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ishimaru

Telescopes in English

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Okay, first question of what may be many :D

I know the basics of telescope types, Refractor, Reflector etc. But why is the image upside down? Or is it? If I looked at the moon through these types, will is b upside down? I find that confusing....

Can someone explain the optical mechanics (if that's a term) is if there is image correction. If I want to look at Pleiades, I would appreciate it being the right way round! :D

Rich

BTW - particulalry interested in Reflectors...

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I think it came as a pleasant SURPRISE to me (having read a lot of books in my distant youth!) that the image didn't have to be "upside down"! The use of a star diagonal in a standard refractor and also many Catadiopteric designs - e.g. a Maksutov Cassegrain, conveniently inverts the image (again), leaving it the "right way up". It is however, like in a (room) mirror, left-right inverted. This isn't so bad...

(I believe) Newtonian reflecting telescopes do have an inverted image. (Doubtless to be confirmed!). :lol:

Use of a special type of (Amici) prism CAN render refractor images both the "right way up" and NON L-R inverted. These have a use in terrestrial / spotting scopes etc. But these increase the light path through glass, so astronomers usually prefer simple mirror diagonals and learn to cope with the/any L-R inversion. Perhaps the "purest" type of scope is the small (refracting) FINDER. This does have a traditional inverted image. Some finder charts are specific to these. Despite that many of us like (e.g. 9x50) prismatic finder scopes of the "RACI" (right angle correct image - Sic) type. Somehow a bit easier for... "finding" stuff! :D

N.B. If you are buying a book, containing charts, it is well worth while checking out the convention used. Many modern books e.g. "Turn Left at Orion" show a view through a star-diagonal-ed refractor or MAK. It does use inverted finder images though. My solution is to buy a second SMALL inverting 6x30 finder for reference. I am assured though that PRACTICE enables any chart to be used with ANY configuration! :D

To answer your question as to the "whys" of the above, try the following link. But a precise understanding is not a prerequisite for telescope use. To get some idea of what's going on, try tracing rays from the "top" of the scope, and see where they end up. These diagrams are not perfect for this - You will have to add a mirror diagonal, eyepiece and an EYEBALL(!) sometimes (c.f. The first diagram). But a pencil and paper can be intructive, if you're still keen to understand... :p

http://www.astro.ufl.edu/~oliver/ast3722/lectures/Scope%20Optics/scopeoptics.htm

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you will find that unless you are using your scope for terrestrial viewing ( in daylight outside viewing, say: a distant landmark) it wont matter whether the image is upside down or 'back to front', it may not make a direct comparison with a view from a book, does that matter? People on here dont correct/invert their photo's that are posted, I think you'll find (they will correct me if I'm wrong :D)

Does what I say make any sense??

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No, we don't tend to correct the images.

I can never remember which type of scope produces which type of image - so I don't worry about it!!!

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If you are struggling to find your way around a star chart or book because your views are inverted in some way then just rotate the book rather than buy a prism or a correcting eyepiece., its much easier..... :D

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The biggest problem with trying to correct the orientation of the image is that it has to be reflected by a mirror or pass through a prism.

By doing that you lose light. And thats one thing you really don't want to do unless you really have to.

And as Daz said, most of the time you don't even notice.

Ant

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With refractors, the image is neither reversed or mirrored as you are simply looking at the light straight through a peice of glass on the other end. However, refractors that use diagonal's have a mirrored image (as the diagonal puts one mirror in the works).

Matt

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