2. Observations and data reduction
All images were taken in a single night (November 11, 1998) using the Wide Field Camera (WFC) at the prime focus of the 2.54 m Isaac Newton Telescope (INT) of the Observatorio del Roque de los Muchachos in the island of La Palma (Spain). The seeing was 1".5 in the first half of the night and 2" in the second one. The WFC has a panoramic detector consisting of four EEV CCD pixels each, i.e. a total of pixels. The pixel size projects to 0".33 in the sky. The whole CCD array covers a field of view of about , while an additional CCD is used for guiding. The arrangement of the four rectangular CCDs makes the geometry of the field of view as shown in Fig. 1. The emission-line filters used have the following central wavelengths and full widths at half maximum (FWHM): 5008/100 Å ([O III]) and 6568/95 Å (H+[N II]). We also used a Strömgren Y filter as a continuum filter, centred at 5550 Å and covering a spectral region where there are no strong nebular emission lines. Almost the whole galaxy was observed by centering the CCD array in two different positions (designated with A and B) in such a way as to have at each position also a galaxy-free region to be used for sky subtraction. At each telescope position and for each filter, we obtained three images with successive offsets of 25" in order to fill the gaps between the CCDs and remove CCD defects. The approximate centre of each telescope positioning and the exposure times are given in Table 1. The night was photometric and we obtained several exposures of the emission-line standard PN PNG 205.8-26.7 (Dopita & Hua 1997).
Table 1. Log of the observations.
The data were bias subtracted and flat-fielded in a standard way using IRAF. All images were aligned to a reference image, correcting for geometrical distortion. Images in the same filter were then averaged so that bad pixels and cosmic rays were removed. Sky background, parameterized by a constant, was subtracted. We present in Fig. 2 our [O III] image of M33.
© European Southern Observatory (ESO) 2000
Online publication: March 9, 2000