Prism or mirror?

[LLsandeLL]

Hi everyone

What's the most appropriate diagonal to use on an f/10 SCT system: prism or mirror...?

Cheers

Paul

[John]

Hi Paul and welcome to the forum

Either will work but most astro diagonals are mirrors these days. The older Celestron SCT's used to come with a prism diagonal but it may well be a mirror one now.

[JamesF]

Agree. I think you need to worry more about the standard of manufacture and accuracy of the diagonal than whether it's a prism or mirror.

James

[barkis]

A mirror is the acceptable choice. The prism does a job, but the light rays have to pass through the prism, and may

result in some loss of light.

Ron.

[JamesF]

A mirror is the acceptable choice. The prism does a job, but the light rays have to pass through the prism, and may

result in some loss of light.

I think that's too simplistic a way to look at it.

It *may* be that good mirrors have better light transmission than good prisms though I'm not convinced of that. A mirror may well also scatter some light, which a prism shouldn't. So a prism may result in more light actually reaching the eye and give better contrast. In either case with a good quality product, the difference is quite possibly going to be no more than a couple of percent and it's unlikely the human eye can discern the difference at that level.

Prisms may well be unsuitable for fast scopes where the angle of incidence of light with the face of the prism can be steep, but where a slow scope is concerned I think it's better not to worry about whether it's a prism or mirror and just buy something that's well-made.

James

[JamesF]

Oh, of course, the other thing is that a prism generally has a shorter light path than a mirror, so should you for some reason be operating near the extremes of your focuser range, one may work where the other won't. Which is not hard to fix if you need a longer light path, but somewhat more tricky if you need it to be shorter.

James

[Capricorn]

Go for a mirror. As the light is not entering the prism perpendicular then there will be splitting of the light into it's components and the chromatic aberrations will increase.

Remember that Newton showed the splitting of white light into it's colours by using a prism, when using a scope it is best not to repeat and confirm the experiment. :grin:

[E621Keith]

Mirror diagonals have one precision polished surface, while prism have two.

Light beam also have to pass through a lot of glass in a prism, but do not enter the glass in mirror. As such mirror can use glass of a lower grade.

Therefore, a mirror diagonal will be much cheaper to make than a prism diagonal of equivalent optical quality. To be honest, you won't really see any difference when you spend over £100 on a diagonal.

[JamesF]

If mirror diagonals are so much better than prisms, and cheaper to make for the same quality to boot, why do prism diagonals still exist? Just for those people who want an erect image?

James

[sologuitarist61]

If mirror diagonals are so much better than prisms, and cheaper to make for the same quality to boot, why do prism diagonals still exist? Just for those people who want an erect image?

James

Probably, but dielectric mirrors seem to be the rage these day. I have two a 2" and a 1.25" version for differing tasks, but cannot really say I have seen a great deal of difference between them and the bog standard diagonals, but that may be down to my not so good eyesight.

[E621Keith]

If mirror diagonals are so much better than prisms, and cheaper to make for the same quality to boot, why do prism diagonals still exist? Just for those people who want an erect image?

James

I believe the main reason for using a non erecting prism diagonal is because they have a shorter light path, which makes them much better for binoviewing. Many standard scope will run out of focuser in-travel with standard mirror diagonal.

A 96% transmission 1.25" non erecting Baader prism cost over £100 where as a 1.25" dielectric WO mirror digonal cost £65.

A mirror diagonal will be better than a erecting prism for astronomy. Erecting prism diagonal uses roof prisms, which splits the beam into two and then recombining them. There are a lot of reports of image break down at high magnification. Roof prism are very difficult manufactured and aligned correctly, and it's the reason why porro prism binocular are generally recommended for astronomy over roof.

[x6gas]

Light beam also have to pass through a lot of glass in a prism, but do not enter the glass in mirror. As such mirror can use glass of a lower grade.

Yeah. Astro mirrors are front coated so the light doesn't actually pass through any glass at all; it's all about the optical flatness of the surface and how well aligned it is...

[E621Keith]

x8gas is right, all astronomy mirrors, as well as most optical mirrors, are first surface mirror. i.e. the reflective coating is in front of the glass.

This exposed reflecting surface will oxidise and loses its reflectivity over time. This was the reason why the mirrors in reflectors needed recoating every few years. Traditional mirrors also have a lower reflectivity, only around 80%.

In comparison, a 90deg prism uses total internal reflection so no light is loss in the reflection process. Absorption in the optical glass and reflection on the two air-glass surfaces are the main cause of light loss in a prism. High quality optical glass are very transparent and an anti reflection coating have been widely available for many decades. Reflectivity over 90% are achievable.

These all changed when dielectric mirrors finally entered the amateur astronomy market (in the last decade, if my memory is correct). Dielectric mirror coating protects the reflection surfaces from air, so they last much longer than standard mirrors. The dielectric coating also enhance reflection and 95%+ are achievable. As such prism diagonal loses their advantage of light through put when dielectric mirrors coating was introduced to the amateur astronomy market.