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Date: Mon, 5 Apr 93 14:03:25 CDT From: gav@astro.as.utexas.edu (Gerard de Vaucouleurs) Subject: SN1993J photometry SN1993J Comparison Stars Except for neglected color terms in the transformation to the Johnson V system, the main source of scatter in the various provisional magnitudes is in the adopted standards, often star B = GSC 0928. The transformation from GSC V magnitudes to Johnson V magnitudes published by Harold Corwin in IAU Circular 5742 is HGC V = 12.87 + 0.969(+/- 0.052)(GSC V - 12.43) from an impartial l.s. line fit to GSC #0434,0928,1023,1031,1123. These stars have all nearly the same color, <(B-V)> = + 0.54, range 0.46-0.63. Stars #0224 (red) and #0308 (aberrant) are excluded. Neglecting the barely significant scale coefficient, the mean systematic difference between the two scales is HGCV-GSCV = 12.87 - 12.43 = + 0.44. This is close to the difference for star B (11.90 - 11.42 = + 0.48). A preliminary reduction to ~ 50 available estimates to the Johnson V system (leaving still a large dispersion ~ 0.2 mag) suggests that the V maximum was reached on JD9077.7 at V = 10.5, B-V = + 0.3, with onset near JD9072.0. G. de Vaucouleurs (gav@@astro.as.utexas.edu)
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