  Simple formulae are often used to estimate the sensitivity of coded mask
X-ray or gamma-ray telescopes, but these are strictly only applicable if a
number of basic assumptions are met. Complications arise, for example, if a
grid structure is used to support the mask elements, if the detector spatial
resolution is not good enough to completely resolve all the detail in the
shadow of the mask or if any of a number of other simplifying conditions are
not fulfilled. We derive more general expressions for the Poisson-noise-limited
sensitivity of astronomical telescopes using the coded mask technique, noting
explicitly in what circumstances they are applicable. The emphasis is on using
nomenclature and techniques that result in simple and revealing results. Where
no convenient expression is available a procedure is given which allows the
calculation of the sensitivity. We consider certain aspects of the optimization
of the design of a coded mask telescope and show that when the detector spatial
resolution and the mask to detector separation are fixed, the best source
location accuracy is obtained when the mask elements are equal in size to the
detector pixels.
