18 replies to this topic

[azure1961p]

Younger folks today in the pursuits may never have even heard of the common Ramsden popular decades ago.  Often they were the oculars they'd sell as part of a package with a scope. Costs kept down, got you *into* the heavens where treasures awaited.  Then , it seemed like to me the classic Kellner filled that bill and some poor barlows to join these entry level ocular set.  And now? Nothing that I can see - and probably because nanufacting costs have come down so low it's just easier to make basic plossls, and that Kellner, now a RANKEN Kellner, some folks also call it a reversed Kellner.  

 

As a lot of folks the Ramsden was a *colorful* ocular, narrow field and don't let your Saturn drift thru the field away from center!  Indeed, Dave Gray enjoys his Ramsden as a kind of anti prismatic dispersion eyepiece such was the Ramsden distortion. 

 

Today though, Ramsden is practically a unicorn and none too pretty.  Does anyone - besides David Gray even have one in their ocular set?

 

Pete

Edited by azure1961p, 12 April 2025 - 09:48 AM.

[petmic]

I do have one and use it quite a lot in the Unitron 114 as a finder eyepiece. It is sharp edge to edge and at 40deg AFOV i wouldn't call it narrow.

[triplemon]

Kepler, Ramsden, Dollond.

The fundamental ingredients to mix to get you 90% of anything you need and still more than half of anything we have today for close inspection of virtual things.

[RichA]

Younger folks today in the pursuits may never have even heard of the common Ramsden popular decades ago.  Often they were the oculars they'd sell as part of a package with a scope. Costs kept down, got you *into* the heavens where treasures awaited.  Then , it seemed like to me the classic Kellner filled that bill and some poor barlows to join these entry level ocular set.  And now? Nothing that I can see - and probably because nanufacting costs have come down so low it's just easier to make basic plossls, and that Kellner, now a RANKEN Kellner, some folks also call it a reversed Kellner.  

 

As a lot of folks the Ramsden was a *colorful* ocular, narrow field and don't let your Saturn drift thru the field away from center!  Indeed, Dave Gray enjoys his Ramsden as a kind of anti prismatic dispersion eyepiece such was the Ramsden distortion. 

 

Today though, Ramsden is practically a unicorn and none too pretty.  Does anyone - besides David Gray even have one in their ocular set?

 

Pete

I don't even remember people wanting to use Ramsdens and Huygens even in the early 1970s.  They were considered as low-end 0.97 eyepieces for entry-level refractors and pity anyone who was forced to use them in Newtonians.  Kellners were considered a step-up, but everyone I ran across wanted orthos.  Brandon and Clave Plossls were out of reach for many, economically.  The Erfle was generally thought of as a war surplus eyepiece adapted to telescopes.

When I got hold of an adapter and put a Celestron 26mm Plossl on a cheapo 60mm refractor, I couldn't believe the improvement.  However, it is possible that a Ramsden might counter-act some negative optical aspect of some telescope, i.e., under/over correction counteracting each other, that sort of thing. 

[jrmacl]

I've a couple of Huygenian/Mittenzwey that came with a '62 Tasco I was playing around with not too long ago as I had just bought some .965" to 1.25" and 1.25" to .965" adapters

[Jay_Reynolds_Freeman]

I have several Ramsdens that have high-grade polish and either no coatings or well done ones with little scatter. I suspect these eyepieces were assembled using military optics left over from World War II. They have essentially no lateral color -- that is a property of the original Ramsden design. And of course, they have only four air/glass interfaces. They are first rate for any use where seeing low-contrast detail is an issue and narrow apparent field of view is acceptable.

 

Clear sky ...

[therealdmt]

I started with two Ramsdens (and a Kellner for low mag) back in the day. No plans to go back, but those Ramsdens did serve their purpose. They were my only high magnification eyepieces for 10 years, and I got lots of good views through them over that time, of course.

 

Nowadays, for my limited (but not non-existent) interest in minimal glass eyepieces, I think I’ll be satisfied sticking with Plossls and Edmund RKEs, which were the two kinds of eyepieces I’d wanted but couldn’t really afford as a kid (I mean, it wouldn’t have been impossible to save up and buy them, but funds were limited and there were other [non-astronomy] things that, when I came down to it, young me simply wanted more).

