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Eyepiece designs

ATM eyepieces Maksutov optics
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#1 Benach

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Posted 12 November 2019 - 08:38 PM

Mike Jones and me have been sending a few mails back and forth last couple of days about a 64mm and 96mm eyepiece with an AFOV of 45 degrees. Why these characteristics: because it would be a nice extention to the already great Brandon line of eyepieces.

 

Currently Mike has designed a 64mm only and a 96mm is underway. This 64mm is more or less a asymmetric Erfle/Super-Plossl and the design is optimized for a 200mm f/15 Rumak. The latter can be found in Telescope Optics Evaluation and Design. Alas it does not use of the shelf optics. To get the best quality, a custom design is neccesary to use custom optics.

 

Another thingy of these eyepieces is that they have a huge fieldstop. It will not work in any conventional 2" focuser but a 3" focuser is the minimum. Personally, I intent to use a modified version of the Zeiss ring bajonet. At the moment I have no commercial plans with these eyepieces. However, if people are interested in these eyepieces, let me know, I might be willing to reconsider making a very small batch of these. To do this, at least five, preferably ten people should be willing to have these.

Below the Zemax data can be found. Underneath the Zemax data you can see a preliminary CAD-design of the same eyepice. 

 

msg-17464-0-42473900-1573395307_thumb.jp

msg-17464-0-14149900-1573395331_thumb.jp

msg-17464-0-85525700-1573395354.jpg

Oculair.JPG


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#2 luxo II

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Posted 12 November 2019 - 11:55 PM

Interested in 64 mm... could accommodate a 3” focusser on the back of my MC 250/3100...

Groan.... I’ll have to put 3” adapters on the rest of my eyepieces ... that’s going to make the eyepiece box weigh a ton...

Edited by luxo II, 13 November 2019 - 12:19 AM.

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#3 Benach

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Posted 13 November 2019 - 03:15 AM

For these eyepieces I am looking for those rubber rings that are commonly used as a grip around eyepieces. Does anyone know where to get these?

 

Luxo II: yup. To give you a spec now: this eyepiece will weigh about 750 grams. Photo of the BigMak?

 

edit: a more detailed drawing of the eyepiece mechanics has been made. Uploaded in this post.

Attached Thumbnails

  • Oculair.JPG


#4 Mike I. Jones

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Posted 13 November 2019 - 09:06 AM

Should be noted that this 64mm eyepiece is designed to work at f/15, and gives super-sharp imagery.  However, assuming a 7mm maximum exit pupil, the lowest f/# it can be used with is 64/7 or about f/9.  I'll have to tweak the design if anyone's contemplating use with an f/9 scope.

 

And if you think this one's big, wait till you see the 96mm!  The lowest focal ratio it can be used with is 96/7 = f/13.7!!

 

Eyepiece focal length = exit pupil x scope f/#

 

Notice that this is independent of aperture.  


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#5 Jon Isaacs

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Posted 13 November 2019 - 09:09 AM

Luxo II: yup. To give you a spec now: this eyepiece will weigh about 750 grams.

 

 

Not bad at all.  And here I was going to suggest adding a lifting eye for the crane.    :)

 

Jon


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#6 Benach

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Posted 13 November 2019 - 09:21 AM

John, PrestonE: can I add you to the list of interested people? I am fond of lightweight designs so this one is quite lightweight.



#7 BKSo

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Posted 14 November 2019 - 03:50 AM

I am just interested how you plan on making the optics. I am coming up with other eyepiece designs.

 

Also I built a 64mm (just 1.25") eyepiece last month for my 4" F/10 SCT and found it very effective for observing diffuse nebulae under dark sky. My experience is, at such large exit pupil the abbreviation is mainly due to the astigmatism of my own eyes (and I have 1.5 vision during the day). So the, coating, polish and quality of glass is probably more important than how the spot diagram looks in a ray tracing program.

 

Cheers.



#8 MKV

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Posted 14 November 2019 - 05:14 AM

I am just interested how you plan on making the optics.

Me too!!!


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#9 Benach

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Posted 14 November 2019 - 06:17 AM

If these eyepieces will ever be made, they will be made with optics that are made for me/us. I have no intention of making these ridiculous optics myself without a Zygo at my disposal. Especially the radii near the eyelens are something I fear for.

I have measured the front of my Ethoses and at least mechanically a Dioptrix will be attachable.
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#10 m. allan noah

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Posted 14 November 2019 - 07:07 AM

Looks like 25mm eye relief? I think you'll need to extend the housing a bit to help with eye placement?



#11 Benach

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Posted 14 November 2019 - 07:28 AM

The distance between the eye and the center of the eye lens's last surface is now 32mm. But what is a good relief? 20mm? 15mm? 17mm? Everything can be designed in this stage but let those who are interested please give their reasonings. BTW, in the latest design, the eye relief is 20mm measured from the top surface of the mechanics, which is approximately the same as if there is a rubber eyecup folded down.

