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Astron. Astrophys. 350, 476-484 (1999)

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2. Observations and data reduction

The observations have been performed in the nights 21.-24. April 1995 with the Danish 1.54m telescope at ESO/La Silla. The CCD in use was a Tektronix chip with 1024[FORMULA]1024 pixels. The f/8.5 beam of the telescope provides a scale of [FORMULA]/mm, and with a pixel size of 24 µm the total field is [FORMULA]. A total number of 66 images (22 in each colour) has been taken during the 4 nights through Strömgren vby filters. Table 2 shows the position of the CCD fields for the two clusters while Table 3 contains information about the observations. Furthermore, 16 frames containing 17 standard stars from Jonch-Sorensen (1993,1994) were obtained as well, using 14 E region stars from the publication of 1993 and 3 faint stars from the one of 1994. All frames were obtained under good seeing conditions (FWHM of [FORMULA]).


[TABLE]

Table 1. Centre positions of CCD pointings for M22 and M55 fields



[TABLE]

Table 2. Log of CCD observations



[TABLE]

Table 3. Coefficients for the calibration equations


The CCD frames were processed with the standard IRAF routines, instrumental magnitudes were derived using DAOPHOT and ALLSTAR (Stetson 1987, 1992). Calibration equations have been determined for [FORMULA], b and v, taking the according airmasses X into consideration:

[EQUATION]

The coefficients for the calibration equations are given in Table 3. The calibration for [FORMULA] and [FORMULA] have been calculated from the values of [FORMULA], respectively. After the photometric reduction and after matching all frames, upper limits for the total photometric errors are 0.015 mag for V, 0.019 mag for [FORMULA] and 0.029 mag for [FORMULA].

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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