 

Regarding Ramsdens in particular though, although I don’t have any particular bad memories or feelings towards them, I don’t feel any nostalgia towards them either. Beyond that, their field of view and eye relief specs aren’t at all enticing to current me, especially as I only use non-tracking alt-az mounts, so all in all I have no plans to revisit Ramsdens purchase-wise. Now that you’ve brought them up though (and another recent thread mentioned them in a historical context, way back when they were the exclusive, limited supply, late 1700s state-of-the-art hotness), I wouldn’t mind having a look through someone else’s Ramsden. Could be kinda fun, and I’m sure they work well enough as always [within their specs limitations], perhaps especially in a classic focal ratio telescope.

 

Final thought is, the main way I might get a Ramsden again would be through a possible DIY project. Could be interesting, although building an eyepiece isn’t at the top of my "maybe someday [but equally maybe, never] ATM projects" list 

Edited by therealdmt, 13 April 2025 - 02:45 AM.

[therealdmt]

This guy is really impressed with his Ramsden, after having compared it to some much more expensive eyepieces:

https://m-lambda.blo.../p/amazing.html

[machinerabbit1031]

Not everyone is fond of the original technologically advanced eyepiece design. People can't appreciate the Ramsden design, especially now because there's a lot of "better" options which are clearly not once you understand what you might sacrifice clarity sharpness, less ca and an all around true view as a history buff knowing i am looking through a original genuine ancient design gives me a sense of authenticity and legitimacy but like many other things in life the demand for more and "better" has only diluted the appreciation for these treasures leaving them behind and forgotten with every generation the Ramsden slowly fades in the past labeled as an "inferior" design when in reality it is irreplaceable.

[Jon Isaacs]

I just read what Sidgwick had to say about the Ramsden. I always enjoy his writings, a nice balance and a precise writing style. 

 

"The so called positive ocular, thus named for the nor very obvious reason that the focal plane does not lie between the two lenses.  It consists of two plano-convex crown lenses of equal radius with their convex surfaces facing one another.

 

If achromatic condition of 2d = f₁+f₂ it is exactly obeyed, the focal plane of the eye lens with coincide with the field lens, and the exit pupil with the eye lens; disadvantages of which are, respectively, that any dust or defects on the surface of the field lens are visible, magnified, in the field; and that since the eye cannot be placed at the exit pupil, the visible field is decreased and some of the illuminating power of the objective is lost.  

 

Hence, d is usually reduced by from 25% to 40%: both focal plane and exit pupil are then external to the system, though at the cost of introducing a sensible degree of chromatic inequality of magnification., Trouble is also encountered from out of focus ghosts due to double reflection in the field lens.  Since the focal plane is external, a Ramsden can be used as a magnifying glass.

 

The Ramsden has considerably less spherical aberration (positive) than the Huygenian, and less field curvature.. It is therefore preferable to the Huygenian for use in objectives of small focal ratio. It has more color due to chromatic inequality of magnification but smaller longitudinal aberration. Slight astigmatism is not troublesome and there is also slight positive distortion. Useful field is in the region of 35 degrees apparent diameter. In the interests of achromatism, the clearance of the exit pupil from the eye lens is not very great, usually about 0.25f. "

 

Page 137, Amateur Astronomers Handbook, J.B Sidgwick, Second Edition, originally published in 1971, 

 

The section on the achromatic Ramsden and the Kellner is also of interest.

 

Personally, the Ramsden does not seem will suited for the telescopes I observe with.  

 

Jon

[Jay_Reynolds_Freeman]

One of the classic Ramsden designs is two plano-convex lenses of equal focal length F, convex sides facing each other, separated by a distance equal to F. In that form the chromatic difference of magnification is essentially zero -- no lateral color. (For the mathematically inclined, what is going on is that in thin-lens mathematics, the eyepiece focal length as a function of wavelength is constant through and including first order in wavelength.) Thus the eyepiece behaves as if it were made with achromatic lenses, even though it is not.

 

Jesse Ramsden was a smart man.

 

For details of the mathematics, look at the end of this report.

 

 

Clear sky ...

[Jon Isaacs]

One of the classic Ramsden designs is two plano-convex lenses of equal focal length F, convex sides facing each other, separated by a distance equal to F. In that form the chromatic difference of magnification is essentially zero -- no lateral color. (For the mathematically inclined, what is going on is that in thin-lens mathematics, the eyepiece focal length as a function of wavelength is constant through and including first order in wavelength.) Thus the eyepiece behaves as if it were made with achromatic lenses, even though it is not.

 

Jesse Ramsden was a smart man.

 

For details of the mathematics, look at the end of this report.

 

 

Clear sky ...

 

 

Your link doesn't work properly 

 

Sidgwick points out the disadvantages of that particular design and seems to suggest that the common Ramsden is of a different design where the focal plane does not coincide with the first lens and the exit pupil is external.

