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Anyone still using Ramsdens?

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#1 jimeh

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Posted 29 January 2025 - 05:56 PM

I found this in a box of random astro gear I'd forgotten about, so I gave it a go last night and it was surprisingly good. Perfect for those nights when you want low magnification and a narrow FOV.

 

Compared to a 40mm UO kellner, I would guess this to be a (+/-) 50mm Ramsden with a FOV somewhere in the 30s.

 

Anyone have any idea what this might have been paired with? It's in good shape, has a 1.25" barrel with no marks, and it's 100% brass and glass

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#2 triplemon

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Posted 29 January 2025 - 06:21 PM

Compared to a 40mm UO kellner, I would guess this to be a (+/-) 50mm Ramsden with a FOV somewhere in the 30s.

Given the optical prescriptions of a ramsden, its effective FL is aproximately equal to the spacing of its lenses. 

 

Anyone have any idea what this might have been paired with? It's in good shape, has a 1.25" barrel with no marks, and it's 100% brass and glass

It was most likely used with a long focal ratio refractor. Even at F/15 they can be pretty usefull.


Edited by triplemon, 29 January 2025 - 06:23 PM.

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#3 Jay_Reynolds_Freeman

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Posted 29 January 2025 - 06:39 PM

Ramsdens used to be very common. They don't work well with fast focal ratios -- below about f/7, forget it. A classic Ramsden, comprising two identical plano-convex lenses with the convex sides facing one another and separated by their focal length, is extremely well corrected for lateral color (also known as chromatic difference of magnification), notwithstanding that neither lens is an achromat: Jesse Ramsden was a smart man. It also helps greatly if the lenses are well-polished, as was the case when lots of optics were military-surplus and had military-specification-grade polish, and are coated.

 

There is an article on my web site that talks a little more about why Ramsdens work so well, and talks a bit about my own set of them.


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#4 RichA

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Posted 30 January 2025 - 01:40 AM

I found this in a box of random astro gear I'd forgotten about, so I gave it a go last night and it was surprisingly good. Perfect for those nights when you want low magnification and a narrow FOV.

 

Compared to a 40mm UO kellner, I would guess this to be a (+/-) 50mm Ramsden with a FOV somewhere in the 30s.

 

Anyone have any idea what this might have been paired with? It's in good shape, has a 1.25" barrel with no marks, and it's 100% brass and glass

I remember in the 1980s a company called "Augen Optics" that made Huygens, not certain about Ramsdens.  Huygens had issues, but i can't help wondering if in some cases of serendipity, the aberrations of they eyepiece might have an opposite effect to the aberrations in say an achromat.Like longitudinal chromatic aberration.



#5 deSitter

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Posted 30 January 2025 - 09:16 PM

I remember in the 1980s a company called "Augen Optics" that made Huygens, not certain about Ramsdens.  Huygens had issues, but i can't help wondering if in some cases of serendipity, the aberrations of they eyepiece might have an opposite effect to the aberrations in say an achromat.Like longitudinal chromatic aberration.

Thomas (Astrojensen) has verified that in some cases, Huygens eyepieces compensate for CA.

 

-drl



#6 deSitter

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Posted 30 January 2025 - 09:19 PM

Ramsdens used to be very common. They don't work well with fast focal ratios -- below about f/7, forget it. A classic Ramsden, comprising two identical plano-convex lenses with the convex sides facing one another and separated by their focal length, is extremely well corrected for lateral color (also known as chromatic difference of magnification), notwithstanding that neither lens is an achromat: Jesse Ramsden was a smart man. It also helps greatly if the lenses are well-polished, as was the case when lots of optics were military-surplus and had military-specification-grade polish, and are coated.

 

There is an article on my web site that talks a little more about why Ramsdens work so well, and talks a bit about my own set of them.

Modern Plossls with 2 identical achromats are essentially "achromatic Ramsdens". Criterion supplied such eyepieces under that label.

 

-drl



#7 Kasmos

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Posted 31 January 2025 - 03:24 AM

The only Ramsdens I have are three of the R-20mms .965" EPs that came with Mayflower 814s, the similar Manons, and I believe the Sky Chief Jr.s. 

 

R-20mm.jpg

While the field is smaller than the small Kellners and MAs that came to me later, I always enjoyed the views thru them. 

Their design is appealling as well.


Edited by Kasmos, 31 January 2025 - 03:25 AM.

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#8 deSitter

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Posted 31 January 2025 - 04:34 AM

The only Ramsdens I have are three of the R-20mms .965" EPs that came with Mayflower 814s, the similar Manons, and I believe the Sky Chief Jr.s. 

 

attachicon.gif R-20mm.jpg

While the field is smaller than the small Kellners and MAs that came to me later, I always enjoyed the views thru them. 

Their design is appealling as well.

Same here, only one I have is branded Monolux R 20mm. Reminds me of a finder eyepiece. Most classic finders had Ramsden eyepieces. In the modern era these became Kellners. My latest finder has a Plossl - er - achromatic Ramsden :) Finders are f/5 to f/7 so shouldn't be great with Ramsdens, but it's too low power to notice.

 

-drl



#9 Terra Nova

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Posted 01 February 2025 - 01:51 PM

The only Ramsdens I have are three of the R-20mms .965" EPs that came with Mayflower 814s, the similar Manons, and I believe the Sky Chief Jr.s. 

 

attachicon.gif R-20mm.jpg

While the field is smaller than the small Kellners and MAs that came to me later, I always enjoyed the views thru them. 

Their design is appealling as well.

I have that same Mayflower 20mm 0.965” Ramsden eyepiece. It came with my 1965 model 814. It’s a decent eyepiece however I prefer the Huygens design for these simple lens designs. While the longer R-designated eyepiece is okay, I can’t say the same for the shorter S.R. (symmetrical Ramsden?) eyepieces of the same period like the 5mm and 4mm. They are just so restricted! Again, I prefer the modified Huygens (H.M.) in the 12.5, 9, and 6mm range. Better still are the Unitron Symmetricals (9 and 7mm) and the Orthos (6mm, 5mm, and 4mm).


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#10 jimeh

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Posted 01 February 2025 - 02:15 PM

Modern Plossls with 2 identical achromats are essentially "achromatic Ramsdens". Criterion supplied such eyepieces under that label.

 

-drl

I don't understand this. Ramsdens are two elements and Plossls are four elements. Plossls also have a much wider FOV,



#11 deSitter

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Posted 01 February 2025 - 02:19 PM

I don't understand this. Ramsdens are two elements and Plossls are four elements. Plossls also have a much wider FOV,

The optical setup is the same - two indentical positive lenses facing each other. They can increase the FOV by using achromats instead of singlet lenses.

 

-drl


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#12 triplemon

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Posted 03 February 2025 - 04:26 PM

A Ramsden can be made to have the exact same FOV as a Plossl. Edge correction isn't much worse and when the Ramsden is used in a scope of long enough focal ratio, they are tack sharp to the very edge.

 

Another thing when making a Ramsden with similar FOV and edge correction as a Plossl, you have to space the two lenses quite far and then eye relief becomes near zero, a small fraction of the FL. While in a Plossl the lenses can be spaced much closer and the resulting aberations compensated for with the extra lenses in them. So the effective eye relief can be still as much as 0.75x FL without compromising edge performance notably.

But few folks use such really long focal ratio scopes any more. Here is a scope suitable for my 60mm Ramsden with 50 deg FOV, eye relief of 0.2x FL, tack sharp to the edge. A 55mm Plossl for size comparison is on the table as well.

 

Hevelian.JPG

 


Edited by triplemon, 03 February 2025 - 11:59 PM.

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