How Do I Choose Lenses For My Telescope?

Even if you already have a few lenses for your telescope, if you have just started out, the likelihood is that you are still using the budget models that are supplied with your telescope.

Something that many people do not consider is the fact that good eyepieces are really important when it comes to the overall performance of your telescope.

Eyepieces come in lots of different shapes and sizes, and there are many different designs to suit all preferences and budgets.

Just like anything else that you might want to purchase, you get what you pay for. However, you can find great products with desirable qualities for a reasonable cost if you know what you are looking for.

Even though the lens that you use might not seem like a big deal, it can either make or break your entire optical system. When you replace a budget lens with a more high quality one, you will definitely notice the difference. 

How Do I Choose Lenses For My Telescope

How Do I Choose Lenses For My Telescope?

Choosing the right lens for you will depend on a few different factors, like the objects that you want to observe, how tolerant you are of visual imperfections, and whether or not you want to narrow or widen your field of view.

It will also be influenced by how much you are willing to spend on the lenses. We are going to explain everything that you need to know below.

Magnification

Every telescope will have its own stated focal length, and this is the distance from the primary lens or mirror to the point at which it forms an image of a very distant object.

You will usually find this number printed or engraved somewhere near the eyepiece focuser.

This number will typically be in the range of 400 to 3000 mm, depending on the aperture and type of the telescope. You should also know that eyepieces also have focal lengths, and their own magnification. 

To calculate the magnification, you will need to divide the focal length of the scope by that of the eyepiece. Note that the same eyepiece used with a different focal length scope will give different power.

Telescope Eyepiece Sizes

There is one specification that is really important to think about when you are considering a new lens for your telescope, and this is its actual size.

The majority of lenses available today will have chromed barrels that have a diameter of 1¼ inches, and these will slide into the push-fit focusers of most telescopes.

Although, you will also find those that are intended to show you wider views, and these will have barrels that are 2 inches in diameter. There are also older scopes that can accommodate 0.965 inch barrels.

Field of View

Aswell as the focal length, every lens will have an apparent field of view, and this is something that will be measured in degrees. This is what tells you the width of sky, in angular terms, that is presented to your eye.

Eyepieces that have larger apparent fields will take in more of the sky than smaller ones. Simpler eyepiece designs will usually have apparent fields of around 45°, whereas widefield designs may be 60° or more.

Eyepiece Types

Sometimes, the name of the lens can give you some idea of its principal characteristics. For example, an ultrawide lens is more than likely going to be a wide angle lens.

However, there are also other types of lenses with more classic names, like Kellner, Orthoscopic, Plössl, and Erfle, or more specific modern variants like Nagler and Lanthanum.

More often than not, the higher the price of the lens, the better the quality will be. 

Although, eyepieces that have more desirable characteristics don’t always have to come at a huge cost to you, and there are some that are more reasonably priced.

If you are looking for a more budget option, you should know that 3 element lenses that are labelled Kellner or MA will usually have pretty good performance with scopes of a long focal ratio. However, they do not usually work well with scopes that have a short focal ratio.

Eyepiece designs with fewer lenses tend to deliver the most light to your eye, but those with complex optical combinations often provide more expansive views and better eye relief.

A Plössl can deliver well-corrected and wide fields of view with good eye relief, and all of this means that the eye can be positioned at a comfortable distance behind the rear lens, and  it can still see the whole field of view.

If you are someone that wears glasses for either long or short sightedness, you probably won’t need to use them when you are using the telescope. This is because you can adjust the focuser to help with this.

If you are planning on looking at planets or similar things, you will likely require the use of short-focal-length eyepieces, which can sometimes be an issue due to the small eye relief of conventional designs.

This is why you can now get 6 to 8 element designs that will have both comfortable eye relief and wide apparent fields across the focal-length range.

While there are lots of people that swear by these types of lenses as they are some of the best that you can get on the market, you will also need to be prepared to pay a higher price for them.

This is something that stops many people from making their purchase. As well as this, their physical size and weight can be a problem for delicately balanced and small scopes.

Barlow Lenses

A Barlow lens would be inserted into the focuser of the telescope before the eyepiece and it basically works to double or even triple the magnification of the lens.

This does result in a small loss of light, but it can be really useful in doubling your lens investment by making it perform at twice the magnification.

Some helpful things to look out for are achromatic and multi coated specifications when you are looking for a lens.

Gordon Watts
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