Relative and Absolute Calibration for Multi-Band Data Collected with the 2.2 m ESO/MPI and 1.54 m Danish Telescopes
We present the strategies adopted in the relative and absolute calibration of two different data sets: U,B,V,I-band images collected with the Wide Field Imager (WFI∈dexWFI) mosaic camera mounted on the 2.2 m ESO/MPI Telescope and u,v,b,y calastrom images
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INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040, Monte Porzio Catone, Rome, Italy; [email protected] DAO, HIA-NRC, 5071 W. Saanich Road, Victoria, BC V9E 2E7, Canada Univ. of Central Lancashire, Preston PR1 2HE, UK Univ. di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
Abstract. We present the strategies adopted in the relative and absolute calibration of two different data sets: U, B, V, I-band images collected with the Wide Field Imager (WFI) mosaic camera mounted on the 2.2 m ESO/MPI Telescope and u, v, b, y Str¨ omgren images collected with the 1.54 m Danish Telescope (ESO, La Silla). In the case of the WFI camera we adopted two methods for the calibration, one for images collected before 2002, with the ESO filters U/38ESO841 and B/99ESO842 , and a different one for data secured after 2002, with the filters U/50ESO877 and B/123ESO878 . The positional and color effects turned out to be stronger for images collected with the old filters. The eight WFI chips of these images were corrected one by one, while in the case of images secured with the new filters, we corrected the entire mosaic in a single step. In the case of the Danish data set, we compared point-spread function (PSF) and aperture photometry for each frame, finding a trend in both the X and Y directions of the chip. The corrections resulted in a set of first and second order polynomials to be applied to the instrumental magnitudes of each individual frame as a function of the star position.
1 WFI Data Set 1.1 Observations and Data Reduction Data have been retrieved from the ESO archive and include 8 U , 39 B, 51 V , and 26 I images of ω Cen . Data include both shallow and relatively deep images, with exposures times ranging from 1 to 300 s for the B, V, I bands, and from 300 to 2400 s for the U band, and were collected in several observing runs ranging from 1999 to 2003. During this period two filters were changed: data secured before 2002 were collected with the filters U/38ESO841 and B/99ESO842 , while later ones with the filters U/50ESO877 and B/123ESO878 . These data were obtained in good seeing conditions, and indeed the mean seeing ranges from 0.6” for the I band to 1.1” for the U band. We accurately selected the best PSF stars uniformly distributed across each chip. A moffat analytical function linearly variable on the chip was assumed for the PSF. The data were reduced with DAOPHOT IV/ALLFRAME [5].
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A. Calamida et al.
1.2 Relative and Absolute Calibration We are interested in providing accurate relative calibration of individual chips of the WFI mosaic camera because the occurrence of positional effects might cause a spurious color broadening of key evolutionary features such as the Red-Giant Branch (RGB) and the Main-Sequence Turn-Off. Moreover, we need to provide an accurate absolute calibration of our data in order to compare theory with observations. The plausibility of this comparison relies on the accuracy of the absolute zero-point of individual bands. This is a fundamental requirement for t
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