UNDERSTANDING AND CHOOSING EYEPIECES

There are a huge range of eyepieces out there to choose from. Selecting your collection can be a very difficult task and last months. In some cases the quest for the ultimate eyepiece collection will last a lifetime! This primer is designed to make that decision a little bit easier!

Purely for demonstrational purposes, I will use an 8” (roughly 200mm) Newtonian telescope with a focal length of 1000mm as an example in this article.


Understanding The Maths:

It is essential to understand the basic numbers and calculations involved with eyepieces. Start by reading Kaptain Klevtsov’s article.

Focal ratio (f/ratio)
The focal ratio of the telescope is calculated by dividing the focal length by the aperture. It follows that our example telescope has a focal ratio of (1000mm / 200mm =) f/5.

Magnification
Eyepieces give different magnifications (or powers) in a telescope of a given focal length, which is determined by the focal length of the eyepiece. This is the primary property of an eyepiece, and it will always be written on the body in mm. For this reason we will only deal with the metric scale. They range from 2mm to 56mm and beyond.

The magnification of a 2mm eyepiece in a 1000mm telescope is (1000mm / 2mm =) 500x, and the magnification of a 56mm eyepiece is ~17.6x. Clearly a shorter f/l eyepiece will yield a higher magnification.

High magnification is good for viewing planets, the moon and double stars. This is about 150x and up. Images at higher magnification will appear dimmer as you are looking at a smaller area of sky. Medium magnification (60-150x) is ideal for smaller deep sky objects (DSOs) and low magnification (under 60x) is useful for finding objects, for large DSOs and for scanning star fields.

There are a number of factors that limit the maximum usable magnification. The first is the seeing conditions. If you use too high power in poor seeing, you will only amplify the poor atmosphere and not get any additional detail. Average seeing in the UK restricts you to 200x, but it can vary from as low as 150x to 400x.

The other major factor is the scope’s size. The rule of thumb is to stick to 50x per inch, or 2x per mm of aperture. Therefore with the 200mm scope you should not exceed 400x. Of course, you will anyway mostly be limited by the seeing in this case. Personally, I take this rule of thumb with a pinch of salt. On my ED80 I should technically not exceed 160x, but I find 200x is easily manageable. Good optics (such as those in the ED80) may get away with higher mags.

Another factor comes in to play if you use a Dobsonian telescope. The lack of slow-motion control on these mounts mean tracking your object at powers above 2-250x can get quite tricky.

So that’s the high-mag limit – is there a low-mag limit too? Yes, but for this we need to explain the significance of:

Exit Pupil
The exit pupil is the diameter of the disc of light which exits the eyepiece, and this is calculated thus: aperture / magnification OR eyepiece focal length / focal ratio. A 10mm eyepiece will yield 100x, so our exit pupil will be 200mm / 100x = 2mm OR 10mm / f/5 = 2mm.

When observing at night, the pupil in your eye dilates. In a healthy adult the diameter of your dilated pupil is around 7mm. In a young person it may be a bit more and in an old person maybe 6mm. Having an eyepiece exit pupil exceeding your eye pupil means light is lost outside your pupil. Thus your low-power limit is governed by a exit pupil of 6-7mm. In an f/5 scope, this is (f/5 x 6/7mm =) 30-35mm.

Furthermore, viewing through a telescope with a central obstruction (such as the secondary mirror in our Newtonian) at too low power, the central obstruction begins to become apparent as a black shadow in the middle of the FOV. Sticking to a 7mm exit pupil should avoid this.

True Field of View (TFOV)
This is the actual area of sky you are looking at through the telescope. The moon occupies about 0.5° of sky. The TFOV is calculated by dividing the apparent FOV (more on this later) by the magnification given by the eyepiece. For example, a 20mm eyepiece (50x mag) with 70° AFOV will give (70 / 50 =) 1.4° TFOV, or almost three times the moon’s diameter.

Calculations In Summary

Focal ratio = ^{focal length} / _aperture

focal length = f/ratio x aperture

magnification = ^{telescope f/l} / _{eyepiece f/l}

exit pupil = ^aperture / _{magnification} OR ^{eyepiece f/l} / _f/ratio

true FOV = ^{apparent FOV} / _{magnification}


Choosing the focal lengths of your eyepieces

Now we understand all those figures, you can start to decide which focal lengths are ideal to use in your telescope. This may be biased by your primary interest. For example, if you are keen on planetary observing, you might want more high-power eyepieces than low, or if you are into deep-sky, low to medium power eyepieces will suit you better. I will assume an all-round interest.

