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[3.06] HST NICMOS 3 Photometry of Globular Clusters NGC~6287 and NGC~6341 (M92)

J.-W. Lee (UNC), B. W. Carney (UNC), L. K. Fullton (Observatoire de Geneve), P. B. Stetson (DAO)

We present {\em HST} NICMOS Camera 3 (NIC3) photometry for globular clusters NGC~6287 and NGC~6341 (M92). Our measurements range from the RGB-tip to \approx 3 mag below the main sequence turn-off. A comparison of color-magnitude diagrams for these two clusters reveals that they have similar horizontal branch populations but slightly different red giant branch morphologies. Our relative age estimation using the reddening-independent difference in color between the base of giant branch and turn-off point (VandenBerg, Bolte, & Stetson 1990) shows that these two clusters essentially have the same age. We expect to also have results available on the elemental abundances in NGC~6287 in comparison with those in M92, based on high-resolution echelle spectra of two red giant cluster members and the published results for M92 of Sneden {\em et al.} (1997).

We also discuss the intrapixel sensitivity of the NIC3 detector. We show that the variation in the sensitivity across individual pixels is the dominant source of the photometric errors. This error can be as large as 0.3 mag even for isolated bright stars in both F110W and F160W bandpasses. Applying a photometric correction based on the ratio of the flux in the peak pixel to the flux in the whole point-spread function (Storrs {\em et al.} 1999) generally improves the accuracy of the measurement, but it does not fully compensate for the loss of information, especially in F110W bandpass.