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Eyepiece designs

ATM eyepieces Maksutov optics
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#26 luxo II

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Posted 15 November 2019 - 10:57 PM

OTOH this is never going to appeal to those with fast dobs, not even the f/7 refractor types and being 3" it’s going to be very much a niche product just as Sieberts are, nor is it something that will be sufficient to influence a company bottom line. It just might appeal to those with SCTs big enough to take the 3” backend - C11 and larger.

In this respect it's probably safe from competitors.

Now... as to the design... by my calculations a focal length of 64mm and AFoV of 47.9 means the field stop is 53.5mm diameter. In my scope (250/3100) that gives a field of view of 1.0 degree and a 5.1mm exit pupil, which indeed would be quite nice.

At present I use a 2" 38mm SWA which has a field stop that gives about 0.6 degree, just enough for the moon plus a tad, and an antique 2" 30mm erfle of some sort. And I'll admit I do miss the 50mm LV. Star clusters looked like diamonds on black velvet.

OK... so some pics for you, apologies - I've set it up in my basement as the weather here is insane, either 39 C with bushfires or a wild southerly gale and overcast.

- OTA on the mount, under that Matthias Wirth lid is the most exotic soupbowl I'll ever own;
- Assembled with finder, diagonal and the 30mm erfle;
- The business end - focuser is a TS UNC 2" steeltrack crayford, built like a tank. I also have a motor drive for the focuser, but haven't fitted it yet. This thread is making me regret not getting the 3" version, but hey it's possible to retrofit it because the way Matthias has built the OTA is like a Meccano set, it is easy to modify.

In the third pic you may notice I have mounted the finder forward on a vixen dovetail - this turns out to be way, way, more convenient than having at the backend of the scope. With careful alignment of the finder and main scope, I can calibrate the mount in altaz mode well enough that GOTO's will put targets in the field at 200X so most of the time the finder isn't needed.

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Edited by luxo II, 16 November 2019 - 12:51 AM.

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#27 luxo II

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Posted 16 November 2019 - 12:07 AM

And one more... with a van Slyke turret, loaded with Fujiyama ortho's and a finder eyepiece...

Interestingly that turret require slightly LESS backfocus than the APM diagonal with rotolock.

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#28 Benach

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Posted 16 November 2019 - 02:31 AM

Luxo II: Like!

Bob: will check out Surplusshed. Thanks.

Mitch: Huygens eyepieces are limited to a AFOV of about 25°. Apart from that, the outer field is often not well corrected, even at f/15. These will have a AFOV of 45° and are well corrected over the entire FOV.

MKV: Luxo II and Jon just nailed what the purpose of these eyepieces is. Currently most eyepieces tend to be used on fast Newtonians and use more lenses to achieve a huge TFOV. These eyepieces use less lenses for a modestly large TFOV, something that was not possible with current widefield eyepieces. Make for example the calculations for the magnification, TFOV and exit pupil for a 200mm f/15 and a 21mm Ethos eyepiece and you see where I am going at.
"Chinese" in Dutch is proverbial for any low-cost East-Asian producing country: Korea, China, Taiwan etc.
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#29 luxo II

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Posted 16 November 2019 - 02:39 AM

An old-school Erfle scaled up to 65 mm should be quite good too - and give a field of view as much as 60-70 degrees.

Now if only I could find one of those old war surplus tank eyepieces ...

Edited by luxo II, 16 November 2019 - 02:40 AM.


#30 BGRE

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Posted 16 November 2019 - 03:05 AM

Erfles tend to have significant astigmatism at the edge of the field. Scidmore eyepieces are better but have more elements.



#31 luxo II

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Posted 16 November 2019 - 04:52 AM

True...

#32 Ed Jones

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Posted 16 November 2019 - 01:20 PM

I had the same problem with my 10 inch F/17 window scope needing a larger field and lower power.  I ended up making a 3 inch OD eyepiece with lenses I had on hand.even though I had the ability and access to optical machines, mine was good enough.  It's an achromatic Huygens and has some aberrations in the outer 1/3 of the field that covers about 3/4 degree.  

