In a forum, someone write, "the number of waves that enter both faces of the prism is equal". When light waves of a star come from just above, pointed out may be right. But when light waves come from a diagonal direction, the number of waves that enter both faces of the prism may not be equal.
i add, this experiment must be done in the vacuum of the outer space (such as moon's surface). It's because of the effect "extinction" of the air (about the rest, please see my web-site).