 

Jon

[Jay_Reynolds_Freeman]

Your link doesn't work properly 

 

Sidgwick points out the disadvantages of that particular design and seems to suggest that the common Ramsden is of a different design where the focal plane does not coincide with the first lens and the exit pupil is external.

 

Jon

The link works for me -- I will be curious if others have problems with it, and would like to hear about it if so: I can't fix it if I can't make it break.

 

I am not sure which Ramsden design was or is most common. It turns out that the high degree of correction for chromatic difference of magnification obtains for any two-thin-lens eyepiece in which the lenses are made from the same glass type and are separated by half the sum of their focal lengths; thus the problems of the two-identical-lens variant can be addressed. Common Huygenian eyepiece variants also have correct-through-first-order chromatic difference of magnification.

 

 

Clear sky ...

[therealdmt]

The link works for me -- I will be curious if others have problems with it, and would like to hear about it if so: I can't fix it if I can't make it break.

 

I am not sure which Ramsden design was or is most common. It turns out that the high degree of correction for chromatic difference of magnification obtains for any two-thin-lens eyepiece in which the lenses are made from the same glass type and are separated by half the sum of their focal lengths; thus the problems of the two-identical-lens variant can be addressed. Common Huygenian eyepiece variants also have correct-through-first-order chromatic difference of magnification.

 

 

Clear sky ...

The link you posted in post #11 works for me.

 

First paragraph reminded me that my Ramsdens were 1/2 inch and 1/4 inch sized 

[Jon Isaacs]

Jay:

 

I will try your link on another device.

 

I thought Sidgwick's comments here were relevant:

 

"If achromatic condition of 2d = f1+f2 it is exactly obeyed, the focal plane of the eye lens with coincide with the field lens, and the exit pupil with the eye lens; disadvantages of which are, respectively, that any dust or defects on the surface of the field lens are visible, magnified, in the field; and that since the eye cannot be placed at the exit pupil, the visible field is decreased and some of the illuminating power of the objective is lost."

 

He goes on to say that the achromatic condition is not normally obeyed for those reasons.

 

I've always loved his comments on the old oculars since he was writing at a time when they were current. In my situation, Ramsden are not practical. 

 

Jon 

Edited by Jon Isaacs, 13 April 2025 - 02:37 PM.

[tcifani]

It's just a site security warning (The site isn't using a private connection - http instead of https), probably not a big deal but some browsers might prevent from loading like Chrome.

[j.gardavsky]

The Ramsden eyepiece is an extrafocal assembly of the two planoconvex lenses, facing the convex surfaces against each other. The lenses can be either the same, or different. The eyepiece design os extrafocal.

Another 2-lenses eyepieces is the Huygenian, which is intrafocal.

 

See also https://www.quora.co...uygens-eyepiece

 

The Ramsden eyepiece used to be the popular choice for the early19th century draw tubes telescopes made by Ramsden.

Not much later, most of the draw tubes telescopes (Dollond and many others) came with the Huygenian eyepiece.

(As documented in my collection of the draw tubes telescopes)

 

A high quality remake of the Ramsden eyepiece might be of interest, especially when the maximum possible contrast and transmissivity are required, and for those who otherwise reject using the glass ball eyepieces,

 

JG

[azure1961p]

A lot of goodinfo in this thread.  I must say I was never a Ramsden fan looked through it's 4mm ocular which originally came with an Edmunds Scientific 6" F8 reflector.  Returned it after a few nights of use - something unattainable these days in the NE with endless clouds and rain seeing sadly. But that's another tangent.

 

Pete

[Jay_Reynolds_Freeman]

Final thought is, the main way I might get a Ramsden again would be through a possible DIY project.

Such a project is easy to start: If you ever find a commercial source of plano-convex lenses with focal lengths in the eyepiece-focal-length range and diameters reasonable for the focal length, just buy two of them and you are set to go.

 

If you are into fancy machining or 3D printing, you can go that way for mechanical parts. If not, get a 1-1/4 inch sink trap extender, some sheet plastic -- or even wood --an eighth of an inch or so thick, and a decent glue. (I like Pliobond.) Saw off a hunk of the extender, cut wood or plastic discs that fit into it (where the field lens goes) and over it (eye lens), make holes a bit less than the lens diameters in them, paint lots of surfaces black, and glue everything in place -- including the lenses. Be prepared to fiddle to get everything centered. The likely hard part of this is finding lenses in the first place.

 

 

Clear sky ...