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Astron. Astrophys. 342, 687-703 (1999)

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2. Observations

2.1. The sample

The observed galaxies are listed in Table 1, along with their 1950 coordinates, redshift z, apparent V magnitude, date and place of observation. These objects have been selected from Véron-Cetty & Véron (1989) to fulfil the following criteria: a) [FORMULA], b) [FORMULA] to reach photometric signal to noise of about few thousands in a few minutes of integration, c) size [FORMULA] arcmin to be limited by photon noise and not by readout noise, d) there must be 3 or more possible comparison sources in the arcmins CCD field, thus allowing us to identify and discount any of the comparison stars that are themselves variable on short time-scales. Some of these objects have at least 2 stars closer than 1 arcmin and were thus suitable for the infrared camera CAMILA of San Pedro; they were observed simultaneously in the visible and the IR. Some objects, like NGC 4051, NGC 4151 and MCG+08-11-11, did not fulfill all these criteria particularly the c) one since the size of these galaxies was about the third of the field of view. But, firstly they are well known objects, already observed for search of microvariability for two of them (Lyutyi et al. 1989, for NGC 4151; Done et al. 1990, for NGC 4051) and so interesting to study. Secondly, due to their proximity, their flux were high enough to be rapidly limited by photon noise and not by readout noise in a few seconds integration time. We could not satisfy criterion d) for NGC 4151 as well, and only one comparison star was in the CCD field. Consequently, we have treated this galaxy differently (see Sect. 3.2.3). One or two photometric standard stars, selected from Landolt (1992) were also observed before and after each galaxy run to estimate the mean brightness of the object, needed to deduce physical constraints (see Sect. 5).


[TABLE]

Table 1. List of observed galaxies. Coordinates and magnitudes are taking from Véron-Cetty & Véron (1989)


2.2. The observational campaigns

We have carried out two campaigns of observations in Mexico. The first one was done in the I band during 7 nights (7-13 May 1996) at the 1.5 m telescope of the Observatorio Astronómico Nacional at San Pedro Mártir (Baja California). We used a [FORMULA] Tektronix CCD with 6 electrons readout noise and [FORMULA] field of view. Simultaneous observations in the J band were performed at the 2.1m telescope during the 3 last nights. We used the CAMILA [FORMULA] infrared camera with 40 electrons readout noise and [FORMULA] field of view (Cruz-Gonzàlez et al. 1993). The second campaign was done at the 2.1m telescope of the Guillermo Haro Observatory in Cananea (Sonora) during 8 nights (1-9 December 1996) in the I band. Only a useful [FORMULA] pixels part of a [FORMULA] CCD Tektronix, with 8 electrons readout noise and [FORMULA] equivalent field of view, was read. Galaxies with several comparison stars with comparable brightness in the field of view are observed as a priority. The exposure time was chosen to use the CCD at about half of its dynamic in order to prevent saturation due to rapid changes of seeing. The acquisition program was automated to take an image with a period equal to the exposure time plus the backup time.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999
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