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  • Oculair.JPG

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#12 luxo II

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Posted 14 November 2019 - 11:46 PM

20mm would be nice, 15 is ok.

I’d like to suggest something to protect spectacles from being scratched - a groove around the eyelens with a fat rubber O-ring in it, like the old Vixen LV 50mm has.

And no undercut on the barrel, please.

Edited by luxo II, 14 November 2019 - 11:48 PM.


#13 Bob4BVM

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Posted 15 November 2019 - 01:11 AM

For these eyepieces I am looking for those rubber rings that are commonly used as a grip around eyepieces. Does anyone know where to get these?

 

Luxo II: yup. To give you a spec now: this eyepiece will weigh about 750 grams. Photo of the BigMak?

 

edit: a more detailed drawing of the eyepiece mechanics has been made. Uploaded in this post.

Benach,

For the grip rings, try folks who make camera lenses, they source them from somewhere.

For my limited (amateur) DIY EP projects, I always seem to find the grip ring I need from my collection of old camera lenses.

Below is one of mine in the 3" focuser on my 6"f5 frac. Not near as complex as yours but performs well in that scope as a RFT. In its present configuration it is a takeoff of Martin's "di-plossl" config (2,2,1,1,2) ~ 45mmFL with a AFOV of just under 60*.

Has some really nice Nikon ED glass inside which I got lucky on. I weighs a bit over 3# but the scope can handle it.

I will say that the grip ring is essential on this beastie smile.gif

CS

Bob

 

IMG_5056s.jpg


Edited by Bob4BVM, 15 November 2019 - 01:13 AM.


#14 Benach

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Posted 15 November 2019 - 05:01 AM

Luxo II: the undercut you see is for the rubber ring. This protects the thread that is needed to assemble the two parts. The conical part on the left is the barrel. I do not use conventional barrels on my telescope but a derivative of a Zeiss bayonet. If these will be made, I can make it in such a way that this will become interchangable for a 3" barrel with(out) undercut.

 

Photo of the old 50mm LV? We have a 50mm LV at my club but I do not recall seeing something that fits your description.

 

Bob4BVM: looks nice. Do you have a contact adress of Nikon/Canon/Whoever?


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#15 luxo II

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Posted 15 November 2019 - 05:41 AM

OK ... I sold mine a few years ago (and regretted it afterwards) but there are some pics on the net.

 

In essence, around the eyelens is a plane flat flange, maybe 8mm wide,  perpendicular to the optical axis. In that face a groove is machined around the eyelens, and in that isa fairly thick o-ring is simply pressed in (maybe 5mm thick) arranged so that anyone wearing specs (ie me) won't scratch the specs up close to it in the dark.

 

Considering coated prescription lenses cost $500 a pop here its worth having. It also just might help if the eyepiece is placed eyelens down on a hard surface.

 

The o-ring is clearly visible in the two pics.

Attached Thumbnails

  • LV50 - 1.png
  • Vixen LV's.png

Edited by luxo II, 15 November 2019 - 05:47 AM.


#16 Benach

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Posted 15 November 2019 - 06:12 AM

Okay, thought the same. Will check out at my observatory if our eyepiece has this. Never noticed it anyway. But this feature is now included in the design. But please: photo of the BigMak?



#17 steveastrouk

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Posted 15 November 2019 - 01:14 PM

What's wrong with a knurl instead of the rubber parts ? Too cold outside ? Not enough friction on cold fingers ? I could easily make a mould for a two part rubber casting, if the idea takes off.



#18 Benach

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Posted 15 November 2019 - 01:32 PM

A knurl is doable but I like to wrap my hands around rubber instead of bare aluminium when it is cold.



#19 Jon Isaacs

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Posted 15 November 2019 - 02:00 PM

I do not wear eyeglasses at the eyepiece but those who do seem to like 20mm of real eye relief, i.e. the way you are measuring it.

 

Since everything is still flexible, have you consider increasing the AFoV at bit, maybe to 100 degrees?  :)

 

(Sorry, I just couldn't help myself.. I just can't resist a good straight line.)

 

Jon



#20 Benach

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Posted 15 November 2019 - 02:18 PM

Jon: go to Mike and ask him to design this. But I think this will consist of at least ten lenses and dimensions that are way beyond any amateur means. :)


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#21 MKV

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Posted 15 November 2019 - 04:55 PM

But I think this will consist of at least ten lenses and dimensions that are way beyond any amateur means. 

 No kidding? :o) You wanna to take an honest guess how much would a single eyepiece of this type cost to produce? Clearly, someone would have to grab the design (since it's public knowledge) and start a mass-production to make it cost-effective, but then the overseas companies with cheap labor would quickly catch up and take over the business. So, what's the point?   


