AAS 207th Meeting, 8-12 January 2006
Session 128 (Extra) Galactic Globular Clusters

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[128.06] Strömgren CCD Observations of NGC 6397

A. G. D. Philip (ISO and Union College), O. Pintado (Univ. Nacional Tucuman), R. Janusz (Vatican Obs. and Ignatium, Krakow), R. Boyle (Vatican Observatory)

On June 9 - 11, 2004 CCD frames were taken of the globular cluster NGC 6793 with the 2.15-m telescope at CASLEO in Argentina by Philip and Pintado. In a previous paper we reported on a preliminary determination of the error of the flat. In August this year Philip, Janusz and Boyle worked together at the Vatican Observatory, Castel Gandolfo and made a more complete analysis of the y, b and v flats. The flat frames are obtained by taking exposures of an illuminated white spot, attached to the dome. To test how flat the spot exposures are central frames are taken through each filter and then the telescope is moved north and south and then east and west and four more sets of exposures are taken. A data base is built up where we have the magnitudes of stars as they appear at different positions on the CCD frame. The differences between these sets of magnitudes are calculated and mapped as a function of where they appear on the chip. Plots of these differences, as a function of the x and y coordinates on the chip, are made. The ideal result is a graph showing a scatter of 1% about a rms error = 0. If a gradient of any kind is found in the plots then an equation can be found to correct the trends found. There were about a dozen standard stars [We used Graham and Doremus 1968, AJ 73, 226.] well spaced over the frame. We excluded any stars that were close to the edge of the frame. Frames that had poor seeing introduced false tilts so they were not used. The star in common in the pairs of frames had to have similar intensities on each frame. Then, remembering that the photometric differences increase as the magnitudes go fainter all the fainter stars were removed from the calculations. In the preliminary report we found that for the North and East frames the scatter was a bit higher and there were gradients of up to 2%. But now, after rejecting inferior frames, we find that the scatter is 1% across the entire frame for each of the y, b and v frames. We were able to construct a color-magnitude diagram for the stars that had photometric errors of 1% or less. A well-defined horizontal branch is seen as well as the asymptotic giant branch and five blue stragglers. The main sequence can be seen to a y mag of 15. Stars fainter than this limit had rms errors greater than 1%.


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