How Are Telescope Eyepieces Measured?

How Are Telescope Eyepieces Measured

You need to keep track of several technical parameters in order to properly understand how telescope eyepieces are measured. In this article, I am going to talk about all of these parameters so that, the next time when you are looking for an eyepiece, you actually understand what you are getting.

So what are these parameters? I am glad you asked. Here are these:

  • Magnification
  • Field of View
  • Eye relief
  • Eyepiece sizes

At first, before starting out with any of the parameters, we need to understand what ‘focal length’ is.

So, What is Focal Length?

telescope focal length

To understand anything related to telescope and eyepieces, you need to have a clear idea of the focal length. It is a MUST. every telescope has a mirror or objective lens.

In very simple words, focal length simply means the distance from the mirror or objective lens to the particular point where the telescope will get focused. This particular point is known as the focal point.

The focal length is typically measured in mm. Generally, the longer the focal length a telescope has, the more powerful it is to be considered. With higher power, comes higher magnification, lower field of view. Don’t worry, we’ll get to each of these parameters shortly.

Just for now, remember this:

A telescope with 2000m focal length will have 2x power but .5x field of view than a 1000mm focal length telescope.

Now that we know what is the focal length, let’s start with our first technical parameter, the Magnification.

For measuring each of these parameters, I’ll consider that I have a telescope having 1000mm focal length. This uniform consideration will help us to understand each of the parameters better.

Magnification

eyepiece magnification
Lower Magnification vs Higher Magnification

To measure magnification, all we have to do is perform a division math. We already know that our telescope comes with a focal length, which is 1000mm in our case. Similarly, the eyepieces also come with a focal length, like 10mm, 5mm, 25mm, 50mm etc.

To measure the magnification, all we have to do is divide the focal length of the telescope with the focal length of the eyepiece.

Magnification = (Telescope Focal Length/ Eyepiece Focal Length)

For example:

if my eyepiece focal length is 10mm, then the magnification will be: 1000/10 = 100x

if my eyepiece focal length is 50mm, then the magnification will be: 1000/50 = 20x

I think you get the point.

Now let’s focus on some points:

  • Considering a fixed focal length telescope, the longer focal length an eyepiece has, the lower magnification it will produce. On the other hand, the shorter focal length of the eyepiece will produce larger magnification.
  • The magnification an eyepiece produces is not fixed. It varies from telescope to telescope. The larger focal length a telescope has, the more it will magnify.
  • It is always better to have 3 eyepieces in your collection: one for higher magnification, one for medium and the last one for lower magnification. Later in the article, you’ll know why we may need all ranges of magnification.

Upper Limit

Human eyes have many limitations. One of them is, they can’t apprehend magnification lower or higher than a certain point. This is known as the upper limit, or the point up to which we can push our telescope. Beyond this point, you won’t get any satisfying view.

To measure the upper limit, we’ll need to know the aperture size of the telescope. Generally, you’ll find the aperture size on the labeling.

The formula to measure the Maximum Usable Upper Limit is: 60 x Aperture Size (in inches)

This means, if you have a 4 inch aperture telescope, then the maximum magnification you can go for is 60 x 4 = 240x. Beyond this point, you can’t apprehend what you are seeing.

Always remember that, higher magnification provers lower sharpness and resolution. On the other hand, lower magnification will offer you a crisper view of the object.

I hope from now, you’ll keep this upper limit in mind when buying a telescope. If you have a telescope which has a 240x upper limit, there is no need to get an eyepiece that will provide 300x magnification.

You get the point, right?

Eye Relief

eyepiece eye relief

Eye Relief is the second parameter we need to look at. Eye relief mainly indicates how comfortable will the eyepiece be to look at.

In simpler words, eye relief means the distance from the eyepiece to your eye for observing any object. The shorter the eye relief is, the more difficult it will be to observe an object. Similarly, longer eye relief means better comfortability. Also, shorter eye reliefs make stargazing extremely difficult for people with glasses.

Our target is to find an eyepiece with longer eye relief.

Keep in mind these points:

  • Generally, longer focal length eyepieces have longer eye relief. Similarly, shorter ones have short eye relief. Which in other words means, low power eyepieces have longer eye relief and vice versa.
  • In these days, eyepieces are specially made for achieving longer eye relief.

