
10.3.
The Near Perfect Image 257
and 0.246 for the apertures with
£
= 0 and s = 0.33, respectively, hence a smaller
Airy disk roughly compensates for the smaller encircled energy fraction.
For another example assume a passband of lOOnm centered at
A
= 550 nm,
and a star of apparent magnitude 25 imaged by HST. In this case we find a photon
flux of
4.03
photons/sec passing through the HST aperture and 2.63 photons/sec
on the Airy disk. Assuming a reflectance of 0.9 for the primary and secondary
HST mirrors, a more accurate flux value is approximately 2.1 photons/sec for a
star of apparent magnitude 25 at the //24 focus of HST. The detected photo ...