27 replies to this topic

[KarlL]

Is a ball lens eyepiece likely to do well in an f/8 reflector? I had success with Galilean singlets in a C6 a few years ago. 

 

The second question is that since most ball lens eyepieces are DIY, How do most constructors fix the lens in a housing?

 

Thanks to all, as always.

[tony_spina]

good question Karl... This is one of the few eyepiece types I have never tried but would love to do so.. hopefully we get some good replies

[macdonjh]

If you do not want to build one, check out Siebert Optics. He makes them.

[junomike]

IME Tracking it almost a necessity for such a small FOV.

[KarlL]

The Sieberts look good, but they’re too expensive for an experiment. Anyway, I like small DIY projects.

[KarlL]

Mike -

 

The AFOV of the Galilean singlets was probably about 10 degrees. So, yes, I was constantly moving the tube. The clarity & resolution exactly down-the-middle made it worth It on Jupiter, especially using Brandon magenta & salmon filters (with an adaptor). It was like looking down a soda straw. Not an eyepiece for everyone, by any means. For clarity & resolution, it’s the best eyepiece I’ve used, and I’ve looked through a number of excellent pieces of glass. I would put my Brandons second & Baader BGOs third. ALL of these work best doing SLaP.

Edited by KarlL, 24 April 2019 - 09:28 AM.

[KarlL]

Tony -

 

I believe one constructor used a drilled-out rubber stopper. I have a small drill press, so that would work. I also have threaded eyepiece barrels. I might attach the stopper using E6000 adhesive, because it remains somewhat flexible after drying, and that would compensate for temperature changes affecting dissimilar materials. I’m thinking of this inexpensive route because the lens itself is not cheap from EO.

[leonard]

Is a ball lens eyepiece likely to do well in an f/8 reflector? I had success with Galilean singlets in a C6 a few years ago. 

 

The second question is that since most ball lens eyepieces are DIY, How do most constructors fix the lens in a housing?

 

Thanks to all, as always.

 

           Hello ,

 

                     F8 , yes they will do OK at that focal length.

For the one I have I used an old Unitron 9mm sym.ach , that’s what printed on the eyepiece which I take to mean

 symmetrical achromat . Just by luck my 10mm ball fits the eye lens opening well and just protrudes about 1mm

above the top surface . The thing that really makes it work for me is with the old guts removed it has a screw in

if I remember correct , a field stop plate that is screwed up to the bottom of the ball or I made , out of washer material

a round plate with a hole drilled into it that presses up to the bottom of the ball with the field stop plate screwed up to it to hold in place. Guessing lot of ways to do this . 

[KarlL]

Leonard -

 

Excellent!

 

Where did you get the lens? How does it perform?

 

Regards,

 

Karl

[leonard]

Hello ,

 

 

             Edmund Optics , N-BK7 10mm .     I wish they made a 12mm.

 

     How does it perform ?   Small FOV for sure , useable maybe 12 degrees , but that’s fine for planets .

Eye relief is , well short maybe 2mm or so . I personally would never recommend using it without a tracking

mount. Image quality is very good , have I performed tests and compared it to say Tom Back Monocentrics

or Brandon’s ?  No I have not . Why ? at my age I would never be a good judge , that’s for others more capable

to say . Like you maybe . The big factor will be as always the seeing , if you have less than a Pickering 7 night

of seeing IMO your wasting your time trying to judge high power images . Quite a few people have praised the

ball lens as producing some of the highest contrast images they have seen .

   PS - you may want to try an achromatic doublet on planets , I have used a couple and they do perform well also .

[RichA]

Is a ball lens eyepiece likely to do well in an f/8 reflector? I had success with Galilean singlets in a C6 a few years ago. 

 

The second question is that since most ball lens eyepieces are DIY, How do most constructors fix the lens in a housing?

 

Thanks to all, as always.

