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2 Observations and data reduction

The thirteen films of the South-East area of the Virgo cluster of galaxies were taken with the UKST between 1991 and 1994 using the Kodak TechPan emulsion with the OG590 filter. The filter has a lower wavelength cut-off around 590 nm (10% transmittance at 580 nm and 90% at 600 nm), while the emulsion has an upper wavelength cut-off around 690 nm. This particular combination of filter and emulsion has a very small colour term compared with standard R band (Phillipps & Parker 1993). The field of the plates covers an area of $6.2\hbox{$^\circ$}
\times 6.2\hbox{$^\circ$}$, affected by vignetting. The films used, dates and exposure times, are given in Table 1. The first eight films were from the UKSTU archive, while the remaining five were taken specifically for this project. A sixth film (OR16137) was also taken for this project, but has double images and so was not used.

Table 1: Details of the films

{\vert\vert c\vert c\vert c\vert}
Name of Film &
 ...94 \\ OR16073 & 60 & 11/5/1994 \\ OR16167 & 60 & 1/7/1994 \\ \hline\end{tabular}

The $\approx\!
 6.2\hbox{$^\circ$}\times 6.2\hbox{$^\circ$}$ area of the films were scanned using the APM machine at the Royal Greenwich Observatory, Cambridge, at a pixel size of 32 $\mu$m ($\approx$2$\hbox{$^{\prime\prime}$}$). The size of the digitized files was approximately 10500 $\times$ 10500 pixels, which occupied about 222 MBytes.

The thirteen raw frames were combined into a single co-added image using applications from the Starlink KAPPA and CCDPACK packages (see Starlink User Notes 95 and 139, available on the World-Wide-Web through the Starlink home page at Some of these applications were modified to handle the very large image size by breaking the input image up into "chunks'' of $512\times512$pixels for processing. The facilities of the NDF subroutine library were used to do this (see Starlink User Note 33). The following steps were involved in creating the final co-added image:

Multiple R-band CCD exposures of a $\approx 6^{\prime} \times
6^{\prime}$ field containing the IC 3481 and IC 3481A galaxies were taken in photometric conditions on 1995 Dec. 22-23, using the CCD camera with Tektronix CCD on the 1.0 m Jacobus Kapteyn Telescope (JKT), La Palma. The total exposure time was 70 min and the resultant co-added exposure is about as deep as the co-added array in terms of surface brightness. Standard applications from the Starlink KAPPA and CCDPACK packages were used for the reduction of this data and the resulting image (along with others) will be used for calibrating the co-added UKST data. Preliminary results from this calibration are presented below.

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