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  • binocescope OT

    Saw a binocular the other day and tested it in the store. Imaging was pretty good but aligning them to my eyes was very critical. It was difficult to get both eyes seeing the entire field without blackout. I've owned binoculars for years, so it's not just a matter of using them properly. You have to be right on for these to work ok, so I suppose that's one reason they're on sale at liquidation world.

    Anyway, I bought one with the idea of making two telescopes from it. The optics are good enough for what I want, and now I'm considering putting a right angle in the project so the eyepiece can be at 90, and I'll use a front surface mirror to bend the rays. Question is- where in the optical path is the best place for the mirror? I'm thinking that if it is at a point where the rays of light converge, then the roughness of the mirror will degrade the image quality. If I place the mirror somewhere else, then more of the mirror is in use and it will have less effect on image quality. One thing that I'm not too clear on (sic) is whether the mirror will distort the image as in bending the corners, or interfere with whatever color correction has been designed in.

    The objective lens will remain in the 'front end' so I can take advantage of the anti-glare, anti-fringing, etc that's offered by that tube. That part is roughly 1/3 the length of the focal distance, so I could choose a point somewhere between that and the eyepiece for the mirror. I won't be using the prisms from the binocular, so currently the image my eye sees is upside down and reversed left to right. If I'm thinking right, I'll be restoring the image to upright with the mirror- not sure what will happen to left/right. Maybe I could use a second mirror to fix that, and still get my eyepiece to be at 90 to the objective. I'm not too concerned with the orientation of the image though, more so with maintaining the image quality.


    My main questions are- what's the best place for the mirror, am I screwing with color correction by leaving out the prisms, and will I be introducing a pin cushion effect (warped corners) by using the mirror?

    I realize this won't be a great telescope, and will not take the place of my good binoculars. My main intent is to have a 7x50 view of the sky without having to crink my neck back- nothing too involved or expensive. It will be easy enough to make the tube with the bend in it, mount the mirror at that bend, and cater to the focus adjustment.


    Comments and suggestions?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Penta-prism

    The light is probably already "bent" through 90 degrees at least twice in each monocular - by using penta-prisms which are very accurate and very forgiving as regards optical alignment.

    Many optical instrument shops (Surveyors) will have a 90 degree "look down" adaptor that can be fitted to instrument eye-pieces/lenses.

    http://en.wikipedia.org/wiki/Pentaprism

    http://en.wikipedia.org/wiki/Binoculars

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    • #3
      Originally posted by darryl
      I'm thinking that if it is at a point where the rays of light converge, then the roughness of the mirror will degrade the image quality. If I place the mirror somewhere else, then more of the mirror is in use and it will have less effect on image quality.
      Roughness in the mirror will degrade the image no matter where the mirror is. Wavefront error is wavefront error no matter where it originates.

      The larger the area of the mirror you use, the more critical the overall flatness of the mirror becomes.

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      • #4
        Here is my binoscope, two rare Tak FC-100's.

        http://www.pbase.com/smokedaddy/image/108885369

        http://www.pbase.com/smokedaddy/image/108885370

        http://www.pbase.com/smokedaddy/image/116176748

        My binobacks came from here;

        http://www.page.sannet.ne.jp/mazmoto/index-e.htm

        -SD:

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        • #5
          Hmm. I sort of got lost for awhile in those links. Some pretty ingenious configurations there. I'm humbled.

          I've decided to go ahead with the 90 degree eyepiece and correct the image both left/right and up/down. I'll need two pieces of mirror for this, which I can cut out of a larger piece I have on hand. This is pretty thin stuff so I should be able to score and snap it pretty easily. One inch square pieces will be large enough to handle and fasten down without marking up the central area where the light will reflect from.

          I'll have to make the body with two 90 degree corners on it. Each corner will have one mirror mounted within, and I'll have to keep the corners as close together as possible. Somehow I'll make the calculation for focal length so I'll have the right spot to put the adjustment range for focussing the eyepiece. I've already made up a jig to cut the pvc tubing at 45 degrees to form each corner, and I'm about to experiment with a test corner to get the right placement for each mirror. I plan to spray the whole inside flat black, and I'll possibly mask a central area on each mirror so I can paint the unused part of each mirror as well.

          Two minutes from now I'll be cutting the second corner on my test piece to see how close I can get the two mirrors to each other while still being able to build it from the pvc tubing. It looks like distances will be fairly close to what the binocular had with its prisms, so it should all work out ok.

          I just hope I'm not making a fundamental error by using the mirrors. I could simply make a straight tube to mount the objective lens and the eyepiece into and be done with it, but what's the fun in that?

          I've played with the idea of having the eyepiece at 45 degrees to the main tube, but I'm having a problem mapping this out in my brain.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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