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#22 Benach

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Posted 15 November 2019 - 05:09 PM

The point is that the current trend is to design widefield eyepieces that are sharp for a fast telescope. These are often also good for slow telescopes but have the disadvantage that the TFOV and exit pupil is relatively small for slow telescopes. Let me give an example: Let's use the 200mm f/15 Maksutov Mike has used for calculations. With a 21mm Ethos, I will have a magnification 142x and a TFOV of about 42 arcminutes. With a 64mm eyepiece with a AFOV of 45 degrees, I will have a magnification of 47x and a TFOV of about 1 degree. Because the magnification is so much lower, the exit pupil is about three times as big. I do not have to explain the advantages of a big exit pupil, do I?

 

That those Chinese companies can copy this design, sure, but then they will be making something for a niche (slow high contrast telescopes). This is against most trends of current telescope use.

 

And about the costs, I do not speculate yet. First I want to know what the design should be, then we can look for people that are interested in having one and then we can see what various companies are offering for making and assembling these. If all that is well, and I still like the whole idea, I might make a small batch. Until then, this is a modern version of Lovin' Spoonful: "what a day for a daydream".



#23 MitchAlsup

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Posted 15 November 2019 - 08:07 PM

I seem to recall that either Lick or Yerkes has a 100mm 4" EP of Huygens architecture.

Both of these telescopes are long focal ratio scopes, similar to the design at hand, although not Cassegrainian.

Why not use such a design here?



#24 Jon Isaacs

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Posted 15 November 2019 - 08:15 PM

The point is that the current trend is to design widefield eyepieces that are sharp for a fast telescope. These are often also good for slow telescopes but have the disadvantage that the TFOV and exit pupil is relatively small for slow telescopes. Let me give an example: Let's use the 200mm f/15 Maksutov Mike has used for calculations. With a 21mm Ethos, I will have a magnification 142x and a TFOV of about 42 arcminutes. With a 64mm eyepiece with a AFOV of 45 degrees, I will have a magnification of 47x and a TFOV of about 1 degree. Because the magnification is so much lower, the exit pupil is about three times as big. I do not have to explain the advantages of a big exit pupil, do I?

 

That those Chinese companies can copy this design, sure, but then they will be making something for a niche (slow high contrast telescopes). This is against most trends of current telescope use.

 

And about the costs, I do not speculate yet. First I want to know what the design should be, then we can look for people that are interested in having one and then we can see what various companies are offering for making and assembling these. If all that is well, and I still like the whole idea, I might make a small batch. Until then, this is a modern version of Lovin' Spoonful: "what a day for a daydream".

 

Hi:

 

I think I understand the motivation behind these eyepieces.  I am one of those fast scope guys who uses large (7mm+) exit pupils to maximize the brightness of the image.  At F/15, you are not going to get the combination of the wide field of view and bright image possible at F/5 but at least you can get that bright image when that is an advantage. 

 

With this eyepiece, you will get a 4.3mm exit pupil, similar to the 21mm Ethos at F/5.  With the 96mm Eyepiece, you will get a 6.4mm exit pupil, similar to the 31mm Nagler at F/5. 

 

People who use fast scopes tend to think in terms of field of view and forget image brightness because it's so easy.  But when one is using slower scopes, then it requires a different mindset.

 

In the eyepiece world, the 40mm SWA and the 55 mm Plossl max out the field in a 2 inch format, these both provide a 53 arcminute field of view, with 2.7mm and 3.7mm exit pupils.. Still dim.. You and Mike are designing and building eyepieces that are better suited for these long slow scopes.

 

Jon



#25 Bob4BVM

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Posted 15 November 2019 - 08:18 PM

Luxo II: the undercut you see is for the rubber ring. This protects the thread that is needed to assemble the two parts. The conical part on the left is the barrel. I do not use conventional barrels on my telescope but a derivative of a Zeiss bayonet. If these will be made, I can make it in such a way that this will become interchangable for a 3" barrel with(out) undercut.

 

Photo of the old 50mm LV? We have a 50mm LV at my club but I do not recall seeing something that fits your description.

 

Bob4BVM: looks nice. Do you have a contact adress of Nikon/Canon/Whoever?

NO addresses. The rubber grips I have used are all from surplus/salvaged camera lenses. Not what you'd do in a production setup for sure. You might give Fred a call at Surplus Shed, he has some hooks on such & may be able to supply you.

Regards cost, I'd guess what you are proposing would land in the $3-5Grand range based on what is out there. Which is why the only 3" EPs I own are of my own making. Something special about an EP like that to me especially in a RFT, where the big barrel opens new possibilities for the TFOV ! :)

Anyway what you are doing is very interesting, I wish you every success.

CS

Bob




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