Direct Measurements of Black Hole Charge with Future Astrometrical   Missions
  Recently, Zakharov et al. (2005) considered the possibility of evaluating the spin parameter and the inclination angle for Kerr black holes in nearby galactic centers by using future advanced astrometrical instruments. A similar approach which uses the characteristic properties of gravitational retro-lensing images can be followed to measure the charge of Reissner-Nordstrom black hole. Indeed, in spite of the fact that their formation might be problematic, charged black holes are objects of intensive investigations. From the theoretical point of view it is well-known that a black hole is described by only three parameters, namely, its mass M, angular momentum J and charge Q. Therefore, it would be important to have a method for measuring all these parameters, preferably by model independent way. In this paper, we propose a procedure to measure the black hole charge by using the size of the retro-lensing images that can be revealed by future astrometrical missions. A discussion of the Kerr-Newmann black hole case is also offered. 
A.F. Zakharov (NAOC, ITEP, ASC LPI & JINR), F. De Paolis (Lecce   University), G. Ingrosso (Lecce University), A.A. Nucita (...