When you get down to it, a telescope is optically made up of two parts: The objective lens or mirror and the eyepiece. There are other optics involved, of course (a secondary mirror is rather critical in a reflector telescope), but for the most part it is the part you aim at the sky and the part you actually look through. We are going to discuss the latter today.
We’re not going to discuss the various types of eyepieces, that would involve several posts on its own. But we will discuss the issue of eyepiece size, and how compatible they are with other eyepieces.
So if there is a good guideline with regard to telescope eyepieces it is this:
Telescope eyepieces are remarkably standardized, very few other products work with other brands as well as telescope eyepieces!
This may seem a bit odd to a person used to other products. After all the parts of a Ford car will likely not fit in a Toyota. But one shouldn’t think of it in those terms, think instead of a DVD player or Blu-Ray player. There are plenty of brands to choose from, but they will all play DVDs, or Blu-Ray (of course, a regular DVD player won’t play Blu-Ray discs, so our metaphor might actually carry better than we think at first – read on:)
So in discussion of actual eyepieces, there are 3 sizes to deal with – barring a few very rare exceptions. Those sizes are .965″, 1.25″ and 2″. All the sizes refer to the barrel end of the eyepiece. The have nothing to do with the length, diameter, or focal length of the eyepiece. The size is the diameter of the part that actually goes into the telescope.
So a little bit about those sizes:
This used to be referred to as the ‘Japanese Standard”. It has a barrel less than an inch in diameter (.965″ is equal to 25mm) and at one time was ubiquitous as most telescopes were designed and built in Japan. However, this size was generally seen as being inferior. You couldn’t but any decent amount of glass in it (needed for high-end eyepieces) and even Japan started moving on to 1.25″ eyepiece designs. The eyepieces faded out slowly and few sellers still had them as of 10 years ago. However some legacy equipment, old telescopes in ‘Dad’s attic’, and low-end telescope lines appear now and then and require the old lenses to work (or an adapter) so they haven’t completely gone yet.
Far and away the most popular diameter these days. The 1.25″ used to be moderate-to-high end, but now even most beginner telescopes use this standard. It wouldn’t be too far out to suggest that 95%+ eyepieces today are this size. They come a wide variety of sizes and designs.
What this means is that if you have an eyepiece for a telescope that was made in the past 20 years, odds are it will work with almost any telescope that was built in the last 20 years. Even if they weren’t the odds are still pretty good they will be compatible unless the telescope was low-end and old.
The high end of eyepieces, these are gaining in popularity as they make for very comfortable viewing. The large glass apertures mean less struggle to center your eye along with optical comforts. 2″ eyepieces are gaining in popularity but will not likely surpass the 1.25″ as the standard as they are a) too bulky and heavy for smaller telescopes, b) hard to design for shorter (higher magnification) focal lengths.
Other designs – Occasionally, some obscure telescope maker tries to reinvent the wheel and makes a telescope that uses some kind of proprietary eyepiece. This never succeeds but does result in some odd telescopes and eyepieces rolling around the marketplace. They are extremely rare, however, so don’t worry too much about buying one by accident.
So when you get down to it, it is fairly amazing that telescope’s use so much of the same standards. Especially given that no national or international standards were ever officially set such as happened with microscopes. In the end, it is probably simply a matter that no vendor ever wants to have to tell a customer that the $500 worth of accessories are now useless. You might get away with that in a large, everyday-use market like computers (at least over a period of a few years you can), but not with a hobby like astronomy.
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