The Basics of Eyepieces

The Basics of Eyepieces

I would like to start by saying that I am far from an expert on the subject of eyepieces, I would also like to state that I have no affiliation with any eyepiece manufacturer or reseller. The views expressed in this article were written based on readily available information and on my own experiences, and therefore is merely my opinion.

Some of the following information is very basic, but we all must start somewhere, and what is the point of having an article like this if you have to be a physics professor to understand it.  

With this in mind, I will not be delving into the realm of optical physics, or the structure of different types of eyepieces, merely touching on some of the differences you can expect to notice when you look through them.

First we need to know a few common terms and abbreviations used when discussing eyepieces and what they actually mean.

Focal Length

Focal length is a term used to describe one property of not only eyepieces, but any optical system. Your telescope has a focal length, a magnifying glass also has a focal length, and of coarse so does your eyepiece. Focal length is the distance at which an optical system focuses light. For example, when you hold out a magnifying glass and allow the sun to shine through it, the suns rays refract through the lense and form a cone of light behind it. When you move the magnifying glass away from or towards a surface, you can form a small spot of concentrated rays at a certain distance. I’m sure we’ve all burnt a piece of paper at one stage of our childhood with one of these instruments. The distance between the lense and the point where the suns rays converge is called the focal length. Basically when you adjust your focuser on your telescope, you are adjusting the eyepiece in or out in a similar way to the magnifying glass above, to allow your eyepiece to focus.

Aperture

This is simply the diameter of your main light collecting lense. With refractors it is the size of the lense at the large end of the scope. With reflectors it is the size of the large mirror at the bottom of the telescope. This measurement can be expressed in either inches or mm. You’ve all heard of a 4 inch telescope, or a 100mm telescope. Both these measurements refer to the aperture. (I realise this is a telescope term not an eyepiece term but you need to know what it is in order to calculate exit pupil later on)

Magnification

Well I’m sure we all know what it is, but how do you calculate the magnification of an eyepiece? The amount of magnification an eyepiece will give you, will differ from one telescope to another in most cases. The formula for working out magnification is as follows.

Magnification = Telescope focal length / Eyepiece focal length

If you use a 20mm eyepiece in a telescope with a focal length of 1000mm, that eyepiece will give you a magnification of 50 times. The same eyepiece in a telescope with a 2000mm focal length, will produce 100 times magnification.

So this is why I stated it would differ in different telescopes in most cases. If you had a reflector for instance with a focal length of 1200mm, and a refractor with the same focal length, any given eyepiece will produce the same magnification in both telescopes.

FOV (Field of view)

The field of view actually has two parts, AFOV and TFOV.

AFOV (Apparent field of view)

This refers to the width of sky an eyepiece design will allow you to see when you are looking through the eyepiece only. Usually expressed in degrees, it is a way of estimating which eyepiece design will show you a larger field in any particular telescope. For instance two eyepieces of the same focal length will give the same magnification in the same telescope, however if one has an AFOV of 65deg and the other an AFOV of 50deg, the 65deg eyepiece will show a larger portion of the sky. Let me explain this further. Lets say you were looking at Jupiter in an eyepiece with a 50deg AFOV. Lets also say this eyepiece gives a magnification of 100 times in your telescope. Lets say that this combination lets you see Jupiter but not its moons, because they were too far apart to all fit in the eyepiece view at once. Now let’s change the 50 deg AFOV eyepiece for a 65deg AFOV eyepiece of the same focal length. This combination will still give you 100 times magnification however because it shows you a larger piece of sky, you can now see the moons and Jupiter all at once in the eyepiece.

TFOV (True field of view)

This refers to the actual width of sky, you can see through a particular eyepiece and telescope combination. The TFOV is a product of your eyepiece focal length (mm), your eyepiece AFOV (deg), and your telescope focal length (mm). The formula for calculating the TFOV for any eyepiece telescope combination is as follows.

TFOV = AFOV / Magnification

If you use a 20mm eyepiece which has an AFOV of 65deg, in a telescope with a focal length of 1000mm, the TFOV of that eyepiece and telescope combination is as follows.

Magnification = Telescope focal length / Eyepiece focal length

  • = 1000mm / 20mm
  • = 50 times magnification

TFOV = AFOV / Magnification

  • = 65deg / 50 times
  • = 1.3deg

Therefore you would see 1.3deg wide piece of sky through this combination. As you can see from this calculation AFOV and TFOV are very different. However AFOV is an indication of how large your TFOV will be in a particular telescope. To explain this further, if the eyepiece in the above example had a 50deg AFOV you would only see a 1deg piece of sky.

