Astron. Astrophys. 349, 55-69 (1999)

2. Observations

The observations were performed with the direct CCD camera attached to the Dutch 0.91m telescope of the La Silla Astronomical Observatory (ESO) during six runs from October 1995 to October 1998. The available CCD detector was the ESO chip No. 33, which is a TEK CCD with 512x512 pixels, pixel size of and spatial resolution of 0:0044, providing a field of view of 3:077x3:077. Given the limited size of the field of view, the need to observe not too far from the meridian and at the same time to be able to get two images of the same field in the same night, we were forced to limit our programme to 4 selected fields of IC1613. The fields are displayed in Fig. 1. Most of the observations were performed without filter (white light, hereinafter Wh) in order to get the best photon statistics for the study of faint Cepheid light curves; few images were taken in Johnson V and R filter for comparison purposes with other works and to get an indication on the colors of the relatively bright stars. Bias and twilight flat field frames were gathered in each useful night.

Fig. 1. The four surveyed field of IC 1613. The background image comes from the STScI Digitized Sky Survey (north is up and east to the left), and measures approximately 13:05 on each side

In this paper we present the results regarding field A, which is an interesting field because it contains the largest number of previously known variable stars. During the allocated telescope time the field has been observed for 46 nights and a total of 67 images were collected. The complete log of the observations is reported in Table 1. The table contains, for each image, the date of the beginning of the night, the Heliocentric Julian date of midexposure, the mean FWHM PSF value (which takes into account both seeing and possible non-perfect focussing of the telescope), the airmass, and the number of stars detected in the image, which is an indication of its overall goodness because it depends both on the PSF (point-spread-function) and sky background level. Each Wh, V and R image is the sum of two or three successive exposures for a total of 1800 sec; only for the image No. 61 the total exposure was shorter (600 sec).

Table 1. Log of observations of field A of IC 1613

The observations without filter imply the dependence of the resulting photometry on: a) the adopted instrumentation; b) a color term which is related to the different airmasses. The effective wavelengths of V and R bands are compared in Table 2 with that of Wh band, for stars with B, A, F and G spectral type observed with the adopted instrumentation, that is optical telescope and back-illuminated CCD-TEK detector. The Wh-band effective wavelength for late-type stars is intermediate between that of V and R bands. The color-effect due to different airmasses in the present case is negligible (in comparison with the expected photometric precision) since the observations were performed not far from meridian.

Table 2. Comparison of effective wavelengths (nm)

© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999
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