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#33 MKV

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Posted 17 November 2019 - 03:57 AM

MKV: Luxo II and Jon just nailed what the purpose of these eyepieces is. Currently most eyepieces tend to be used on fast Newtonians and use more lenses to achieve a huge TFOV. These eyepieces use less lenses for a modestly large TFOV, something that was not possible with current widefield eyepieces. Make for example the calculations for the magnification, TFOV and exit pupil for a 200mm f/15 and a 21mm Ethos eyepiece and you see where I am going at.
"Chinese" in Dutch is proverbial for any low-cost East-Asian producing country: Korea, China, Taiwan etc.

Benach,

 

I didn't ask what's the purpose of these eyepieces but what's the point of revealing their design. Why hand competitors someone else's work for free so they can profit from it?  You already know what the eyepieces are supposed to do. The only way you'll know if they will is if you actually make them, and you know very well that making numerous custom  lenses of that type will be expensive. So, so do you exactly plan to make them?

 

Regarding "those Chinese" expression and the Dutch language, I am not concerned with idiomatic nuances of Dutch because this is an English language Forum. My objection was to your use of "those Chinese" in your reference to what I wrote earlier, and I never wrote such a phrase, nor did I mention the Chinese specifically in any context. 

 

All the best,

Mladen


Edited by MKV, 17 November 2019 - 03:58 AM.


#34 jjack's

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Posted 17 November 2019 - 03:59 AM

Have you ever try to make a plossl or a 3 element eyepiece with 50 mm objective lenses from binoculars ?

I had seen this at a star party and it work pretty well.



#35 luxo II

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Posted 17 November 2019 - 04:58 AM

Yes I did, long ago - I had several surplus objectives from 30mm binoculars. Two achro doublets with convex sides face to face did indeed make quite a nice plossl - but that's expected as that's exactly what that eyepiece consists of. With a third doublet it also worked but not so well.

 

Also made a Ramsden, Huygen and Kellner eyepieces in my glass-grinding days... more out of curiosity than anything else.


Edited by luxo II, 17 November 2019 - 05:01 AM.


#36 Jon Isaacs

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Posted 17 November 2019 - 09:24 AM

Now... as to the design... by my calculations a focal length of 64mm and AFoV of 47.9 means the field stop is 53.5mm diameter. In my scope (250/3100) that gives a field of view of 1.0 degree and a 5.1mm exit pupil, which indeed would be quite nice.

 

At present I use a 2" 38mm SWA which has a field stop that gives about 0.6 degree, just enough for the moon plus a tad, and an antique 2" 30mm erfle of some sort. And I'll admit I do miss the 50mm LV. Star clusters looked like diamonds on black velvet.

 

 

Something doesn't seem quite right. The 38 mm SWAs like the Orion Q-70s have a 45.7 mm field stop. That would provided a 0.84° TFoV. Maybe you have a different eyepiece? If so, the Q70 might be worth buying.

 

As an aside, I've always wanted to look through a 38mm Q70 in a slow scope. I had a 20 mm Expanse which was quite messy at F/5 but quite magical at F/11.3... I even put together an 80 mm F/11.7 With a 2 inch focuser just to see but never came up with the eyepiece. 

 

So here's a question:

 

The barrel of a 2 inch eyepiece is about 48 mm in diameter.  This eyepiece has a field stop of ~53 mm.  There are 1.25 inch eyepiece designs that sneak in field stops in the 30-31 mm range by placing the field stop behind the barrel and accepting some vignetting. 

 

It seems a difficulty with this eyepiece is the need for a 3 inch diagonal etc. Maybe at least a version compatible with 2 inch stuff is worth considering.

 

Jon



#37 Mike I. Jones

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Posted 17 November 2019 - 10:17 AM

Eyepieces have considerable distortion.  Field stop diameter and eyepiece focal length are not directly related to angular FOV.  You have to account for distortion, which is not linear with image height. 



#38 Benach

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Posted 17 November 2019 - 02:15 PM

MKV: I may be making them but if and only if people are interested in joining me. If people aren't, then also too bad for me. But we will see if people will show any interest.


