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  1. #1
    akscubaduck's Avatar
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    Default 2" wide angle eyepiece



    Looking for opinions on a 2" wide angle eyepiece for my XTi 10 Telescope. I don't want to spend 500 dollars but I do want a good eyepiece. Any ideas?
    Terry

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    Quote Originally Posted by akscubaduck View Post
    Looking for opinions on a 2" wide angle eyepiece for my XTi 10 Telescope. I don't want to spend 500 dollars but I do want a good eyepiece. Any ideas?
    Terry
    Hi Terry,

    I have found the GSO Superview range of oculars to provide excellent views for money spent. My scopes 14" f5.1 and 22" f5.36 (still in testing stage) provide a viewing feast with these relatively inexpensive eyepieces. I started out with the 2" 42mm and 30mm pieces. The 30mm is very good with its 67deg FOV for low power wide field work, and the exit pupil is ok at around 6mm. The 42mm I dont use that much as the exit pupil is a little too big for me at around 8.2mm. I have more recently aquired the 1.25" 20mm and 15mm 67deg and 70 deg units also and no regrets.

    Stephen.(44deg.S.)

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    I have read good things about explore scientific 82degree nitrogen purged series the 31mm is the one I was looking at. About half the price of the 31mm nagler. I also saw the Baader planetarium Aspheric modular ep. They sell a 31 & 36mm. I have an Orion q70 32mm and a 26mm. They are not bad for 100 bucks, but silly me got to involved in cleaning the 32mm. I had it nice and clean but saw a little spec on it, and being to nit picky, took it apart. Which you should never do especially if it's under warranty. I was doing fine but forgot that I didn't tighten it back down. So when I flipped it over, glass came a fallen. Oops! Oh well just a few suggestions for you. I would like the nagler, but think I might end up with the Explore scientific.

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    Default

    Both the Explore and the Aspheric would be nice eyepieces. The GSO superview mentioned above is a very good value and quality for the low cost.

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    Check out Russell optics on ebay. I've had good experiences with his optics.
    Dennis Wilde
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  10. #6
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    Quote Originally Posted by akscubaduck View Post
    Looking for opinions on a 2" wide angle eyepiece for my XTi 10 Telescope. I don't want to spend 500 dollars but I do want a good eyepiece. Any ideas?
    Terry
    Your scope -- I have one too! -- has a focal ratio of 4.7.

    That means that it does not give a terribly clear, sharp, flat, well focused image across a very wide field with some eyepiece types.

    This is due to the angle of the light cone, from the primary parabolic mirror, and the conical section that forms the "prime focus" of the scope (without an eyepiece.) This conical section is the 'virtual image' of the primary, and it's NOT a flat plane.

    So, if you want to get the widest field of view of this plane, you pose great challenges to the eyepiece designer.

    If the mirror had a f/ratio of, say, f/10 (like a lot of Schmidt-Cassegrain models) or f/15 (like a 'classic' old long refractor) you could employ "simple" minimalistic eyepieces with few glass elements, and get a pretty decent wide field.

    But, if you attempt to get the full optimal field of view of the XT10 scope, at night, using a 7 mm relaxed exit pupil, you are really going to see the drawbacks of the simple eyepiece designs. The 'optimal' wide field of this scope -- by my calculations, using both 1.25" and 2" barrel oculars -- for no more than a 7.2 mm exit pupil, would be about these values:

    Widest Estimated Field w/1.25" Barrel Size <= 1.4 Degrees or 86 Arcminutes;
    Widest Estimated Field with 2" Barrel Size <= 1.98 Degrees or 119 Arcminutes (with a 34 mm, 70 Degree Apparent Field, 2.0" Barrel eyepiece; exit pupil=7.2 mm.)

    or, perhaps:

    Widest Estimated Field with 2" Barrel Size <=127.1 arcmin or 2.1 degrees (with a 31 mm 82d apparent field 2" barrel eyepiece ; exit pupil=6.6 mm.)

    Those are two eyepiece that are/were commonly available now, or at the time I wrote that program.

    So, let's just say, generally, you CAN get about a 2 degree field, with no light loss, if you can employ an exit pupil of 7.2 mm.

    Now, it's very hard to design an eyepiece that can accomodate the field curvature of a simple f/4.7 Newtonian AND provide this entire wide field of view.

    You get what you pay for.

    2-element Huygenian designs, which *actually work ok* in a long focus scope, will be just HORRIBLE in this scope!

    3-element Kellners might be acceptable at medium powers, not the widest field. As you go lower and lower, the aberrations will be more obvious; the stars will be more distorted; focusing will be harder--as you can focus on ZONES but not the entire field.

    4-Element, and 5-Element ~50 d apparent field Ploessls, well made, look fine! So do Orthoscopics, narrower field oculars.

    5 Element "Enhanced, modified Erfles" don't look so hot. The edge will be pretty cruddy; you see the focus being VERY different if you want to look at something dead-center, or at the side.

    The 8 and 9 element modern designs, like the Nagler (or the very old Bertele ultra wide field, which you surely cannot find anywhere!) might be *considerably* better.

    You can clean up the view with a 2x Barlow; but of course you are doubling the power and reducing the FOV.

    You can buy a Parracorr, and fine-tune it; used with an 8 or 9 element super wide, flat field eyepiece, you get marvelous vistas: elegant, lovely.

    You can spend more for the above combo than the total initial price of the scope. Some people *recommend* that! I *do not*!!!

    I recommend getting a different TYPE of scope for wider fields--a fairly high quality doublet apochromat "fast" refractor. Less light gathering; but wonderful with Naglers (etc.--including all the competing brands & designs) on star clusters. You can now knock about $150-200 off the prices of the bottom end apo refractors by buying a standard achromat...the better ones, like the Orions I own, are 'acceptable' if you can tolerate false color on bright stars & planets.

    If this is not practical, then you must choose one of these options:

    1. Buy a 2" barrel modesly priced wide field and LIVE with the edge distortion, fussy eye positioning, astigmatism--and "seagull stars" at the edge.

    2. Buy a nice 2" wide field eyepiece that does NOT give you the ultimate widest field--but gives higher power and a nice wide field view, with a smaller exit pupil that is less than 6.6 mm.

    3. Find somebody's old, obsolete & discontinued 2" barrel Kellner, and accept the ~45d apparent field. Cheap; you get sort of halfway between 1 and 2, above. Not as much astigmatism or seagull effect as the Erfle "wide field"; but not as wide a field!

    I've done 1, 2, and 3; but have long ago given up the old obsolete long focal length Kellners I used to own for my C-8.

    Now I just stick with option 1. When I want "beautiful star vistas with less distortion"... I use my refractor!

    Steve W/8HHaggis
    Last edited by 8HHaggis; 03-18-2012 at 12:13 AM.

 

 

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