Re: Star Diagonal

Why are telescope diagonals called star diagonals? What is the history of this name?

Chris Wenske said:

Why are telescope diagonals called star diagonals? What is the history of this name?

There are basically two types of diagonals, simple mirror and prism diagonals which provide an image which is correct in one direction but is reversed in the other direction. If the diagonal were mounted so you were looking down through it as is normal, then the image would be upright but reversed left to right. These are called star diagonals because they are preferred for stargazing because they are generally better at high magnifications.

The other type of diagonal is considerably more complicated and provides an image the is correctly oriented. This would normally use an Amici prism and is more commonly used terrestrially where the orientation is more important and the high magnification image quality less important.

Jon