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#39 Jon Isaacs

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Posted 17 November 2019 - 02:25 PM

Eyepieces have considerable distortion.  Field stop diameter and eyepiece focal length are not directly related to angular FOV.  You have to account for distortion, which is not linear with image height. 

 

True. But with a 48 degree AFoV, the distortion is usually quite limite.

 

What is the field stop diameter?

 

Jon



#40 Benach

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Posted 17 November 2019 - 03:04 PM

Jon: 53.8mm.

#41 Jon Isaacs

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Posted 18 November 2019 - 08:33 AM

MKV: I may be making them but if and only if people are interested in joining me. If people aren't, then also too bad for me. But we will see if people will show any interest.

 

My suggestion is to try to make the eyepiece workable with a 2 inch focuser, that way you just might get more people interested.  Vignetting is not always so easy to see.

 

Jon



#42 macdonjh

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Posted 18 November 2019 - 09:07 AM

My suggestion is to try to make the eyepiece workable with a 2 inch focuser, that way you just might get more people interested.  Vignetting is not always so easy to see.

 

Jon

I've been interested in Mr. Siebert's really long focal length eye pieces, but have never acted on my curiosity because of the cost.  It's not just the eye pieces: I'd have to buy a 3" diagonal and convert at least one of my slow scopes to a 3" focuser, too.  I realize my f/12 and f/15 won't ever be mistaken for RFTs, and am fine with that.  I think what I really want is outside the realm of what's possible in optics: a 2" barrel without noticeable vignetting, focal length of 70mm or longer, AFOV of 50o or so and eye relief of 20mm or less (I don't want to have to stand across the room to see the image).  Actually, Gary Russell talked me out of one of his Super Plossls (I think the 80mm) because of really long eye relief and an image that wouldn't be that much different than the 50mm Konig I already have.

 

Good luck with this project.  It's interesting to follow.


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#43 Jon Isaacs

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Posted 18 November 2019 - 09:18 AM

I think what I really want is outside the realm of what's possible in optics: a 2" barrel without noticeable vignetting, focal length of 70mm or longer, AFOV of 50o or so and eye relief of 20mm or less (I don't want to have to stand across the room to see the image).

 

I am thinking of the Celestron Ultima 35mm eyepiece.

 

Typically 1.25 inch eyepieces are limited to 27mm field stops because of the 28mm filter thread/barrel diameter.  The 35mm Ultima is a 1.25 inch eyepiece with a 30mm field stop.  This is accomplished by placing the field stop behind the barrel.  I have never looked through one but apparently the vignetting is acceptable. 

 

This eyepiece isn't your 70mm 50 degree but it is a 64mm 48 degree eyepiece and if I could be fit in a 2 inch format, it would be something more people could use. 

 

Jon



#44 macdonjh

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Posted 18 November 2019 - 09:58 AM

 

This eyepiece isn't your 70mm 50 degree but it is a 64mm 48 degree eyepiece and if I could be fit in a 2 inch format, it would be something more people could use. 

 

Jon

Depending on eye relief, the 64mm might be interesting to me.  True field wouldn't be that much different from what I have available (0.58o compared to 0.49o for my 40mm and 0.57o for my 50mm), but exit pupil would "balloon" to 4.3mm.

 

Just doing some simple math: for the 96mm: magnification of 55x, TFOV 1/2o or so (assuming a 30o AFOV is possible) and a 6.4mm exit pupil.  That would be something. I would still want eye relief in the 15mm- 20mm range rather than the 60mm- 75mm range.

 

For my scopes it would still be a specialized eye piece.  Kind of like an OIII or H-B filter.  I wouldn't use it that often since it would really be trying to make my scopes something they're not.  But, I can say sweeping the Veil with a 6.4mm exit pupil might be something to get excited about.  I'm only capable of doing that with 10" of aperture at this time.



#45 luxo II

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Posted 18 November 2019 - 03:57 PM

Jon, The point of going to 3" focuser is to accommodate a wider true field of view (ie the field stop) than will fit in a 2" barrel. My scope and the MK91 can comfortably fill a field stop more than 60mm, as could a C11 or C14.

