Thus I0 dA0 dT,0 = I0 dA0 AT /r 2 = Ii dAi dT,i = Ii dAi AT /f 2 , (11) but solid angle sub-tending image from telescope equals solid angle sub-tending object from telescope so dA0 /r 2 = dAi /f 2 , (12) and as expected, Ii = Io . Do telescopes measure ux or intensity? If resolution of telescope is crude, and we cannot resolve object, then we measure ux. If telescope can resolve object, then we measure intensity. Why?