Field Of View

eye piece field of view
Wider Field Of View vs Narrower Field Of View

Field of View (FOV) means how much of the sky we can see through our telescope. It is generally measured in degrees. The larger the field of view is, the more we can see through the telescope at a particular point. That means, we can see more with a 60 degree FOV than a 45 degree FOV.

Field of views are of two types: Apparent Field of View (AFOV) and True Field of View (TFOV)

Apparent Field of View (AFOV):

The apparent field of view is a design characteristic that comes with the specifications of an eyepiece. Some eyepieces have wider AFOV while some have narrower. If the magnification is kept constant, the wider AFOV will reveal more part of the sky than a narrower one.

True Field of View (TFOV):

True Field of View is actually the measurement of how much of the sky we see. The AFOV is a design characteristic and predefined. However, we’ll have to measure the TFOV which varies with magnification. Thus, the TFOV will give us an actual representation of the degree of revealed sky.

Here’s how to measure TFOV

True Field Of View  = Apparent Field Of View / Magnification

Suppose, we have a 10mm eyepiece with a 50 degree apparent field of view. As mentioned earlier, our telescope is 1000mm.

First, we need to measure the magnification which is (1000/10 = 100x) in this case.

Now, the TFOV= (50 degree/ 100x) = 0.5 degree.

So, for this particular setting, our true field of view will be 0.5 degree.

A 0.5 degree field of view will snugly fit the Full Moon!

Here are a couple of pointers:

  • For observing larger objects, such as nebula or galaxy, we’ll obviously need a larger field of view. Can you guess why? It is because these objects are spread across a larger region than, say a moon. So, we’ll need a wider field of view which means, lower magnification.
  • For observing tiny objects like a star or the moon, we’ll need a narrower field of view. It will help us to get a closer look at the details of the object we are seeing.

Eye Piece Sizes

eyepiece barrel size
Credit: Eyes on the Sky YouTube Channel

Eyepieces these days come with 2 standard sizes: 2” and 1.25”. This measurement refers to the diameter size of the eyepiece barrel. There is also a 0.965” one which is now mostly avoided.

2” Eyepiece:

As the name suggests, the barrel diameter will be 2 inches of these eyepieces. Most telescope these days accept a 2” eyepiece. However, some may require an additional adapter for the attachment.

Generally, 2” eyepieces offer a wider field of view and lower magnification.

Other accessories such as Barlow lens will fit perfectly with a 2” eyepiece.

1.25” Eyepiece:

There is hardly any telescope these days that doesn’t accept a 1.25” eyepiece. Even most telescope kit comes with a 1.25” eyepiece to begin with. You can also attach other accessories like a barlow lens or filter easily with these eyepieces.

Barlow Lens:

barlow lens

Barlow lens is a wonderful gift to us, fellow stargazers. This tiny gadget will reduce your eyepiece collection by half. And it will also save a ton of money.

A barlow lens is nothing but a lens that can be attached with your eyepiece. It generally multiplies the magnification power of an eyepiece.

For example, a 2x barlow lens will double the magnification of an eyepiece.

If a eyepiece has 10mm focal length, by attaching a barlow lens, it will turn into a 5mm focal length.

So, by having a barlow length, we can have two magnifications with only one eyepiece.

Thus it reduces the need for having more eyepieces. By having only 3 eyepieces, we’ll get 6 different magnifications by having a barlow lens.

Barlow lenses are be available in different powers, such as 2x, 1.5x etc.

When buying eyepieces, keep your barlow lens in mind. Having a 10mm and a 5mm eyepiece will be redundant if you already have a barlow lens.

I hope you got the point!

Final Words

Lastly, all I want to say is, understand your eyepiece before getting one. Know what you are getting. And of course, get a good quality eyepiece. Doesn’t matter how costly and good your telescope is, if the eyepiece is bad, you’ll get a poor result.

The Telescope Is Only As Good As Your Eyepiece Is!

 

 

Muntaseer

I have wondered how the Stars and Moons look like for many years. I’ve fallen in love with Cosmology since I was a boy. I am writing these articles to share my love for astronomy with you.

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