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Description
We present deep V, I photometry of the globular clusters NGC 288, 362, and 1851 obtained during a single observational run under strictly homogeneous conditions. We use the bimodal horizontal branch (HB) of NGC 1851 as a "bridge" to obtain the optimum relative match between the HBs of NGC 288 and NGC 362. In this way we can effectively remove the uncertainties associated with distance, reddening, and inhomogeneities in the absolute calibration, thus obtaining a very robust, purely differential estimate of the age difference between these two clusters. According to the bridge test, NGC 288 is found to be older than NGC 362 by 2+/-1Gyr. The observations were carried out during the nights of 1997 January 2 and 3 at the 2.2 m ESO/MPI telescope at La Silla (Chile), with the EFOSC2 camera equipped with a Loral/Lesser 2048x2048 pixel CCD. The pixel scale is 0.26"/pix, and the effective field of view is 8x8arcmin^2^. The gain is 1.63 e^-^ADU^-^, and the readout noise is 6.3 e^-^rms. The seeing conditions were average during the January 2 night (~1.3") when the NGC 1851 and NGC 288 observations were carried out, and they worsened (~2") during the second night when we observed NGC 362.
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