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CLASS: a well-dened sample of radio-selected
gravitational lens systems for cosmology
I.W.A. Browne & S.T. Myers
Jodrell Bank Observatory & NRAO
To exploit gravitational lensing for cosmology large, reliable and
statistically complete surveys are required. With the Cosmic Lens
All-Sky Survey (CLASS) we have set out to achieve these goals.
We pre-select targets to be at spectrum radio sources and map
every source with the VLA at 200mas resolution. Candidates
having multiple compact components with ux density ratios 
10:1 and separation if the range 0.3 to 15 arcsec are followed up
with high resolution MERLIN and VLBA observations, eliminat-
ing those candidates which do not match strictly dened surface
brightness and morphological criteria. More than 15,000 sources
have been observed and the radio follow up of the candidates is
complete. Eighteen lens systems have been found. We summarize
their statistical properties.
Optical follow up of the lens systems with HST reveal the lensing
galaxy in all but one (questionable) case, indicating that dark
lenses are rare. The image separations are closely peaked around 1
arcsec. By comparison of the radio image ux ratios with those in
the optical/IR we concluse that the eects of microlensing and/or
extinction are common.
The frequency of lensing in CLASS constrains the cosmological
constant (0 ) and this is to be discussed in the invited contribu-
tion by Helbig et al.
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