37/0.0 and 966.75/0.0. For files with reference wavelength = 1245 Å we took the mean of 8 spectral scans with XCEN/YCEN between 963.37/0.0 and 970.12/0.0, and for the files with reference wavelength = 1352 Å the mean of 18 spectral scans with XCEN/YCEN between 959.19/0.0 and 976.12/0.0.

Thus the LIMB-A data refers to observations taken up to and including the limb, whereas the LIMB-B data refers to data taken several arc sec off-limb. In Fig. 1 we show the N V 1242 Å line flux as a function of position in arc sec from disk center for the limb observations derived by integrating over the entire slit for each scan position.

[FIGURE] Fig. 1. The N V 1424 Å line flux as a function of position in arc sec from disk center for the limb observations

Each spectral scan was checked for the presence of explosive events as these can cause localised line broadening. Considering that the integration time was 300 s per spectrum, detection of such events is difficult. Although some spatially small brightenings ([FORMULA] arc sec) are present in one or two spectra, these do not affect the average, as for example in the disk center spectrum we average over an area of [FORMULA] arc sec2 for data involving O VI and [FORMULA] arc sec2 for data involving N V .

2.2. Data reduction

For the SUMER instrument, the process of data reduction involves three main steps; destretching, radiometric calibration and slit effects correction (these datasets were automatically flat-field corrected onboard the satellite). Destretching of the SUMER datasets is necessary, particularly for the data towards the end of the slit due to various wavelength and spatial distortions in the detector. After ruling out unusable files, due to errors in the headers, bad data, etc., all the images obtained at different scan positions (for a determined wavelength range and solar position) were averaged in order to improve the signal to noise. Due to problems towards the end of the slit, only the spatial pixels [FORMULA] were considered in deriving the averaged spectrum. Conversion of the SUMER count rates into physical units was via the radiometric calibration of Wilhelm et al. (1997). The final step in the process was the correction to the measured line widths for the effects of the slit used. The magnitude of this correction is small, although it can result in a correction of [FORMULA] km s-1 in the derived non-thermal velocities (see later). The above corrections were done using various IDL routines from within the SUMER software tree (e.g., [FORMULA], [FORMULA], [FORMULA], and finally, [FORMULA]). Spectral line fitting was via [FORMULA] from the CDS software tree.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998
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