You can buy ball lenses from Edmund Optics, glass or sapphire.  If you have an old eyepiece with a short f.l. it can be adapted easily enough to hold the ball, or if you have access to someone with lathe, can make one.  Read also about Leeuwenhoek microscopes, which used small ball lenses.  It's also possible that the crude way the single lenses were made resulted in inadvertent aspheres being created which would have produced a more usable field.

[BillP]

As Leonard said, small usable AFOV.  In my f/8 it is between 10-15 degrees.  The AFOV is a lot larger just that the central 10-15 degrees is the area where it is sharp without aberrations.  I use the Edmund 8mm BK7 Ball which gives an effective focal length of 5.9mm.  I just mounted it in an old volcano top Ortho I had.  Went to hardware store and got a small o-ring gasket so the Ball fit well.  Contrast is unbelievably good...very much beats my 6mm ZAO-II.  6mm eyepiece gives me about 140x in my TSA-102 and I am fine without tracking even with its small usable FOV.  While the ER is very short, it feels ok for me as the ball is in a volcano top design eyepiece and the ball protrudes well above the top surface so ER feels ok even though short (1.4mm).

 

Using the BK7 Ball here is the effective focal length for each diameter Ball:

 

4mm Diameter Ball = 2.9mm FL Eyepiece
5mm Diameter Ball = 3.7mm FL Eyepiece
6mm Diameter Ball = 4.4mm FL Eyepiece
8mm Diameter Ball = 5.9mm FL Eyepiece
10mm Diameter Ball = 7.4mm FL Eyepiece

[Kent10]

"very much beats my 6mm ZAO-II"

 

Bill, that is impressive.  Contrast is better.  Is there also less scatter?  Is the ball sharper?  Does this mean you would choose the ball over your ZAO II when viewing planets or do you only use it rarely?  If rarely, is that because you prefer the wider FOV or more comfort?

 

I will be using a tracking mount soon, so I may have to try some of these even though I have Zeiss and TMB monos.  I like that Siebert has focal lengths in 0.5mm steps.

[BillP]

On-axis they both are as crisp and resolute.  Never took notice of scatter differences but would guess less since only 2 polished surfaces vs 8 surfaces on an Abbe.  Contrast is very noticeably starker which is what makes planetary details pop all the more.  Use it strictly for planetary as not suited for other targets given the very small usable AFOV. 

 

Not sure how Siebert is achieving the .5mm increments unless the ball glass types are all different index of retractions (so not all BK7) or he is getting custom ball diameters.  The formula to calculate the effective focal length of a ball singlet is:

 

EFL Ball Singlet = nD/[4(n - 1)]

n=glass index (1.515089 for N-BK7)

D=diameter of ball in mm

ER Ball Singlet = EFL-(D/2)

Edited by BillP, 25 April 2019 - 07:40 PM.

[Kent10]

Thanks Bill.  Siebert has BK7 and Fused Silica.  I was thinking the other day, that I wished I had a nice 4.5mm ortho or mono but perhaps a Ball will have to do.  Too bad it won't be good for anything but planets.  I may still take a look at a double star or small cluster though.

 

https://www.sieberto...lanesphere.html

[BillP]

If you use a good Barlow with a good Ortho or Mono you will not lose anything really.  So if you have a good 2x then figure out the offset distance you will need to seat the eyepiece a little higher to get it to operate at 2.2x, then put a 10mm Mono in there and you will have your 4.5mm effective focal length eyepiece.

[Kent10]

Yes, thanks Bill.  I have done this with my Zeiss 2X Barlow and 10mm Zeiss mono.  I don't know what the powers were that night.  But I started at 2X and kept adding extensions.  I had the best views of Saturn that night.  Still haven't seen better.  I intend on doing that again when the seeing is exceptional.  The problem is exceptional seeing doesn't usually last for me.  It is usually only at dusk.  I haven't viewed in the middle of the night when it might also be great.

[NinePlanets]

OP: Yes, ball lenses provide excellent contrast. I have used a ~5mm EFL in both a Dall-Kirkham as well as in an f/6 Newtonian push-along scope and, even though the apparent FOV is small, it only gradually gets worse off-axis so it is not that big of a deal to keep the push-along action going.

