EYEPIECES

The way an eyepiece performs depends upon the arrangement of its lenses. Nowadays all eyepieces are made of at least two lenses. Eyepieces that are of a simple design, perhaps with just two singlet lenses, tend to have some colour fringing and image distortion towards the edge of the field of view. If such an eyepiece is well made, however, it will give a reasonably wide field of view and bright images that are free of the ‘ghosts’ that can arise from internal reflections in the more complicated modern eyepieces. The best place to use these simple types of eyepiece is in long focal ratio refractors (like ours!).

The Ramsden is an example of a simple eyepiece that works well for planetary observation. The Ramsden is made of two identical planoconvex lenses set apart by the same distance as their focal lengths. The two lenses are arranged with their curved surfaces facing each other.

That is the original design of the Ramsden eyepiece, but it suffers from the disadvantage that any dust particles or scratches on the inner lens (the FIELD lens) will be in sharp focus when the eyepiece is in use. Also, it will have almost zero eye-relief. To overcome these two problems, the lenses can be moved closer together, to about 75 % of their focal lengths.

The Huyghenian is also a common type of eyepiece which uses two, widely separated, simple lenses. This time the curved surfaces of both lenses face toward the front of the telescope. The Huyghenian eyepiece is referred to as a ‘negative’ eyepiece, because its focal point lies between its two lenses. Huyghenian eyepieces have a very narrow field of view. They are fine in a microscope but not very useful in a telescope.

You can tell if an eyepiece is a Huyghenian by its narrow field of view and the fact that you cannot use it as a magnifying glass. (If you poke your finger inside the eyepiece, you cannot get it to focus the image of your finger.) This is because, as we said earlier, the focal point lies between the two lenses, and so is inaccessible. Probably the eyepiece will have the letter H on the outside. If your telescope has these eyepieces it may be time to set them aside, treat yourself to a more modern type of eyepiece (like one of ours!) and discover that the performance of your telescope has been transformed!

The Plossl (also known as the dial sight or the symmetrical eyepiece) consist of four lenses, arranged as two doublet lenses. Its more complex design overcomes the narrow field of view and the short eye-relief of the simpler types of eyepieces. The Plossl eyepiece gives a wide, flat field of view that is free from rainbow fringing right to the edge of the field of view. Its apparent field of view (the angle through which you need to move your eye to see from one side of the eyepiece view to the other) is about 55o and it has a long eye-relief, which means that spectacle wearers need not remove them while observing.

An almost identical performance is achieved with the orthoscopic eyepiece, which also has four optical elements, this time a triplet lens and a singlet.

The Erfle eyepiece usually consists of five or six elements. It always has a concave field lens, which aids identification. It has an even wider field of view, typically 76o, so the effect is rather like looking through a porthole into space! Its disadvantage is that, when looking at bright objects such as Jupiter, it suffers from internal reflections that can cause you to see a ‘ghost’ planet moving towards the real image. Two other disadvantages are its short eye-relief and its long price tag.

EP. 1 Plossl eyepiece, 35mm, brass

We make these Plossl eyepieces in polished brass. The body has been coated in clear lacquer to prevent it tarnishing and the entire item is very handsome indeed. This is our best selling eyepiece and it has an excellent reputation. If you are starting in astronomy and can only afford one eyepiece – choose this one.
Standard fitting 1¼” or 2"
Focal length 35mm
Clear aperture through the lenses 23mm
Optical construction two cemented doublets

£90.00

EP. 2 Plossl eyepiece, 55mm, brass

This is the big brother of the previous eyepiece. We make it in polished and lacquered brass and its lenses are huge. They give a comfortable view and a long eye-relief. We commend these Plossl eyepieces to those observers who wear spectacles at the telescope.
Standard fitting 1¼” or 2”
Focal length 55mm
Clear aperture through the lenses 34mm
Optical construction two cemented doublets

£90.00

EP. 3 Rich Field Military Eyepiece, 37mm, in focusing mount

This is astonishing! It is a fully anti-reflection coated eyepiece with a wonderfully flat, wide field of view. It works well in long focal ratio telescopes (as you would expect) and it performs just as well in the modern, short focal ratio telescopes, such as the apochromatic triplets. Many modern eyepieces do not do this.

As a bonus the eyepiece has a chunky focussing ring which, when rotated, moves the lenses smoothly in or out, to alter the fine focus. In addition it is provided with a soft rubber eyecup with a rubber dust cap.

We have fitted it with a 2" (50.8mm) nosepiece for standard telescopes.

This military eyepiece is of a quality that is far superior to most commercial optics.

Focal length 37mm
Field lens diameter 37mm (yes, really!)

£120.00

EP. 3A Rich Field Military Eyepiece, in original format

This is the same item as above, but without the 2" nosepiece. You can build it into a telescope of your own construction by using the six threaded holes in the base of the body. This would be superb in a finder or spotting scope.

£85.00

EP. 8 Eyepiece with smooth-action focus control

This eyepiece has a short focal length and is designed to view the screen of an image intensifier tube. The innermost lens is spring loaded so it can press against the object being viewed. The eyepiece is a chunky, almost rectangular casting, made of aluminium with the focus control knob at the side.

Field lens diameter 16mm
Eye lens diameter 18mm
Focal length dunno, but fairly short.

You could use this eyepiece for its original purpose or to examine images on a photographic transparency. Please note that this eyepiece does not seem suitable as a telescope eyepiece. (If you can work-out how to use it on a telescope, just let me know and I’ll put up the price.)

£10.00