3. Observations and data calibration
For the tracking of the calibration of the SUMER instrument during the SOHO mission, a set of five emission lines had been selected that have repeatedly been observed in quiet-Sun areas near the disk centre. Since March 1996, these observations have been performed roughly once every month and are continued. The lines measured during the observing sequence are He I 584 Å, Mg X 609 Å and 624 Å, Ne VIII 770 Å, N V 1238 Å. The He I and the Mg X lines are in second order, while the N V line is in first order. The Ne VIII line is observed in second order with the A detector whereas it is seen in first order with the B detector.
To exclude any contributions from active or bright regions, an area close to disk centre was always selected that was devoid any solar activity by referring to the He II 304 Å images of the Extreme-Ultraviolet Imaging Telescope (EIT) of SOHO. An area of was registered by a raster scan of the telescope with a step size of 0.76" in east-west direction. After September 1996 the raster scan mode was given up, and instead the slit was kept at a fixed position, letting only the solar image drift across the slit by the solar rotation. Each line was registered for 21.3 minutes, taking 80 exposures of 16 sec. Each raster was registered once during one run, while the Ne VIII line was registered four times. The area sampled by solar rotation was . So the area sampled was much smaller, and thus the sample of the chromospheric network structure was only marginally representative, when the raster scan was not invoked. But nevertheless, the size of this area warranted that always a part of network and cells was sampled. Recently, in August 1999, the raster scan mode was resumed.
An additional set of measurements was obtained by an observing sequence of the hydrogen Lyman continuum, performed independently but with similar regularity. In these observations a spectral window of 40 Å of the H I Lyman continuum at 880 Å was sampled for three hours at a quiet-Sun area, corresponding to a raster area of about .
© European Southern Observatory (ESO) 2000
Online publication: February 9, 2000