 

The night I got to use this lens was one of exceptional seeing at high altitude but in there was quite a bit of forest fire smoke in the sky. With normal high-quality eyepieces, Saturn's crepe ring was intermittently visible. With the ball lens it was starkly and continually present.

 

Yes, it is tempting to have one in the eyepiece case for special occasions.

Edited by NinePlanets, 25 April 2019 - 08:26 PM.

[KarlL]

The clarity of the singlets I experimented with a few years ago has stuck with me. Looking at sunspots in white light was memorable, as was Jupiter.

 

Now it’s a matter of seeing what I have or what I can get to mount a ball lens. I’m considering a few very cheap 23MM barrel-diameter microscope eyepieces from Surplus Shed. (I have custom adaptors, threaded for 1.25” filters.) These are Huygenian, with a small eyelens hole. Alternatively, I could 3D-print a volcano top at our library, bore it out, and attach it to a 1.25” threaded barrel I have.

[rhcrooks]

Ball eyepieces are just singlet lenses right? Like the cheap hand-held common magnifying lens?

[KarlL]

It’s a spherical lens used as a singlet in an appropriate eyepiece holder. Most use the BK-7 lenses from Edmund Optics. The 10MM diameter lens is ~ $42 & has a focal length in the neighborhood of 7.5MM. They are precision optical elements.

 

Have you worked with any kind of singlet?

[rhcrooks]

Thanks for the info - no I've never had the chance to see or look though one. Perhaps one day.

[Fivemileshigh]

As Leonard said, small usable AFOV.  In my f/8 it is between 10-15 degrees.  The AFOV is a lot larger just that the central 10-15 degrees is the area where it is sharp without aberrations.  I use the Edmund 8mm BK7 Ball which gives an effective focal length of 5.9mm.  I just mounted it in an old volcano top Ortho I had.  Went to hardware store and got a small o-ring gasket so the Ball fit well.  Contrast is unbelievably good...very much beats my 6mm ZAO-II.  6mm eyepiece gives me about 140x in my TSA-102 and I am fine without tracking even with its small usable FOV.  While the ER is very short, it feels ok for me as the ball is in a volcano top design eyepiece and the ball protrudes well above the top surface so ER feels ok even though short (1.4mm).

 

Using the BK7 Ball here is the effective focal length for each diameter Ball:

 

4mm Diameter Ball = 2.9mm FL Eyepiece
5mm Diameter Ball = 3.7mm FL Eyepiece
6mm Diameter Ball = 4.4mm FL Eyepiece
8mm Diameter Ball = 5.9mm FL Eyepiece
10mm Diameter Ball = 7.4mm FL Eyepiece

Please forgive my bringing this back from the past and the daft question, but where is the EFL measured from? The center of the sphere or the surface? I would like to make a pair of these and I'd like to know where to put the field stop.

 

Many thanks,

 

Dumitru

 

EDIT: Nevermind, I found the answers:

 

https://www.edmundop...ing-ball-lenses

Edited by Fivemileshigh, 13 August 2022 - 10:13 PM.

[SteveC]

Please forgive my bringing this back from the past and the daft question, but where is the EFL measured from? The center of the sphere or the surface? I would like to make a pair of these and I'd like to know where to put the field stop.

 

Many thanks,

 

Dumitru

 

EDIT: Nevermind, I found the answers:

 

https://www.edmundop...ing-ball-lenses

I hope you are using a tracking mount, otherwise your efforts will be in vain. When you say a pair,  I hope you're not going to use them in a binoviewer. 

[Fivemileshigh]

I hope you are using a tracking mount, otherwise your efforts will be in vain. When you say a pair,  I hope you're not going to use them in a binoviewer. 

Yes, there is (or rather will be) tracking.

 

And yes, I did plan to use a binoviewer. Is the extra glass going to delete the advantages?

 

Thanks!