
With the success of the Event Horizon Telescope, identifying new
observational signatures of black holes are of increasing interest. In
this talk we investigate blueshift of photons from an equatorial
accretion disk of emitters orbiting on stable circular orbits
terminating at the ISCO (innermost stable circular orbit). We analytically calculate the blueshift of
photons that escape to the celestial sphere (instead of falling into the
black hole) and numerically calculate the maximum blueshift received by
an observer at fixed angle on the celestial sphere.
