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Astron. Astrophys. 344, 36-42 (1999)

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2. Observations and data reduction

2.1. Observations

Mkn 620 was observed with the 2m Ritchey-Chretien-Coud[FORMULA] (2-m RCC) reflector of the Bulgarian National Astronomical Observatory (BNAO) at Mount St. Spirit near Rozhen, Rodopa mountains. The observations were carried out on December 12, 1992.

The narrow-band images were taken with the Focal Reducer of the Max-Planck-Institute for Aeronomy (MPAe). The technical data and the capabilities of the MPAe Focal Reducer are described by Jockers (1992).

The telescope / reducer configuration and the CCD's square 22 [FORMULA]m pixels provide an image scale of 0[FORMULA] 8 px-1 and an unvignetted field of view (FOV) [FORMULA]5 arcmin at an effective focal ratio f/2.86. This scale is quite well adapted to the typical seeing conditions at BNAO (2"-2[FORMULA] 5 FWHM). One arcsec corresponds to a distance of 125 pc at Mkn 620 assuming [FORMULA] km sec-1 Mpc-1.

Images through interference filters centered near the wavelengths of [N II ] [FORMULA] Å,[O III ] [FORMULA] Å and [O I ] [FORMULA] + [Fe X ] [FORMULA] ÅÅ ("on-line") and on the emission free continuum windows at [FORMULA] Å and [FORMULA] Å ("off-line") were obtained. The "off-line" images were used to subtruct the continuum contribution contained in the "on-line" images. Moreover, we utilized the "off-line" images to form the color map.

A broad band image was also obtained with a Gunn r interference filter.

The observing log is presented in Table 1 where the central wavelengths [FORMULA] and the effective width [FORMULA] of the interference filters, and the spatial resolution of the images in terms of the point-spread function (PSF) are listed.


Table 1. M 620-observing log and related data.

Two exposures of Mkn 620 were obtained through each filter to eliminate cosmic ray events and to increase the reliability of the measurements. Between exposures the telescope was slightly offset to avoid permanent defects of the CCD. Flat-field exposures were obtained using dusk and dawn twilight for uniform illumination of the detector. No dark correction was required.

2.2. Data reduction

The images were reduced following the usual reduction steps for narrow-band imaging (Haniff et al. 1988; Perez-Fournon & Wilson 1990; Tsvetanov & Walsh 1992).

The raw images contained a small amplitude pattern, periodic along the rows. Analysis of the overscanned area showed that a pattern with the same amplitude and phase existed therein. Therefore it was possible to use the overscan for creating bias frames with the systematic additive noise, characteristic for each particular image. Afterwards the bias and periodic pattern have been removed in one step.

After flat-fielding the frames were aligned by rebinning to a common origin. The final alignment of all the images was estimated to be better than 0[FORMULA] 1. As an unwanted by-product of the rebinning procedure a small decrease of the resolution ([FORMULA][FORMULA] 15) was noticeable. The two images taken through the same filter were averaged and the cosmic-ray signatures were removed.

Unfortunately, small tracking errors of the 2-m RCC telescope caused residual ellipticities in the PSFs of our frames. Therefore, a convolution procedure was performed in order to match the PSFs of each line-continuum pair. The same procedure was applied to those images which later were used in the mapping of continuum emission ratios. This degraded the final resolution to a mean value of [FORMULA]3[FORMULA] 3.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999