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Ball eyepieces


John

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There has been quite a lot of talk on another, US based, forum about single element "ball" eyepieces. They seem to have established a niche with some users looking for ultra sharp and contrasty high power eyepiece but who are not bothered about field of view or eye relief (there is not much of either with the ball type eyepieces I believe).

Personally I don't fancy them but I've never tried any. Have any SGL members tried one of these ?.

I believe they are mostly DIY devices as I'm not sure any manufacturer actually produces them although I thought that the US company Siebert was thinking of producing them :)

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I made a few, donkey's years ago, from the lenses incorporated in prefocus torch bulbs. They gave amazingly high magnifications even with my 2" F8 achromat first home made telescope. The Moon was quite impressive but not much else, the field was miniscule and the off axis aberrations were horrid. I read somewhere that William Herschel made almost spherical eyepieces by dropping blobs of molten glass into water with which he achieved his often quoted very high magnifications.

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  • 6 years later...

To revive an old thread, because I've been searching eyepiece designs for planetary observation. And a search for Siebert brought me here. Siebert released the Planeshpere eyepiece line and a Monocentric eyepiece line. 

The price for the Planesphere is $139, which for a unique high quality one man band product strikes me as a good deal. Quite a few people over on the other forum made there own version ball eyepieces. The general consensus was that the ball eyepieces only have a useful on axis performance of 5-10 degrees. So probably only really useful on planets. Most report very very good on axis performance, some stating that they perform better than TMB Supermonocentrics. One guy reported they performed the same. Still for $139, I think that is exceptional performance, if comfort and eye relief aren't to much of a concern for an individual. One thing most people that tried the ball eyepieces stated that a guided electronic mount is a must at high magnifications, because of the small AFOV.

Now I really want to try the Planesphere, but then I don't have a guided tracking mount, hell I don't even own an EQ mount. So it could be quite a gamble with an manual Azimuth mount. I guess I will really get a feel for the old times with a telescope at F15, manual AZ mount, and ball eyepiece! :D

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The contrast of the central image in a singlet sphere is simply outstanding.  I made myself a 6mm focal length sphere by purchasing the ball, around $30 USD if I recall, then finding an old eyepiece that I took apart that it would fit in.  If the opening is too big in the eye lens housing then just went to hardware store and looked for a rubber washer the ball would not slip thru in and trimmed that to fit in the eyepiece housing.  Then used more of those washers under it to hold in place until they extended to the retaining ring to lock in place. 

AFOV is extremely small.  Sharp central region is only about 10 degrees or a little more in my f/8 Apo.  After that the view is stretched and skewed...but usable for finding the target at least.  It really does need a tracking mount.  But if you are a fanatic at planetary then might be worth it for you.  Once, when I was trying for a specific feature on Mars that I had not been able to catch, I was finally able to see it with my 6mm focal length ball whereas it would not show up in my 6mm ZAO-II.  So the little bit of contrast gain can definitely be worth it.

I got the BK7 ball singlet, which was a precision spec called Techspec, at Edmundoptics.com - https://www.edmundoptics.com/optics/optical-lenses/ball-condenser-lenses/n-bk7-ball-lenses/.

Formulas to use are:

EFL = nD/[4(n - 1)]
ER = EFL-D/2

EFL = Effective Focal Length of Lens (mm)
ER = Eye relief (mm)
n = Index of refraction of the glass type (N-BK7=1.515089; S-LAH79=2.0)
D = Diameter of the ball (mm)

Basically, for different sized BK7 balls here are the results:

04mm Diameter = 2.9mm EFL / 0.9mm ER
05mm Diameter = 3.7mm EFL / 1.2mm ER
06mm Diameter = 4.4mm EFL / 1.4mm ER
08mm Diameter = 5.9mm EFL / 1.9mm ER
10mm Diameter = 7.4mm EFL / 2.4mm ER

Don't let the small ERs worry you too much.  Since the balls are rounded and typically extend above the surface of the housing, I find that they do not feel as bad as they look.  Not sure I'd spend the $140 or whatever for one of these given their very specialty use and ease in making one using some old eyepiece housing (and the Siebert build is below standard of even inexpensive consumer eyepieces, so less expensive and better build just to buy a GSO Plossl and use that housing).  Much more fun and rewarding to just make them.  Lots of sources besides Edmund Optical for these since they are used in a lot of manufacturing applications with laser sensors on assembly lines.

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Wow thanks Bill for a fantastic write up, very informative. I am tempted to have a go at building my own. I can find cheap eyepieces on Astroboot for less than £10. So that would serve as a eyepiece housing, get in contact with Edmund Optics UK for the ball lens, then I can get to work!

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You also might get some contrast improvement by taking an old symmetrical Plossl apart, removing one doublet, and flipping the remaining doublet so the convex surface is towards the eye and the flat surface toward the field (make sure it is the standard kind with flat and not concave surface).  This arrangement is evidently a Dolland eyepiece according to Thomas (aka Astrojensen from Denmark on the U.S. board).  So I just did it to a 20mm GSO Plossl I have.  When you do this it changes the focal length to maybe 30-35mm and the sharp portion of the AFOV maybe hits 15 degrees.  Barlows well on outdoor targets.  Will try it on celestial ones my next outing, Barlowing it and comparing that to some other premium eyepiece I may have of what looks to be similar magnification.

For that one what I did was to:

- first unscrew the barrel,
- then unscrew the retaining cylinder holding the lenses in,
- taking out the two doublets and spacer,
- replacing one doublet reversed,
- putting the spacer back,
- then cutting a strip of cardboard so its width about same as the doublet I removed, curling that cardboard into a cylinder and inserting that into eyepiece to fill the space left by the removed doublet,
- then putting the retaining cylinder back in which puts pressure on the cardboard cylinder I made which in turn puts pressure on the one doublet,
- then screwing barrel back on.

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Thanks again, now I will be ordering two cheap eyepieces to experiment. A Dolland eyepiece sounds very interesting indeed, and a name most astronomy history buffs should have heard of.

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