 

This is not an eyepiece for fast scopes or small SCT's - such as those in your sig,

 

OTOH to fit a 64mm eyepiece in a 2" barrel it would have to be limited to something like 35-40 degrees AFoV which is a bit pointless because there are several other 2" eyepieces with no field stop - I have two - an Erfle and 38mm SWA, so this really isn't going to show a larger true field of view.

 

Secondly if this happens it will be a very small volume and at high cost - there is simply no point making "yet another 2" hand-grenade" when there are already plenty of others in 2" barrels for far less - the TMB Paragon, Vixen LV50, LVW42, a couple of Masuyama's and several Televues come to mind for starters.


Edited by luxo II, 18 November 2019 - 04:07 PM.

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#46 Benach

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Posted 18 November 2019 - 04:37 PM

Luxo II: you nailed it again. Besides: I want these eyepieces to be a logical extension to the Brandon series of which I am very fond of. That is why the optical properties are a direct extention of this design.

 

To be honest: I still have no idea about the prices. But I know this: if you start comparing the costs of this eyepiece to a regular eyepiece, I am sure you will think these eyepieces will be extremely expensive. Couldn't agree more. But that is mostly because normal eyepieces (and camera objectives) are made for a bargain if you would compare it. Yes, this even applies for TeleVue Ethoses/Naglers. Normal prices of custom lenses is in the order of $500.- per element if the batches are small so go figure.


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#47 MKV

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Posted 18 November 2019 - 07:04 PM

To be honest: I still have no idea about the prices. But I know this: if you start comparing the costs of this eyepiece to a regular eyepiece, I am sure you will think these eyepieces will be extremely expensive. Couldn't agree more. But that is mostly because normal eyepieces (and camera objectives) are made for a bargain if you would compare it. Yes, this even applies for TeleVue Ethoses/Naglers. Normal prices of custom lenses is in the order of $500.- per element if the batches are small so go figure.

That's exactly the point I was making earlier, Benach. I didn't want to be the bearer of bad news, because I know how much special order lenses cost. The price tag of about $2500 for the five lens elements alone is not unreasonable. Of course, the housing will add a couple of hundred more per EP (because it will be a special order no matter how many you order); it's not a contract for an ongoing production, and setting up production (templates, jigs,etc.) is what drives up the cost.

 

If the eyepieces turn out to be a success, the competition will jump on it immediately and sell the entire eyepiece for about $500, just as Ethos EPs sell for. How's that going to make you feel for having paid several times that price? And for what? It's a shame, because they didn't have to design or invent anything! The idea was served to anyone who wants to run with it on a silver platter for free. This is why prudence calls for patents, but if a design is made public knowledge, all bets are off.

 

It may be a worthy project to investigate if one can put together a decent EP of this type with stock lenses. That would cut down the cost significantly. Anyway, good luck to you and anyone else who wants to try this otherwise very interesting idea.

 

Mladen


Edited by MKV, 18 November 2019 - 07:06 PM.

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#48 Benach

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Posted 18 November 2019 - 10:02 PM

Seeing that this is is going against current trends, I think it will not go so fast. Besides, a copy of an eyepiece is quickly made without even having to disassemble it. So yeah, I see your point but I doubt it will happen. You see now already how many people are interested. Barely a big fuss, which I expected. Not in the least because 3" eyepieces are rare for most amateurs.

#49 MKV

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Posted 19 November 2019 - 03:16 AM

I certainly hope you're right Benach. It would be a shame if it didn't, but it's unlikely in my opion too masny will join you in procuring an EP that's just shy of $3K.

 

 

I agree that eyepieces can be copied (you buy one and take it apart and then return it), and so can just about any product, but that's why copyright laws and patents exist -- as a theft insurance. 

 

http://astronomie.ra...ethos_x-ray.jpg



#50 Benach

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Posted 19 November 2019 - 04:13 AM

Check out the endless number of eyepieces that already have been copied, eg. it is no coincidence that a 55mm Televue Plossl exists next to a 56mm Bresser Plossl, and you know how difficult optical patents are these days. But that is another discussion.


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