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Are Telescope Eyepieces Universal? [Can you interchange them?]

Posted by Ryan DeLange on

Many people purchase a used telescope (or clean the cobwebs off the one in the basement) and wonder if new eyepieces will work with it. Or they plan to buy a new telescope but aren’t sure if the eyepieces they already own will be interchangeable with it. Let’s explore the answer to this common question and find out if telescope eyepieces are universal.

Telescope eyepieces are interchangeable as long as they have the same barrel size (the diameter of the eyepiece’s insert tube). The universal standard is 1.25” eyepieces and most modern telescopes accept them; however, even models that accept 2” eyepieces usually come with an adapter to use a 1.25” eyepiece. 

That means you absolutely can use your Celestron 1.25” eyepiece with your new Sky-watcher telescope (or any brands for that matter). 

But what about the older telescopes that only take 1” eyepieces? And will an eyepiece get the same result when used with a different telescope? Find out this and more as you read on. 

Can I use 1.25” eyepieces on my old telescope?

Many older telescopes are designed for 1” eyepieces (.965” to be precise). Newer eyepieces, unfortunately, do not fit in these telescopes. There are, however, adapters that do allow these telescopes to accept a standard 1.25” barrel eyepiece. 

But before you buy one, be sure to ask the manufacturer or seller of the adapter if it will work with your telescope, because sometimes your telescope won’t be able to achieve focus with the extra length of the adapter. Also, an adapter may not work with longer focal length eyepieces due to vignetting, where some of the light rays are not able to pass through, resulting in darkness at the edge of the image.

How do I know which eyepiece fits my telescope?

If you’ve purchased a used telescope, you may have no idea what size eyepiece it is compatible with. In this case, you’ll simply need to measure the distance across the opening in the focuser where the eyepiece goes. That measurement is the barrel size of the eyepieces you’ll need.

If it is a two-inch opening, then of course you can use 2” eyepieces. But see if your telescope came with a 1.25” adapter. If you don’t have one, you can probably purchase an inexpensive one. Then you’ll be able to use both size eyepieces. 

Eyepieces perform differently when used in different telescopes

As we’ve seen, you can swap eyepieces between modern telescopes usually with no problem. But you’ll need to consider how an eyepiece might give very different results when used in different scopes.  

Swapping between telescopes with different focal lengths

First of all, the same eyepiece will produce different magnifications when used in telescopes with different focal lengths. Focal length is the distance the light travels from the front opening of the telescope to the eyepiece. Longer focal lengths give higher magnifications. The formula for magnification is:

Magnification = telescope focal length / eyepiece focal length

So, if you put a 10mm (focal length) eyepiece in your Dobsonian telescope which has a 1000mm focal length, it will give you 100x magnification. Swap that same eyepiece into your 550mm focal length refractor and it drops to only 55x magnification.  

Not only that, you will experience different fields of view in these two scenarios. If you stick a particular eyepiece in the telescope with the 1000mm focal length, your field of view will be much narrower than when you swap the same eyepiece into the telescope with the 550mm focal length. 

Swapping between telescopes with different focal ratios

The second difference to consider is how an eyepiece performs in telescopes with different f-ratios. An f-ratio indicates the relationship between the telescope’s focal length and its aperture (the diameter of the front opening). The formula is expressed as:

F-ratio = telescope focal length / telescope aperture

The important thing to consider when swapping eyepieces is that telescopes with low f-ratios (about 3-6) usually require higher quality eyepieces. This is partly because of the steep angle at which the light enters the focal plane, which causes problems only solved by eyepieces with better glass and more lenses. 

So, while you may enjoy magnificent views with your cheaper eyepiece in your f/15 Maksutov-Cassegrain telescope, you could be quite frustrated when using the same eyepiece with your f/5 Newtonian. 

When you need a 2” eyepiece

Dobsonian telescopes often are designed to accept 2-inch eyepieces. The reason for this is that the owners of Dobs are often looking to view deep space objects, which are faint and wide in the sky. Once they get one of these faint fuzzies located in the eyepiece, they don’t want to have to keep readjusting the position of the telescope as the earth spins.

To get this experience - sometimes called the space-walk effect - you need an eyepiece with a long focal length (low magnification) and a wide field of view. The only way to get this combination is with a 2-inch barrel. 

Maintaining a crisp image across larger fields of view is a technological feat. In fact, until recent years the technology didn’t exist on the consumer level. Good 2-inch eyepieces can cost several hundred dollars. 

So, if you own a Dobsonian telescope and only have 1.25” eyepieces, you’ll be fine. You can still view just about anything you want to see with these smaller barrel eyepieces. But, if you really get the bug and find yourself wanting a more immersive experience, you’ll want to buy a more expensive 2” delight. 

Conclusion

In short, yes, telescope eyepieces can be interchanged between different telescopes for the most part. You can use the universal 1.25” eyepiece in pretty much any modern telescope. Things may get trickier if you have a very old scope that needs .965 inch eyepieces, but it is possible to adapt these scopes to use modern eyepieces. 

But remember - any particular eyepiece will perform differently when paired with telescopes of different optical designs. You’ll need to consider this when planning to acquire a set of eyepieces that work with your telescope (or telescopes).

Let me know if you think I missed something important. Your comments are certainly welcome. 


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