45 Deg mirrors or prisams

[ve1drg]

I was told the other day that since I have a mirror in my right angle viewer on my esprit 120mm skywatcher scope,  that all images would be upside down.  But if I used a prisam instead it would all look normal?

Is this true?  

[Laurie61]

With a refractor, from memory, if you just use an eyepiece it gives an upside down image. With a mirror diagonal and the lens pointing straight up,  the view is the right way up but flipped left to right. A prism will correct for both effects and give a real image ?    

[ve1drg]

With a refractor, from memory, if you just use an eyepiece it gives an upside down image. With a mirror diagonal and the lens pointing straight up,  the view is the right way up but flipped left to right. A prism will correct for both effects and give a real image ?    

So it IS true  that a prism gives the correct image over that of a mirror-diagonal. 

This is good to know.  I just need to find a prism diagonal now for my Esprit 120mm refractor.

Thanks for your input.

[Stu]

With a refractor, from memory, if you just use an eyepiece it gives an upside down image. With a mirror diagonal and the lens pointing straight up, the view is the right way up but flipped left to right. A prism will correct for both effects and give a real image ?

Yes, that's correct. You can get either 45 degree or 90 degree erecting prisms which give a 'correct image' ie right way up and not left to right reversed.

Normally the image quality through a mirror diagonal is better than via a prism, so for astro use most people stick with a mirror diagonal as it doesn't really matter which way round things appear. The exception is for finderscopes where a prism can help by giving you a RACI (Right Angle, Correct Image) finder which makes finding things easier. Daytime viewing is obviously another very valid application!

Stu

[Peter Drew]

A 45 degree or 90 degree prism diagonal will give a correctly orientated (naked eye) image only if it is a roof prism. A standard 90 degree prism diagonal will give the same right way up, left to right reversed image as a mirror diagonal. The extra faces on the roof prism diagonals, for the price, are the main reason for them being somewhat inferior to standard diagonals due to the added optical difficulty in manufacture. 

[ronin]

I would have said that the mirror would produce a better image then the prism, however on CN there was a recent comparison (3-4 days back) the prism won for clarity and sharpness, and not just from the person doing the comparison, others that had used both agreed with the conclusion.

As the person doing the comparison was BillP author of choosing and using astronomical eyepieces then worth taking into account.

The prism used was a TV one, so good to start with, didn't get much information on the mirror, but do not think Bill would pick a top prism and a cheapo mirror as any comparison is rendered meaningless.