Peter sent Chris a file called “ROLO_765nm_Vega_psf.dat”
which contains the PSF of Vega measured by Tom Stone.
Here is the profile:
the horizontal scale is log of r in arsecs.
the inner part is something like a Gaussian, like King finds.
the halo outer part goes at r^-2.4, in the range 1.2 < log(r) < 2.0,
so steeper than King but not as steep as our steepest profiles
to date (these are r^-3).
The very outer parts are certainly affected by sky subtraction,
as the light will be well below sky at log(r)>2, one could fix this
probably by choosing the sky so that the halo continues at the
same gradient, but this is of course arbitrary.
It seems that for a wide range of telescopes and instruments,
the distant stellar light falls off as a power law — we see this
in the ES telescope, a 35 mm camera, King’s profile, and a
range of other instruments/telescopes summarised in a paper
by Bernstein (ApJ, 666, 663, 2007).
The slope of the power law can change even on the same
telescope/instrument! We see different powers on different
nights. This is important!