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Data reduction was carried out using the
MIDAS
software package (Grosbøl [1989]).
Procedures included bias subtraction,
cosmic-ray removal and flat-field correction.
The flat-field correction was produced with the normalization algorithm
described by Shergin et al. ([1996]).
After the wavelength mapping and night sky subtraction,
each 2D frame was corrected for atmospheric extinction and was flux
calibrated.
To derive the sensitivity curves, we used the
spectral energy distributions of the standard stars from
Bohlin ([1996]). Average sensitivity curves for grisms were produced
for each observing night.
R.m.s. deviations between the average and individual sensitivity
curves are
1.5%, with the maximum deviations of
4%
in the spectral region 3800
4000 Å.
The 2D flux-calibrated spectra were then corrected for atmospheric
dispersion (see Kniazev et al. [2000]) and averaged.
Finally, the 1D averaged spectrum was extracted from a
08 region along the slit, where
> 2
(
is the dispersion of
a noise statistics around this line) (see Fig. 2).
Redshift and line fluxes
were measured applying Gaussian fitting.
For H,
[N II]
6548,6583 Å and
[S II]
6716,6731 Å a deblending procedure
was used, assuming gaussian profiles with the same FWHM as for single
lines.
The errors of the line intensities in Table 2
take into account
the Poisson noise statistics and the noise statistics in the continuum
near each line, and include uncertainties of data reduction.
These errors have been propagated to calculate element abundances.
The observed emission line intensities
,
and those corrected
for interstellar extinction and underlying stellar absorption
are presented in Table 2. All lines have been normalized
to the H
intensity. The H
equivalent width EW(H
),
the absorption equivalent widths EW(abs) of the Balmer lines,
the H
flux, and the
extinction coefficient C(H
)
(this is a sum of internal
extinction in HS 0822+3542 and foreground one in the Milky Way)
are also shown there.
For the simultaneous derivation of C(H)
and EW(abs), and
to correct for extinction, we used a procedure described in detail
in Izotov, Thuan & Lipovetsky ([1994]).
The abundances of the ionized species and the total abundances
of O, Ne, N, S, and He have been obtained following
Izotov, Thuan & Lipovetsky ([1994], [1997b])
and Izotov & Thuan ([1999]).