Diagonal - reversed or not reversed. That is the question.

[Davidv]

What determines if a diagonal gives a reversed left to right image or not? Is it just whether it is a prism or a mirror type or does it depend on if you are using a refractor, SCT or Mak? What sort of diagonal gives an upright but not reversed image?

I've always thought this reversed image business might be a bit disconcerting.

Thanks in advance. Dave.

[John]

With a mirror diagonal the image is the right way up but left and right are reversed. You can get prism diagonals that give a completely "normal" image but I'd much rather not introduce further glass, of undertermined quality, into the optical path - in astronomy light is too precious to waste !. Purists would probably use no diagonal at all.

It can be disconcerting but you soon get used to it, as you do the inverted image with newtonians.

[Merlin66]

Only special prisms where the image is double internally reflected fix it.

They are used in those "right way up" finders.

The best to use is a high efficiency front surface mirror.

[Davidv]

Quote: "It can be disconcerting but you soon get used to it, as you do the inverted image with newtonians."

I imagine you would, John. I hadn't really thought of it like that.

Thanks for the replies both.

[Ad Astra]

Essentially, each rotation (left/right) and (up/down) requires a reflection off a surface (mirror or prism) and each reflection causes a loss of light and can introduce optical error if the surface isn't totally flat.

The brighter, clearer image greatly compensates for being the wrong way round - As someone said, you will quickly get used to it - much quicker than you will get used to a dim or poor quality image.

Dan

[great_bear]

Purists would probably use no diagonal at all.

Actually, purists prefer prisms*.

- they scatter light less than mirrors do.

(at least the good ones)

rgds,

Jeremy.

* it's also true that uber-purists in Japan use no diagonal at all.