A spatially resolved study of ionized regions in galaxies at different ...

3.3. Data Reduction 47
FlatFielding correction done by the application of the master CCD flat (usually provided by
the Calar Alto Observatory) to all the raw images. Next step consists on the combination
of different exposures on the same target and same pointing. This also performs the cosmic
ray rejection. In the case of the standard stars, we had only one exposure per pointing, so
it was necessary to clean for cosmic rays using other method. This is briefly explained when
describing the flux calibration procedure.
3.3.2 Identification of the position of the spectra
The raw data from a fiber-feed
spectrographs consist in a collection
of spectra, stored as a 2D frame,
aligned along the dispersion axis. For
each wavelength, each spectrum is also
spread along the perpendicular (“crossdispersion”
or “spatial”) axis. As seen
in Figure 3.4, spectra are separated by
a certain width, following a characteristic
profile which may be considered
Gaussian. When the spectra are tightly
packed, as in PPak mode, contamination
occurs among neighbors. This is indicates the dispersion axis. Spectra are separated 5
Figure 3.4: Section of a PPak raw data. The arrow
the so-called cross-talk. It is important ∼ pixels across the cross-dispersion axis, following a
to take into account, not only in raw pseudo-Gaussian of FWHM ∼ 3 pixels, contaminating
data but also in final processed formats, the adjacent spectra.
that adjacent spectra at the CCD may
originate from distant locations in the sky plane. Special care has to be taken to compensate
effects of instrument flexure, optical distortions, etc, which cause that the spectra are not
perfectly aligned along the dispersion axis. This effect is corrected by calculating the shifts
between the calibration frame and the corresponding object frames. Therefore, it is necessary
to find the location of the projection of each spectrum at each wavelength along the CCD in
order to extract its corresponding flux.
The very beginning IFS data reduction step is to identify where the spectra lie on the
CCD. This is done by using continuum illuminated exposures at every location where the
telescope is pointing. An ARC exposure (obtained at the same location) is also needed, since
flexures may also affect the wavelength calibration. The location of the spectra (apertures)
are identified by extracting a slice perpendicular to the dispersion direction, finding where
the peaks are. Each aperture is traced along the dispersion direction usually by fitting a
Gaussian function. It is important that the trace is continuous. The aperture mapping is