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Gas Disks and Supermassive Black Holes in Nearby Radio Galaxies

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3.2.1 Data Reduction<br />

We used the st<strong>and</strong>ard STIS calibration pipel<strong>in</strong>e (calstis, see Brown et al., 2002) to<br />

perform bias, dark <strong>and</strong> flat-field corrections us<strong>in</strong>g the best available reference files.<br />

We used calstis version 2.13 (26-April-2002) throughout the data reduction 1 .<br />

We shifted the rows of alternat<strong>in</strong>g observations by 4 pixels, so that they were<br />

properly aligned with their counterparts <strong>and</strong> comb<strong>in</strong>ed them us<strong>in</strong>g the STSDAS rou-<br />

t<strong>in</strong>e ocrreject. We cleaned co-<strong>in</strong>cident cosmic rays <strong>and</strong> negative bad pixels, which<br />

would not be caught by ocrreject, us<strong>in</strong>g the NOAO/IRAF task cosmicrays. At<br />

each step we carefully <strong>in</strong>vestigated the effects of vary<strong>in</strong>g task parameters to <strong>in</strong>sure we<br />

were not damag<strong>in</strong>g valid data while remov<strong>in</strong>g most cosmic rays.<br />

We made use of the STIS calibration pipel<strong>in</strong>e tasks wavecal <strong>and</strong> x2d to perform<br />

wavelength calibration <strong>and</strong> image rectification respectively. The error <strong>in</strong>troduced by<br />

rectify<strong>in</strong>g after shift<strong>in</strong>g one of the images is ∼ < 0.05 pixels (which is ∼ < 1.3 km s −1 at<br />

6750˚A).<br />

Panel (b) of Figures 3.2 to 3.22 (see key <strong>in</strong> Figure 3.1) shows (i) the central strip<br />

1 We found that significant variations <strong>in</strong> measured parameters could be <strong>in</strong>troduced by us<strong>in</strong>g different<br />

calstis versions. Versions 2.4 <strong>and</strong> 2.13 produce consistent results, while <strong>in</strong>termediate versions<br />

do not.<br />

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