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4.2.5. Flux Calibration

The final accuracy of a surface brightness measurement depends on four independent facets of the calibration: point source calibration; aperture correction, compensating for flux not recovered in the point source calibration; the calibration of the fiducial standard star system; and the solid angle subtended by each pixel. The solid angle of the pixels is, of course, very well known and is a negligible source of error for the WFPC2 data. However, the other three calibration steps require quantities of data and observing time only available to the WFPC2 Instrument Team. Details of the WFPC2 calibration are thoroughly discussed in Holtzman et al. (1995a, henceforth H95a), H95b and subsequent STScI Instrument Science Reports. Below, we summarize their results.

Sensitivity to the same fiducial standard is stable to sigma < 1% for the F555W filter, and sigma < 1.5% for the F814W and F300W between February 1995 and March 1997 (STScI TIR/WFPC2 97-01). Observations of the same standard may be variably affected by CTE due to variations in the peak flux of the observation which will arise from variations in the position of the PSF within a single pixel. The real response of WFPC2 is, therefore, likely to be more stable than this result. The synthetic photometry package (SYNPHOT), including system throughput curves and synthetic zero-points, is based on the HST secondary standard system and is estimated by H95b to be accurate for point sources to roughly 1% for a source with typical stellar colors. Point-source calibration should therefore be good to roughly 1%.

Aperture corrections based on the encircled energy curves are given in H95a (see Figures 5a & 5b, and Tables 2a & 2b in H95a). For the optical filters, the aperture correction is nearly 10% in moving from a 0.5 arcsec aperture to an infinite aperture, with uncertainties of 1% for the redder bandpasses, and 1-2% for the F300W.

Finally, the secondary standard star system which is used for all the HST instruments is "conservatively estimated" to be within 1% of the Hayes 1985 optical calibration of Vega (Bohlin 1995; Colina & Bohlin 1994; Hayes 1985), with internal agreement of the same order. The agreement between the HST secondary system and the Hamuy et al. (1992) secondary system to which our ground-based observations are tied is estimated to be 1-2%, as discussed in Paper II.

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