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Message: <blockquote data-quote="Jay Turberville" data-source="post: 199404" data-attributes="member: 2746">The erecting prism is also left off because the additional optical surfaces and refraction of light that they introduce further degrades the image.  This degradation is fairly minor from a birder's perspective but certainly not worth tolerating from an astronomer's point of view.  Especially, as you point out, since astronomer's usually don't care much about image orientation. On the surface, this seems to be a farily minor thing, but such fine things are part of what separates a mediocre astro scope from a fine one.  The "true crossover" scope will be difficult to make.  One use or the other is bound to suffer compromises in features that many users in the respective groups would consider essential.  Such is the case with the TeleVues.  They are optically excellent, but they are not waterproof.  I suspect they lose some of their optical edge if you add even a very good prism (Televue reccomends their use only at moderate magnifications). And I don't think they are champs in the areas of ergonomics and weight.  As Hugh pointed out, Televue is probably about as close to achieving this "true crossover" ideal as anybody, and their scopes are clearly more oriented to astronomy.</blockquote>