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Porro Prism Angled eyepiece.

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#1 Jon Isaacs

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Posted 06 May 2013 - 12:43 PM

Here's the question:

I recent acquired a decent quality spotting scope with a 45 degree angled eyepiece. In the past, I have used correct image prism diagonals with various astro-refractors, those diagonals have all been Amici/roof prism designs and have the characteristic line visible on bright objects.

Looking at the specs on this spotting scope as well as some others, (not all) they say they use Porro Prisms...

Spotting scope with Porro prisms.

Not knowing much about spotting scope design and prism design, I am interested in what exactly this means and whether one could build a correct image diagonal for an astro-telescope that did not use a roof/Amici prism.

:question:

Jon

#2 GlennLeDrew

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Posted 06 May 2013 - 03:03 PM

Do you see any hint of a roof line-induced diffraction spike on bright objects? Have you looked down into the objective end with a close focus monocular or binocular to see if a roof line can be seen? (Explore the back end optics by adjusting the monocular's own focus.)

#3 don clement

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Posted 06 May 2013 - 03:14 PM

I use a first surface roof mirror for a right angle correct view 80mm finder scope.

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#4 Jon Isaacs

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Posted 06 May 2013 - 05:44 PM

Do you see any hint of a roof line-induced diffraction spike on bright objects? Have you looked down into the objective end with a close focus monocular or binocular to see if a roof line can be seen? (Explore the back end optics by adjusting the monocular's own focus.)


Glenn:

The maximum magnification is 60x so I am not able to really test it but I did take a look at Castor and it provided a very clean split without a hint of a diffraction spike. I did find numerous references to Porro-Prism spotting scopes including some Swarovski;s. Here is diagram:


Porro Prism Spotting Scope

Jon

#5 GlennLeDrew

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Posted 06 May 2013 - 09:45 PM

Jon,
The lower of the two drawings shows immediately ahead if the eyepiece a two-reflection prism exactly like those found in 45 degree stereo microscopes. I should have envisaged this configuration. It is not a roof prism and therefore will not introduce a diffraction spike.

These prisms tend to be a bit small, thus limiting the field of view. And one surface must be silvered/aluminized due to the steepness of the angle at which light strikes it. I don't much like them, although for f/5 and slower systems they do work well enough.

#6 Jon Isaacs

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Posted 07 May 2013 - 03:28 AM

Jon,
The lower of the two drawings shows immediately ahead if the eyepiece a two-reflection prism exactly like those found in 45 degree stereo microscopes. I should have envisaged this configuration. It is not a roof prism and therefore will not introduce a diffraction spike.

These prisms tend to be a bit small, thus limiting the field of view. And one surface must be silvered/aluminized due to the steepness of the angle at which light strikes it. I don't much like them, although for f/5 and slower systems they do work well enough.


Glenn:

I found it interesting that it could be done without a roof prism. The fact that Swarovski uses it gives it some validity...

Most spotting scopes are around F/6 and compared to a astronomical refractor have a limited field of view, a 2 degree TFoV at 20x is very typical. My 80mm F/7 apo is capable of 4.7 degrees.

Spotting scopes seem to use a 24mm - 8mm zoom to provide a range of 20x-60x, I am guessing that's what this one is. Assuming the 480mm focal length, the field stop would only be 17.5mm based on 2.1 degree TFoV with an apparent field of view of about 42 degrees. At 60x, it's about 60 degree AFoV...

The ED objective seems sharp and free of chromatic aberration. Viewing a bright, fluorescent light at night, there is no CA and the light is sharp and it's construction can be seen...

Jon

#7 BillB9430

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Posted 07 May 2013 - 10:54 AM

Jon, I think you and Glenn are correct that most commercial spotting scopes are limited in field because of their small prisms (and narrow field zoom eyepieces). Upscaling the erecting prisms would quickly lead to a heavier spotter than most users want to carry.

It IS possible to "ATM" a larger erecting system without a "roof" for astro (or dual) use. Years ago, Surplus Shed had 50 mm aperture Porro-Abbe prisms for the bargain price of three for $50. Just one of those makes a nice 90 degree erect image diagonal, though the viewer must stand at right angles to the side. See attached photo for a home-mounted one. A big Porro-Abbe prism can still be assembled from two large RA prisms using optical cement.

Commercially available Amici (90 degree) and Schmidt (45 degree) roof prism diagonals I've seen are only 32 mm clear aperture and are a little small for maximum widefield views, compared to 2" mirror and simple RA prism diagonals. Of course, it is not necessary to fully illuminate the edge of a wide field for visual use. For most astro-views, this is all kind of a moot point, since R-L reversed views don't matter. It is nice to see the North American nebula looking like the map rather than its mirror image, however. -Bill

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#8 Jon Isaacs

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Posted 07 May 2013 - 11:24 AM

Bill:

Thanks for your input. I am not big on roof prisms for telescopes because of the spike they produce at high magnifications so the idea that one could erect the image without a roof prism was quite nice.

As far as looking like a map, I am OK with the reversed left-right image of a star diagonal, correct images actually confuse me and in any event, I do most of my dark sky observing with Newtonians so the erect image of a star diagonal in a refractor is luxury enough.

Jon






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