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I'm a little confused about the relative differences between a 90-degree Star 'Di-electric' diagonal and a 45-degree erecting prism? Can anyone enlighten me on the merits and pit-falls of both.

:)

I quite like the look of a WO 2" 45-degree jobbie but wanted to know whether this is a good choice for both visual and photographic imaging when attached to my lovely Megrez 110mm APO :(

Many Thanks,

Gary

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OK well basically the 45 degree prism of the type that comes with a scope is really only suited for low power use. You can get better ones will also do medium power observing but they'll cost more. The WO 2" one is OK but it will never match the peformance of even a cheap dilectric.

A 90 degree dielectric will be better at higher powers, and is the most cost effective alternative for the performance you will get. So for general nighttime use stick with the 90 degree one.

Dielectric just refers to the protective coating on the mirror but generally this is only used on decent quality diagonals.

John

Edited by johninderby
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Thanks John. Not sure whether it makes a difference to the description but the WO model is an 'erecting' prism? I assumed these units did not reverse the image seen through the eyepiece as seen with 90-degree mirror diagonals, or am I barking up the wrong tree altogether here?

Gary

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First - you don't want a diagonal in the way when imaging - use an extension tube if required. The less mirrors/glass the better!

A 90 degree mirror simply reflects the light. The latest dielectric ones have a very accurate surface and very high reflectivity so are great for astronomy.

The 45 degree erecting prism passes the light through a glass prism losing more light and introducing more aberrations. The main reason for one is terrestrial viewing when you want to see things the right way round (which doesn't really matter for astronomy)

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Hi Gary,

A 90º star diagonal (like this one) allows you to view stars at, or near, the zenith without having to contort yourself into unnatural and uncomfortable positions.

A 45º erecting prism (like the WO) places the eyepiece at a shallower angle for daytime terrestrial observing. It'll also orient the image so that it appears the right way up - a 'scope's optics flip or rotate whatever you view.

For imaging the fewer optical surfaces in the light path the better, so most imagers do it straight through. Also, by lengthening the light path, you may make it impossible to reach focus with the camera.

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I'm a little confused about the relative differences between a 90-degree Star 'Di-electric' diagonal and a 45-degree erecting prism? Can anyone enlighten me on the merits and pit-falls of both.

:)

The 45-degree erecting prism is intended for daytime terrestrial use, if used for astronomy it would result in a left-right diffraction spike off bright stars. The 90-degree erecting prism is designed to fix that, it does not produce diffraction spikes and places the eye at a more comfortable angle.

Hope that helps,

Steve :(

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The degredation introduced at night by an erecting prism is very obvious. I craved a correct view of the moon but gave up on it! But by day, to my eye, the difference (other than in orientation) is negligible. So spot with a prism, do astro with a diagonal and image with an extension tube as stated above.

Olly

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The 45-degree erecting prism is intended for daytime terrestrial use, if used for astronomy it would result in a left-right diffraction spike off bright stars. The 90-degree erecting prism is designed to fix that, it does not produce diffraction spikes and places the eye at a more comfortable angle.

Hope that helps,

Steve :)

Thanks Steve - you just sorted something that's been in the back of my mind as to whether the horizontal diffraction spike was inherent to the design or just down to a cheap prism diagonal.

Edited by haitch
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If needed...

First Light Optics - Two-inch Focus Extension tubes

I use an 80mm on my Megrez and that achieves focus with my DSLR racked out just 9.1mm so I could use a 50mm. My focuser travel is 80mm so I know the 80mm picks up from where the focuser on its own leaves off.

Before buying one just check to see if your focuser travel is enough to achieve focus without one - it may be that it is long enough when all the way out. It probably won't be but I don't want to be the cause of you buying something you don't need!

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