Amazing high performance eyepiece "SR", SPECIAL Ramsden

 Exploring unknown something seems lovely interesting. A web-shop "AliExpress" is one of lovely sites like deep sky exploring through the Hubble space telescope.
Celestron SR (special Ramsden) 4mm eyepiece
Celestron SR-4mm
I have discovered a great item, that is a high power short focal length eyepiece, then I confirmed its amazing performance.
 The eyepiece is labeled as "SR-4mm", which is "SPECIAL Ramsden" manufactured by Celestron. This eyepiece had unbelievable low-price tag, I paid just only $2.3 for it in fact.
This article introduces this CHEAP but GREAT performance eyepiece.

* "Ramsden" type eyepiece:
The Ramsden type as telescope eyepiece is one of the most simple and classical style, which consists of only two plano-convex lenses. Some textbooks say that a Ramsden eyepiece is cheap but has much remaining aberrations so that it is classified as the lowest grade.
The "SR" type is basically Ramsden but I have never seen its name in textbooks. Someone says that the "S" stands for "Super", "Special", "Standard" or "Symmetrical" Ramsden. But I prefer to adopt capital "SPECIAL" as "S", because of its great performance and high-grade optical design (mentioned in later).

* Visually testing
I didn't have any good expectation for the SR-4. Because this type of eyepiece is commonly bundled with inferior toy telescopes which have catch-phrase of "high power magnification".
However, an authority, Dr. Paolini said in his publication that, "sometimes simple lenses structure brings clear image because of less air-glass interfaces". This is why I've got interested in this cheap and low-graded eyepiece.
Anyway, I examined this SR-4 through viewing Mars which had already become small as 5 arcseconds of its size (Feb.2019). As seeing it through the eyepiece then I was very impressed its contrasty and sharp image with distinct pattern on the Mars surface. In the fact, image around surrounding area of FOV had terrible aberration but its center image through the SR has enough sharp, aberrations seemed negligible.
Then I compared the SR-4 to PENTAX Or-6, Tani-optics Or-7, KSON super Abbe 4.8 and Vixen PL-4. The best high-power eyepiece of mine was the PENTAX Or-6 before the test but the result showed that the SR-4 brings better contrast than the Or, and very similar sharpness in its center of field. That was clearly far better than the PL-4.
Therefore, I needed to take the cheap SR-4 over the high-graded PENTAX Or-6 as my best high-power planetary eyepiece.
(note; textbooks are correct of its description regarding short eye-relief, narrow FOV and much aberrations in surrounding area.)

*Photo imaging test
My experience mentioned above might be seen like a phantom, because most of the textbooks say that the Ramsden type does not bring sharp image because of its chromatic aberration but orthoscopic type shows sharper image without aberrations.
So I decided to take an evidence which is photograph. I found Jupiter in lower sky then I took its photos by using traditional projection method (Apr. 2019) .
Anyhow, look at a set of pictures below. Both were taken by 20cm/F5 reflector with EOS60D as 1 minute movie and its auto-exposure, then the 4% best images were stacked by Registax. Projection length was regulated to obtain similar image size of Jupiter.

A comparison between Special Ramsden SR-4mm and PENTAX Abbe Or-6mm
Can you discern?
SR-4mm vs PENTAX Or-6mm (the order is secret :-) )
[SE200N, D=200mm / fl=1000mm]
*Chromatic blur is not aberration but atmospheric dispersion.

At this moment, I just give you a question.
"Can you discern which one is taken by using SR-4 or PENTAX Ortho?"
One of them is by SR-4, another is by PENTAX Or-6.
I guess it may not be easy to answer the question (but careful observation may introduce you the certain answer).
At least, I cannot find distinguish chromatic and spherical aberrations in both pictures, both look fine, I prefer SR-4 because image taken by it looks having better contrast.
This result means that my impression in the visual testing is right. My best planetary high power eyepiece is truly the SR-4! (My cheapest eyepiece is also the SR-4.)

Added info., competing to higher grade eyepieces.
After the first impression mentioned above, I did some competitions with high-grade eyepieces, such as Vixen HR2.4, Pentax SMC O-6, XP-3.8, XO-2.5 Takahashi TOE3.3 and TMB Planetary II 6mm.
At the center of FOV, surprisingly, I found that resolution by the SR-4 is able to compete these high-grade high power eyepieces.
At least, the SR-4 has clearly better resolution than TMB Planetary II 6mm (genuine version), and has very similar resolution to HR2.4, SMC O-6, XP3.8, XO2.5 and Tak TOE3.3.
Then I examined its performance by using artificial star photographing, and compared to HR and XP. The result gave me a conclusion that the SR's resolution reaches these high grade expensive eyepieces.

 Someone says that the "SR" means "Special low-cost Ramsden", which hires two same double side convex lenses. The two same lenses without right and back side may bring low assembling cost. This opinion looks believably true because of its ultra low-price, however, I found that it is completely untrue through disassembling the SR-4.
 I was surprised the result of disassembling. The surprising point is that the SR-4 has two different both double-side convex lenses. Its field-side lens looks almost plano-convex lens but its field-side surface has subtle curvature as convex. And the eye-side lens is double-side convex lens with different curvatures. The SR's lenses have four independent convex surfaces.
 This fact means that, the SR doesn't have either symmetric, standard and low-cost structure, which certainly brought me impressive sharp and contrasty planetary image! Therefore I must call this outstanding eyepiece with my respect as capital "SPECIAL" Ramsden!! I respect this SPECIAL optical design as same as for Abbe ortho.

Lenses of a "Special Ramsden" eyepiece
Disassembled lenses of the Celestron SR-4mm

Thus, I decided to refine this SR-4, such as black painting of side of lenses and its optical parts to make my best favorite high-graded eyepiece, then I ordered 5 pieces of same one for the refinement with my respect :-).


Troy R. さんの投稿…
SR: Symmetrical Ramsden
Lambda さんの投稿…
Dear Mr.Troy,

Thank you for the comment.
I have over 20 kinds of SR eyepieces and disassembled several of them.
But I could not find symmetrical design in them.
Additionally I found some of description of SR eyepieces in catalogues then it says that SR stands for "Special Ramsden".
匿名 さんのコメント…
What telescope are you using?
Lambda さんの投稿…
Thanks for the comment.
20cm F5 newtonian reflector was used.