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Birding scopes fall generally into two groups: dedicated birding scopes and astronomical scopes that have been adapted for terrestrial or birding use.
Rugged and well sealed against water, dust and weather, dedicated Zeiss birding scopes are designed for heavy field use. Many are waterproof, with their focus mechanism and prisms protected inside the scope body. Astronomical scopes often have their erecting prisms and focusing mechanisms outside the scope body, allowing you to customize the back end.
Instead of using prisms, as most birding scopes do, with the inevitable loss of brightness and resolution that results when light passes through the prisms, you can replace the prisms with a 90 degree diagonal mirror and get better image quality. The eyepiece will then look down at a 90 degree angle into the scope. The picture will be right side up but reversed left to right. Note that astronomical telescopes tend to be somewhat fragile, and they need to be protected from the elements.