Curvature of the Field

This term describes a problem which is present in some eyepieces. It basically means due to the way the light refracts through the various lenses within the eyepiece, you cannot sharply focus the entire FOV. The centre of the field will have sharp pin point stars, yet the further away from the centre you look, the less sharply focussed the view will be. This is a common problem with eyepieces from the lower end of the cost scale. Top quality eyepiece manufacturers spend a lot of money finding and implementing ways to alleviate this problem.

Exit Pupil

This is the diameter of the beam of light that exits the eyepiece. If you can imagine that the end of the eyepiece you look into has a small beam of light exiting it. This is what you put your eye up against when you observe. The diameter of this beam will change with Magnification and Telescope aperture. The reason knowing this figure is useful is because if the size of the exit pupil is larger than the pupil in your eye, some of the light from the eyepiece cannot get into your eye, and therefore is wasted. The pupil in a young persons eye, can only open to about 7mm maximum, and the older you get, generally the less your eye pupil can open. So as you can see when searching for an eyepiece to buy, it is important to consider the exit pupil a particular eyepiece will give you. The human eye gets its best resolution at around 2mm exit pupil, so although for different magnifications in the same telescope your exit pupil will be different, you should always try to get the exit pupil as close as possible to that 2mm mark. Below is the formula for calculating the exit pupil of an eyepiece. Remember, different telescope different exit pupil.

Exit Pupil = Telescope Aperture / Magnification 

I have a 302mm telescope with a 1500mm focal length, and I want to calculate the size of the exit pupil I will get from a 17mm eyepiece.

Magnification = Telescope Focal length / eyepiece focal length

  • = 1500mm / 17mm
  • = 88.2 times magnification

Exit pupil = Telescope Aperture / Magnification

  • = 302mm / 88.2 times
  • = 3.4mm

Therefore a 17mm eyepiece in my telescope will give me an exit pupil of 3.4mm. Although this is a little above the 2mm mark, it is well below the upper limit, before I start wasting light.

Eye Relief

This is the farthest distance away from the eyepiece you can place your eye while still seeing the entire field. In other words, if you put your eye right up to the eyepiece you can see a certain sized FOV. As you move your eye further away from the eyepiece, at a point your FOV starts getting smaller. The furthest away you can move your eye without your FOV getting smaller is called the eye relief of that eyepiece. This is another very important factor to consider when buying eyepieces. It is especially important if you need to wear glasses while observing. The eye relief cannot be calculated, it is like focal length, and AFOV, it is inherent in the design of the eyepiece. When you see eyepiece specifications, they usually state three things. AFOV, Focal Length, and Eye Relief.

Ok, now we have the basics it’s time to put it all into practice.

There are several factors that need to be considered when choosing an eyepiece. Everyone is different when it comes to what they require, the types of objects they like to look at, whether they wear glasses while observing, what their budget is, how old they are, what expectations they have and what faults they are willing to overlook when it comes to choosing the best eyepiece for them. Let’s touch a little on these subjects now.

There are many different types of eyepieces, and their cost can range from $50ea to $900ea for what may seem like the same eyepiece when looking at just the specifications. The difference is almost certainly in the quality of the views and the extent to which the manufacturer has gone to alleviate some of the problems I have mentioned above. In my opinion good eyepieces are more important than a flash top of the range telescope. You can get acceptable views out of a very average telescope with good top quality eyepieces, however average eyepieces will always give you average views no matter how flash your telescope is. To use an analogy, it’s like trying to run a race car on standard unleaded car fuel. To get the best you can from your telescope, you need good eyepieces. Once you have good eyepieces, and you find yourself wanting more, then you can upgrade your telescope. The best thing about this is you already have good eyepieces to go with your new flash telescope.

The types of objects you like to look at also determines which eyepieces you need, because of the size of the object in the sky, how faint it is, and what it is you’re trying to see. For instance, emission nebula and reflection nebula like Eta Carina, The Trifid Neb and The Swan Neb, all require a wide field and low magnification to see the most nebulosity. Objects like Planetary Nebulas, need much higher magnification, because some of them are so small that otherwise they just look like stars. Some star clusters are small, so more mag is needed to appreciate them, while others are so big that even at the outer limit of exit pupil, you still cannot fit them in the eyepiece. A lot of this is subjective, so the best approach is to have a look in lots of different eyepieces and see what you like. Then using the information above, you can make some informed decisions and buy the eyepieces that suit your circumstances the first time, and hopefully alleviate some of the trial and error, which normally costs you money.

I personally like lots of eye relief, very wide AFOV, and different magnifications for different types of objects. I also like good quality eyepieces that have a flat field, and good contrast. Unfortunately eyepieces like this are very expensive, but you know what they say, you only get what you pay for.

I hope you found this to be informative and useful information.

There are several accessories such as barlows, field flatteners, powermates etc, that are available to compliment your eyepiece set, this however is a topic for another article, so until next time, may you have clear skies and great seeing.