Diagonals 45 or 90 degree?

[RonVen]

Do diagonals have any other advantage other than the position that you look into the telescope?

Ron

[ronin]

Basically no, so long as it is a mirror, all they do is make it easier to look into the scope.

If you have a prism diagonal then they can make the image erect but astro scopes are not intended to work with a lump of glass in the path so you will get additional CA from the prism.

We cannot produce a perfect reflector or prism so with either type of prism it will reduce the amount of light that gets into the eye, even a small bit with a good mirror, also the mirror surface and the glass body add some scattering to the optics.

[RonVen]

Basically no, so long as it is a mirror, all they do is make it easier to look into the scope.

If you have a prism diagonal then they can make the image erect but astro scopes are not intended to work with a lump of glass in the path so you will get additional CA from the prism.

We cannot produce a perfect reflector or prism so with either type of prism it will reduce the amount of light that gets into the eye, even a small bit with a good mirror, also the mirror surface and the glass body add some scattering to the optics.

Thanks. A very good analysis. Ron

[Peter Drew]

I don't think you will find a 45 degree mirror diagonal. The angle of incidence equals the angle of reflection, the mirror in a 90 degree diagonal is set at 45 degrees, a 45 degree mirror diagonal would have to have its mirror set at 22.5 degrees which would require a much larger and higher quality mirror to work well. Prism diagonals can be supplied with a "roof" section ground into them which can produce either a 45 degree or 90 degree angle of view and an erect image, these prisms are more difficult to make in astronomical quality.

[E621Keith]

Erecting prism split the beam down the middle to do a left/right flip (roof prism that Peter mentioned). It's very difficult to get the two surface at perfect angle, so it will degrade the image quality.

Standard 90 deg non erecting prism is a different matter. Depending on price, some prisms can be as good as if not better than mirror diagonals in slower scopes. Prism is also much easier to clean. However, prism can induce aberration in scope faster than f6, so it's best to go for a dielectric mirror diagonal if you have such a scope.

Televue makes a 60deg mirror diagonal but I don't think anyone else make non 90deg mirror diagonal. Almost all 45 deg prisms are erecting prisms, but there are a few non erecting 90deg prism as well as erecting 90deg prism