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Astron. Astrophys. 331, 894-900 (1998)

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2. Observations and data reduction

Observations of the optical images were made on October 16, 1996 with the 1.3 m telescope at Skinakas Observatory in Crete. The telescope, an [FORMULA] Ritchey-Cretien, was equiped with a focal reducer, producing an enlargement of the field of view by a factor of 1.9, and with a Thomson [FORMULA] CCD camera with [FORMULA] pixels. This arrangement produces a scale of 0.75 arcsec/pixel and a field of view of approximately [FORMULA]. Assuming a distance of 9.5 Mpc for NGC 891 (van der Kruit & Searle 1981), the pixel size corresponds to 34 pc. The B and V passbands that we use are comparable to those of Johnson's photometric system with effective wavelengths 0.443 µm and 0.564 µm respectively, while the I passband is comparable to that of Cousin's photometric system, with effective wavelength 0.809 µm. In order to calibrate the images, we used standard stars and a photometric procedure described in detail in Paper I. The rms deviations from the least squares fit between catalogue magnitudes and calibrated magnitudes for the standard stars were 0.03 mags in B, 0.02 mags in V and 0.03 mags in I. The total integration time was 80 min in B, 20 min in V and 20 min in I. The seeing was measured to be [FORMULA] for that night.

Observations of the NIR images were made on November 1995 using the 2.5 m telescope at the Wyoming Infrared Observatory (WIRO). The detector that was used was a Michigan Infrared Camera (MIRC) with a NICMOS II [FORMULA] chip. The pixel size at the prime focus was [FORMULA] which at the distance of 9.5 Mpc corresponds to 0.102 kpc. A mosaic of three frames was made for each band (J and K) and the total integration time was 18 mins for each filter. During the observations, "blank sky" frames were taken at frequent intervals and then subtracted after bias and flatfield correction. Calibration of the NIR images was done using the aperture photometry of Aaronson (1977).

Standard photometric procedures were used (described in more detail in Paper I) in order to reduce the images in a way suitable for the model.

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© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998