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Thread: Edmunds 8.4-21mm zoom, etc.

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    Default Edmunds 8.4-21mm zoom, etc.



    I acquired one of these a long time ago. Does anyone know if they had orthos and other designs in the same form? Mine does not say Ortho on it, but an old Celestron version I found on Ebay says Orth on it and it looks identical to mine. More recent zoom EPs do not seem to be Orthos. Have Orthos been surpassed by Plossls, etc of modern day?

    I am also wondering whether I should change the 1.25" focuser on my 6" f/5 Edmunds Newt so I can enjoy some 2" EPs I am starting to add to my collection, which fit some newer fast scopes I have purchased. I realize the Edmunds f/5 system was designed for 1.25", so a 2" is unnecessary, but can it benefit technically with a 2" EP"? I wear glasses, so the eye relief is a big thing with me. I hate having to remove my glasses to view something. Would a 2" 10mm have a longer eye relief than a 1.25" 10mm? Thanks.

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    Default Re: Edmunds 8.4-21mm zoom, etc.

    Could you post a picture of it? Possibly the link also?
    Edmunds did and still does make some fine eyepieces though I have never heard of an Orthoscopic zoom.
    I have an older Edmunds 12.5mm Hastings triplet (Schott glass) that is outstanding.
    The new ones are still very good using Hoya glass.
    A good Orthoscopic is an excellent eyepiece and are my favorite eyepieces for viewing.
    The barrel size does not determine the eye relief, that is determined by the design and focal length.
    Plossl have about 75% eye relief of the focal length and Orthos about 80%.
    If you wear glasses to view you should be looking at eyepiece designs that give you at least 15mm+ of eye relief.
    Last edited by Gabby76; 06-04-2017 at 06:46 PM.
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    Default Re: Edmunds 8.4-21mm zoom, etc.

    That's the right focal range matching my Orthoscopic zoom eyepiece that I got from University Optics about 40 years ago. Still have it and use it from time to time.

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    Default Re: Edmunds 8.4-21mm zoom, etc.

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    Default Re: Edmunds 8.4-21mm zoom, etc.

    Thank you nFA, I have never seem a Orthoscopic zoom before!
    How much does the FOV change when adjusted?
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    Default Re: Edmunds 8.4-21mm zoom, etc.

    Quote Originally Posted by Gabby76 View Post
    Thank you nFA, I have never seem a Orthoscopic zoom before!
    How much does the FOV change when adjusted?
    By a lot. Just eyeballing a comparison with a 26mm Meade "Super Plossl" that is sitting next to it I'd say that at minimum magnification it's about 40 degrees AFOV. As one dials in the high magnification the AFOV becomes larger than 50. It's really hard to tell because the field stop totally fuzzes out at high magnification.

    Now I think some caution about the word "orthoscopic" for this ep is in order. It could be simply a technical use of the term not design heritage. Plossls and the Abbe design commonly called "Abbe Orthoscopic" are both orhtoscopic eyepieces.

    https://en.wikipedia.org/wiki/Orthoscopy

    So I think it much more likely that this design is a Plossl Orthoscopic and thus orthoscopic with a small o.

    The edge of field performance at the 50 deg AFOV is too good for an Abbe design and hints at Plossl heritage.
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    Default Re: Edmunds 8.4-21mm zoom, etc.

    Quote Originally Posted by Kennyboz View Post
    I am also wondering whether I should change the 1.25" focuser on my 6" f/5 Edmunds Newt so I can enjoy some 2" EPs I am starting to add to my collection, which fit some newer fast scopes I have purchased. I realize the Edmunds f/5 system was designed for 1.25", so a 2" is unnecessary, but can it benefit technically with a 2" EP"? I wear glasses, so the eye relief is a big thing with me. I hate having to remove my glasses to view something. Would a 2" 10mm have a longer eye relief than a 1.25" 10mm? Thanks.
    It depends on the eyepiece design details. Some 1.25 eps have jest as much eye relief as 2" eps.

    The thing that is a sticking point in your plan is that a 2" ep might have a wider field stop than the 1.25". So to avoid vignetting you'll have to replace the secondary mirror too! It's not just the eyepiece holder. Only a larger secondary can avoid illumination drop off in low power 2" viewing.
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    Default Re: Edmunds 8.4-21mm zoom, etc.

    Thanks for the heads up on vignetting. Perhaps before I invest ion a 2" focuser for the 6", I'll do a makeshift experiment with a 2" 50mm EP. My zoom EP looks exactly like the one posted in this thread. I need to learn how to put pics in the "gallery" for posting.

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    Default Re: Edmunds 8.4-21mm zoom, etc.

    Quote Originally Posted by Kennyboz View Post
    Thanks for the heads up on vignetting. Perhaps before I invest ion a 2" focuser for the 6", I'll do a makeshift experiment with a 2" 50mm EP. My zoom EP looks exactly like the one posted in this thread. I need to learn how to put pics in the "gallery" for posting.
    You won't like what you see. That will give you a huge 10mm exit pupil and you will see a central shadow of the secondary mirror! If your eyes are young 35mm is the longest practical in an f5 scope. If your eyes are old maybe 25mm is the lowest magnification you can use without essentially throwing away aperture and light gathering power.
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    Default Re: Edmunds 8.4-21mm zoom, etc.

    I've already seen the secondary mirror shadow with the 50mm, and I have an even longer one on the way to me. Good news is, I have a nice little 90mm f/5 refractor (achromat - can't afford the triplets) that likes the 50mm, also a 90mm f/10 that'll probably also like it. In a week or so, I will have a collection of three 2" EPs, including a 35mm or so and I didn't spend a fortune on them. I'll probably get one more shorter one just to see how it compares to a 1.25" of the same FL. Just playing (wasting money) at this point. The cost of education can be high, but what one learns is priceless. Thank you for the help.

 

 

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