37 replies to this topic

[LukaszLu]

I must admit that this is the first time I see something like this and it looks like a home-made invention to me. The lens is protected from the edge of the sleeve by a fiber pad, but it is not attached in any way and simply falls out of the housing. It looks like someone tried to glue it with some cement - to no avail. It is hard to believe that the manufacturer anticipated the need to manipulate something like this in night observation conditions.

 

On the other hand, I'm wondering about the thread. I don't know what the sleeve is made of - but if it is galvanized brass, then making the thread yourself would result in exposing the original metal...

[deSitter]

I must admit that this is the first time I see something like this and it looks like a home-made invention to me. The lens is protected from the edge of the sleeve by a fiber pad, but it is not attached in any way and simply falls out of the housing. It looks like someone tried to glue it with some cement - to no avail. It is hard to believe that the manufacturer anticipated the need to manipulate something like this in night observation conditions.

 

On the other hand, I'm wondering about the thread. I don't know what the sleeve is made of - but if it is galvanized brass, then making the thread yourself would result in exposing the original metal...

Interesting - so the original, without doubt plano-convex lens, was damaged, and a replacement was enlisted. Good theory.

 

So it would be interesting to check out the edge color error of this eyepiece. I have a 20mm Ramsden from around the same time. The field stop is sharp but there is a ton of cyan-blue fringe. This is just color error from the two singlet lenses. Please report what you see near the edge! Maybe this is a case of a compensating eyepiece - a design that cancels out CA in the scope, like a Chromacorr.

 

This is the most interesting thread!

 

-drl

[LukaszLu]

I recently bought a modern Celestron Omni 40 mm out of curiosity - I just wanted to see what this eyepiece was worth. And there is a very clear colored rim at the edge of the field of view. Something that cannot be seen, for example, in the very similar design and dimensions of Bresser's 40 mm Plossls from the beginning of this century. So the mere presence of such an effect does not mean that it is not the work of the manufacturer :-)

 

As for this 'Ramsden', this effect is probably not visible here, but I will take a closer look at the edge of the field of view tomorrow. In any case, chromatic aberration does not seem to be a shortcoming of this eyepiece.

[deSitter]

I recently bought a modern Celestron Omni 40 mm out of curiosity - I just wanted to see what this eyepiece was worth. And there is a very clear colored rim at the edge of the field of view. Something that cannot be seen, for example, in the very similar design and dimensions of Bresser's 40 mm Plossls from the beginning of this century. So the mere presence of such an effect does not mean that it is not the work of the manufacturer :-)

 

As for this 'Ramsden', this effect is probably not visible here, but I will take a closer look at the edge of the field of view tomorrow. In any case, chromatic aberration does not seem to be a shortcoming of this eyepiece.

Just remember, edge color is probably in the eyepiece. CA errors will be the same everywhere.

 

-drl

[LukaszLu]

Here are the results of a simple test performed with a smartphone. Projection from 3 eyepieces:

1. Polarex R25
2. Polarex K18
3. Royal Astro(?) HM25

 

As you can see, both 25 mm eyepieces have a distinct colored rim around the edge of the field of view, but the chromatic aberration in the case of this modified Ramsden begins to intensify towards this edge in a significant part of the field. In practice, it doesn't bother me as much as the photo shows - until I took it, I was convinced that CA was quite acceptable. This is the case when observing objects in the center of the field of view, which offers a very precise image. However, as you can see, compared to the HM25, this home made design gives a relatively small size of the apparent FOV.

 

 

 

 

 

[LukaszLu]

Test participants:

Attached Thumbnails

• 

[LukaszLu]

And one more test: "shortly Barlow" of unknown origin + Polarex K18. Magnification 100x:

Attached Thumbnails

• 

Edited by LukaszLu, 09 May 2024 - 11:18 AM.

[LukaszLu]

And finally Polarex SYM.ACH 9 mm (the same 100x, larger apparent FOV):

Attached Thumbnails

• 

[deSitter]

Here are the results of a simple test performed with a smartphone. Projection from 3 eyepieces:

1. Polarex R25
2. Polarex K18
3. Royal Astro(?) HM25

 

As you can see, both 25 mm eyepieces have a distinct colored rim around the edge of the field of view, but the chromatic aberration in the case of this modified Ramsden begins to intensify towards this edge in a significant part of the field. In practice, it doesn't bother me as much as the photo shows - until I took it, I was convinced that CA was quite acceptable. This is the case when observing objects in the center of the field of view, which offers a very precise image. However, as you can see, compared to the HM25, this home made design gives a relatively small size of the apparent FOV.

 

R25.jpg

 

K18.jpg

 

HM25.jpg

Yes this blue fringe is classic eyepiece lateral color. As you say it's not as bothersome in dark sky, but the better eyepieces minimize it. The Kellner is a Ramsden with one achromat replacing the eye lens, and that removes the lateral color. A Plossl is a symmetrical Ramsden with both lenses replaced by achromats (SYM ACH in Unitron language). For slow scopes, Kellners are excellent performers.

 

-drl

[LukaszLu]

A moment ago I had the opportunity to briefly observe the thin crescent Moon. 'Ramsden' at its modest 36x magnification produced a great image and I certainly wouldn't call it 'spoiled by CA'. I managed to try the SYM.ACH 9 mm (still perfect precision at 100x) and the Moon hid behind the trees that I cleverly planted in my garden ;-)

[petmic]

I have recently obtained one of those Polarex branded R25 and it quicky became my most favourite EP for Unitron 114. I prefer it to Tani K22 and even to Vixen K20 in that scope.

[petmic]

I wonder how would you rate the Mono 40?

No no - Monochro 40+adapter as well as mount and tripod are not the part of the find :-)

[LukaszLu]

Hi, to be honest, F15 refractors are not wide fields instruments and regardless of the advantages of this eyepiece, it is not very useful for me. Compared to the more standard Kenko AH. 40 eyepiece, it has a similar field of view, but a larger and more comfortable AFOV. Especially since this eyepiece has a large eye relief. This is of great importance because eyepieces with a larger focal length are usually sensitive to off-axis viewing. Thanks to the large eye relief, it is much easier to deal with this.

 

An undeniable advantage is its impressive appearance - as a collector's gadget, it works great mounted in a telescope. And only one thing failed: the manufacturer did not predict that in a few decades the standard would be 1.25''. The sleeve is slightly larger and does not fit in today's focusers. It is a pity, because the eyepiece could finally find its intended use, with instruments dedicated to wide fields.