This section describes the data acquisition and instrumental details which are relevant for the interpretation of the data. A more detailed instrument description is given by Wilhelm et al. (1995) and first inflight performance characteristics are given by Wilhelm et al. (1997a) and Lemaire et al. (1997). SUMER is a stigmatic normal-incidence spectrograph operating in the range from 400 to 1610 Å with mirrors made out of SiC and three normal-incidence reflexions. The steep fall-off of the reflectivity below 500 Å determines the lower wavelength limit. Only few lines short of 500 Å have been identified so far.
The off-axis parabola telescope mirror has a focal length of 1302.77 mm.
It can be moved in two dimensions around the focal point to allow
pointing of the instrument in the range of 
arcmin. This
allows us to take disk and off-limb spectra in the lower corona.
Four spectrometer slits with angular dimensions of 
, 
, 
, and 
are
available. The slit assembly allows us to move a selected slit into the
telescope focus and to refocus its position if required. With the slit the
photon input for the detector can be adjusted according to the selected
spectral band and to the solar feature observed. The measurements in this
communication were performed with the 
-arcsec slit, and the
widths of the spectral lines thus are not affected by the slit width.
Two diffraction orders can be observed by SUMER; first order lines and second order lines appear superimposed in the SUMER spectrum. The dispersion of the spectrometer varies slightly with wavelengths from 45 mÅ/pixel (first order) and 22.5 mÅ/pixel (second order) at 800 Å to 41.8 mÅ/pixel and 20.9 mÅ/pixel at 1600 Å.
The instrument is equipped with two photon-counting detectors (A and B)
operating in Cross Delay Line technique (XDL) - for details see
Siegmund et al. (1994). Only one
detector can be operated at a time. Each detector has 1024 spectral pixels and
360 spatial pixels. The pixel size of approximately 
is defined
by the analogue electronics. The central area of the detector is coated
with KBr while both sides are bare microchannel plate (cf., Fig. 1 (click here)).
Figure 1: Detector B
photocathode layout (view from grating to the focal plane).
The limb spectrum discussed in this
paper was obtained with a short slit covering only 120 pixels. Each individual
spectrum extends for approximately 45 Å (1st order) in the dispersion
direction. Some pixels on either side are dark
Figure 2: The limb spectrum around 703 Å (2nd order) on both sections
of the detector photocathode. The O III lines at 702.332, 702.899,
and 703.85 Å are blended with S IV (1404.77, 1406.06 Å),
and O IV (1407.386 Å) lines, if
recorded on the KBr coating (bottom). On the bare part (top),
the O III triplet is prominent
The KBr coating increases the detective quantum efficiency (DQE) up to a factor of > 10 in the range from 900 Å to 1500 Å. Observation of lines in both sections of the photocathode helps to detect second order lines, since they appear in similar intensity on both parts of the photocathode. This effect is demonstrated in Fig. 2 (click here).
Only a few second order lines have been identified in the spectral range below 590 Å where the instrument sensitivity is very low, whilst corresponding first order lines fall into the efficiency maximum. He I at 584 Å is the most prominent second order line in the spectrum presented here.
The spectra presented in this communication were taken with detector B. The spectral ranges of both detectors are different; the B detector range reaches from 330 to 750 Å in second order and 660 to 1500 Å in first order. The range from 660 to 750 Å is covered twice. From the line ratio of the O III line at 703.85 Å in both orders a grating efficiency ratio between first and second order of 1.0 was obtained.
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 
(
) (Wilhelm et
al. 1997b). The detector B efficiency curve has not yet been
validated under operational conditions. We have no indications for
differences in the performance of detector B and applied the laboratory
efficiency curve for this analysis. Below 769 Å this curve had to be
extrapolated, and progressively increasing uncertainties have to be assumed.
Both detectors show non-uniformity effects typical for MCP intensifiers. These effects stem from the MCP structure, the inhomogeneity of the HV electric field, and individual pixel errors. These effects, which are very worrisome for imaging purposes can be compensated to a large extent by a flat field correction; SUMER regularly generates a flat-field matrix which can be applied either on board, or on the ground. The dark signal of the detectors is extremely low and for disk observations scattered light is irrelevant. Thus, it is inferred that continuum contributions of the spectra have a solar origin.
Bright spectral lines, if placed onto the KBr part of the photocathode
lead to a local gain depression even if the
narrow slit is used. As a result the DQE of the detector can decrease locally
and peak intensities might be attenuated in a non-linear fashion (Wilhelm et
al. 1997a). This effect starts at 10 cts/pixel/s and
deteriorates progressively. No corrections have been performed for local gain
depression effects, although the intense lines of C III (977 Å),
H I 
(1025 Å), O VI (1032 Å), O VI (1036 Å) are
affected to some extent.
For the spectrum, taken on January 25, 1996, the instrument was pointing
towards the solar limb at the North pole of the Sun
(cf., Fig. 3 (click here)). With the given
slit of arcsec only a window of 1024 by 120 pixels is read out
by the detector. At that time, the slit image was
not yet adjusted with respect to the read-out window, leaving 
pixels unexposed. The remaining 90 pixels cover approximately 60 arcsec of the
northern coronal hole portion of the solar disk and about 30 arcsec beyond the
photospheric limb. A set of 41 spectral
sections, each offset by 17.4 Å and with a 40 Å gap around H I
Ly 
(introduced for safety reasons in this first spectrum) was
exposed during this observation with increasing wavelength. The exposure time
for individual spectra was 100 s. For this communication 29 exposures have been
analysed. In total, 51 minutes elapsed between the first spectrum, taken around
680 Å, and the last spectrum considered here.
The individual spectra have been corrected for minor telemetry errors.
Figure 3: Detector image centered around 1035 Å. This individual spectrum
is dominated by the O VI resonance lines and the H I 
line
which is broad and shows self-absorption. The position of the photospheric
limb is marked (North is down) and the selected range along the slit is
indicated by a bar. Note, that the image of the slit was not yet well adjusted at that
time