Published online by Cambridge University Press: 25 May 2016
PG1115+080 (Weymann et al. 1980), the first quadruple lens system, remains the most promising for an optical measurement of the Hubble constant through a time delay. Factors contributing to this assessment include the well constrained geometry, the apparent magnitudes of the four components (Christian et al. 1987), the expected time delay (Narasimha & Chitre 1992), and the position of the lensing galaxy (Kristian et al. 1993). There are nonetheless many factors which combine to render this a difficult measurement, among them the small separation of the images and the small variability amplitude and long variability time scale of the “typical” optically selected quasar (Hook et al 1994). The measurement is further complicated by the likelihood of microlensing of the individual components (Witt et al. 1995), which depends upon the degree to which the quasar is resolved and the fraction of the lens mass in microlenses.
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