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Astron. Astrophys. 325, 135-143 (1997)

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3. Reduction

3.1. General image reduction

Standard methods were used to analyse the data. First, the bias subtraction was done using a 40 000 pixel overscan region in each image. Both overscan regions and bias images showed that the bias level was very stable during the observations.

In order to create a flatfield frame in each filter, several (three to five) twilight sky images were taken, all shifted with respect to each other in order to avoid bright stars falling on the same position on the chip. Flatfield frames of the same filter were then combined using the IRAF task FLATCOMBINE. The pixel to pixel variation within the flatfield frame was then found to be less than [FORMULA] in all filters.

3.2. Calibration

After bias subtraction and flatfield correction was applied, photometry was done to standard stars and instrumental magnitudes were obtained. For photometry we used the DAOGROW algorithm (Stetson 1990) embedded in IRAF. Photometric magnitudes were taken from Landolt (1992). A three term calibration equation was then fitted to the data. The transformation equations are:

[FORMULA]
[FORMULA]
[FORMULA]

where b, v, i are the instrumental magnitudes of the stars, [FORMULA] the catalogue magnitudes, [FORMULA] the airmasses, [FORMULA] the zero points in each filter, [FORMULA] the atmospheric extinction coefficients and [FORMULA] the colour correction terms.

After the fit was done, residuals between catalogue magnitudes and computed magnitudes were found to be less than 0.04 mags in B, 0.03 mags in V and 0.03 mags in I. All coefficients have insignificant variations from one night to the next. Atmospheric extinction coefficients were 0.23 mags/airmass in B, 0.17 mags/airmass in V and 0.04 mags/airmass in I. Having derived the coefficients, the transformation equations were inverted so that the galaxy images could be flux calibrated.

3.3. Galaxy image reduction

In order to make the galaxy data ready for the model, several procedures have been done, using the ESO-MIDAS data reduction package.

First, the cosmic rays were removed and the sky level was calculated and subtracted. A difficulty in the sky calculation was a bright star in the SW edge of the field, that produced a gradient in the background level. To solve this problem, a background fitting routine using a two-dimensional polynomial (linear in both directions) was applied. An artificial image of the sky was then produced and subtracted from the original image.

As a next step we had to consider the calculation of the data noise. In order to do this, we have followed the procedure described by Newberry (1991). Thus, sky noise, photon noise as well as readout noise were calculated in each individual galaxy frame. Then, after calibrating the galaxy images via the standard stars, a final image was produced in each filter by adding the individual images. The noise was then calculated for the summed image by taking into account the propagation of error. Finally a noise and a signal-to-noise mask of the galaxy image in each filter were produced. The set of pixels whose values lie in the trust region of three sigma were then selected. In V and I, most of the galaxy, even the faintest part within the dust lane, was above the three sigma level. In the B band, we had to lower our threshold to two sigma in order to include a large fraction of the central region of the galaxy where the dust is dominant.

In order to compare the observed galaxy with the model, we had to rotate the galaxy image in such a way, so that the major axis of the disk is horizontal. To do this, we fitted ellipses to the isophotes of the galaxy and characteristics such as the center and the position angle of the galaxy were determined. After the orientation of one image, all other images were aligned to that reference image.

Another step which proved to be very useful was to fold the galaxy. The model which we are going to use (and will be discussed in the next section) is axisymmetric. A visual inspection of the galaxy itself shows a symmetry around the vertical axis through the center. This led us to "cut" the galaxy into two parts along the minor axis through the center and add them together. This was found to be time saving because the model fit was done using only half of the pixels that cover the galaxy image, without any loss of accuracy. Furthermore, the existing fluctuations in the galaxy image are smoothed out to a large degree.

The final step was to remove all foreground stars. This was done using the IRAF task IMEDIT. In the defined aperture, the pixel values were replaced by zero so that the fitting program could ignore this part of the galaxy.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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