45 or 90 deg star diagonals?

[milkyjoe]

Which is best and what is the difference (apart from the obvious ) as they both do the same job... don't they? lol

[jgs001]

It depends on what you're going to be using the scope for. Generally speaking, you'd use a 90 degree for looking at the sky, and a 45 degree for using the scope as a daytime spotter scope of some sort. The angles work better for those tasks. The 45 degree (the ones I have anyway) are built with a prism instead of a mirror, and this also, in mine, is used to provide a correct way up image, which again is quite handy for use as a spotter. An inverted image (with a mirror) isn't a problem for the stars.

[Peter Drew]

The 45 degree prisms are usually based on the roof prism design, these are very difficult to make to a high quality which means that the common stock variety are not as good as the 90's for astronomy.

[acey]

I have 2 small scopes for travel: an 80mm with a 90 degree diagonal, and a 100mm spotting scope with built-in 45 degree diagonal. The latter is fine for looking at objects that aren't too high in the sky, but as the elevation increases it becomes more and more awkward, and it would be hopeless for looking at the zenith. On the other hand it's nice to have a view that is not inverted or left-right reversed as it makes map reading very easy, and I can use the spotting scope without a finder. With a 90 degree diagonal you get upright but left-right reversed, and reading a star map is very difficult - you need a separate finder in order to find your way around. So the spotting scope gives me a more compact package for travel, but the scope with 90 degree diagonal plus finder is better for all-round use.