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45 degree ercting prisim

stuffedhaggis

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stuffedhaggis

stuffedhaggis

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Hey guys

I decided to get a 45 degree erecting prisim for my 200p dob .

Having a bit of an issue with it. When using i cant foucus on anything at all. All stars i look at go like a donut type shape . I have used a 25mm and 15 m eye peice.

When i remove the prism i can see evey thing perfect with the eye peice . Am i using the thing wrong .. or is it a duffer .

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michael.h.f.wilkinson

michael.h.f.wilkinson

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A 45deg erecting prism has a fairly long optical path, due to the way light is "folded" through the optics. I bet your scope does not have enough inward focus to compensate for that. The diagonals are really only designed for refractors, SCTs and maksutovs, not newtonians with their much smaller focus travel (I have one refractor, serving as 16x70 finder, which will only come to focus with a diagonal)

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LukeSkywatcher

LukeSkywatcher

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A 45deg erecting prism has a fairly long optical path, due to the way light is "folded" through the optics. I bet your scope does not have enough inward focus to compensate for that. The diagonals are really only designed for refractors, SCTs and maksutovs, not newtonians with their much smaller focus travel (I have one refractor, serving as 16x70 finder, which will only come to focus with a diagonal)

I had not considered that, but now you mention it, i dont think ive ever seen a newt with a diagonal in it.

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stuffedhaggis

stuffedhaggis

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Maybe dont insert the prism the full way into the focuser?

I feel pretty bad right now because i was the one who told you about them and suggested you buy one. I hadn't factored into the equation that your scope is a Dob.

Dont fell bad its all good . Lol just another learning curve for me.

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stuffedhaggis

stuffedhaggis

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As well as the light path being folded the refractive index make the path appear longer then it is by whatever the index is, usually in the 1.5-1.6 region for glass. You will have to wind it a long way in.

Cheers . Wish there was away to mod the scope for it but ill need to just luve it

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John

John

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The upside down, back to front view is the norm for newtonians. The diagonals that you use with refractors, maks and schmidt-cassegrains turn the image the right way up but left and right are still reversed. You do get used to this after a while. The idea is not to add any more glass in the light path of an astro scope than its strictly necessary, hence the acceptance of the inverted / reversed views.

You can get finder scopes which show a "normal" image and the red dot / Telrad type finders see things as your eyes do as well of course. Perhaps that would be worth considering ?

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LukeSkywatcher

LukeSkywatcher

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On astro (but not terrestrial) targets the degredation caused by erecting prisms is disastrous anyway. Stars are a chellenge for optical engineers! Keep your prism for when you buy a small refractor and then it makes a good birder scope etc.

Olly

Yeah keep it for when you buy a refractor. Trust me, you WILL buy a refractor at some stage.

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