My advice would be to start with the highest and lowest power eyepieces you will be using. For high power, pick an eyepiece according to the limit, as described above. For scopes above 4”, this should yield 200-250x. As a wide-field eyepiece (low power), pick an eyepiece that gives a <7mm exit pupil, or in the case of slow scopes, as wide as you like.

In our example scope, the low power would be 30-35mm. For high power a 5mm eyepiece will yield a safe 200x, but on those rarer nights of good seeing, a 4mm would serve you well to get the most out of the scope. Of course you would be wise not to spend too much on this eyepiece as it may not get very much use.

A Barlow can be a cheap way of effectively doubling the number of eyepieces in your collection. Barlows go in between the eyepiece and the telescope, and they effectively multiply the focal length of your telescope (or half the f/l of your eyepiece). Anything from 1.5x to 5x are available, but 2x is the most common. This means a 10mm eyepiece in a 2x Barlow becomes a 5mm eyepiece.

The medium power eyepieces should go in equal steps between the two extremes. Two or three in between is a good bet. With this scope, perhaps 21mm (48x), 13mm (77x) and 8mm (125x) would cover the rest. Or if you have a 2x Barlow, a 24mm (42x) can cover 12mm (83x) and a 16mm (62.5) can cover 8mm (125x).

If you have software that can generate FOV indicators, it is worthwhile having a play with this as it can be more accurate than the focal lengths alone. This tool will set a ring against the background showing the TFOV of an eyepiece of a given AFOV in a given scope. You can then set it over a particular target to show how that eyepiece will frame that target. Starry Night can do this.

A carefully thought out collection of focal lengths means you can have all bases covered with just 3 eyepieces and a Barlow. Once you are satisfied with your choices you can consider which eyepieces can take those focal lengths.


Choosing the Eyepieces

Again, many factors affect this and you have to do some research into particular models to work out if they’re right for you. Generally speaking though, you get what you pay for. I will talk through the various properties that affect the cost of an eyepiece.

Barrel Size
There are three different sizes of eyepiece barrel – 0.965”, 1.25” and 2”. 0.965” are not worth discussing as they are fairly rare. 1.25” is the most common, and 2” is only required for wide-field eyepieces whose views would otherwise be blocked by the diameter of the 1.25” barrel. In a 1.25” eyepiece you can get up to a 32mm 50° eyepiece (or a 24mm 68° or 20mm 82°). There is no optical advantage to using 2” barrels for eyepieces with FOVs smaller than this.

Type
There are a number of different optical designs which have particular properties. A good summary of the basic types can be found here. To generalise – orthoscopics are excellent for high power as they are cheap, bright and with excellent contrast. They have small AFOVs, but you do not usually need a large AFOV for high power work. Plössls are good for general medium power. Wide angle eyepieces (65°+) can be used to good effect for any power range, but generally they are more of a luxury and hence cost more.

Simple eyepieces have fewer elements. This usually means they will have more contrast and brightness. Complex eyepieces have more lens elements so they can obtain more AFOV, eye relief etc. (see below). These will lead to slightly less light transmission and contrast, and will also cost more.

Zoom eyepieces are very convenient, and can be a cheap way of getting a range of focal lengths in one eyepiece. However, quality and AFOV can be compromised in budget zooms.

Eyepiece sets can be a very good way to go. A set of 6 plössls, for example, will likely cover every eventuality. They may be parfocal (see below), they will come at a deal price, and you will get a nice case too.

Apparent Field of View (AFOV – often just FOV)
AFOV is what the FOV appears to be when you look in the eyepiece. A very small AFOV is like looking through a keyhole whereas a large one could be compared to a porthole on a spaceship! An average AFOV is about the 50° of a standard plössl. 68° is the most the eye can take in. Beyond that you have to pan around the FOV to take in the whole lot. This gives that immersive spacewalk feeling.

Needless to say, the larger the AFOV, the more the eyepiece is likely to cost. However, AFOV is not the only factor. There are some eyepieces which are marketed on their 80° fields, but they are in fact poor performers, and only 40° will be sharp. This brings us onto:

Optical quality
I’ll get it over with right now - no eyepiece is perfect. Every eyepiece will show lack of sharpness towards the edge of the FOV, or maybe a little colour cast, or colour fringing etc. There are many different aberrations that can be visible and I will not go into them.

You need to decide for yourself if you can live with them or if you want to spend extra to bring them to a minimum. My personal advice is to try to ignore them! As soon as you start getting bothered by them, it can become very expensive to eliminate them.

It’s easier to correct these aberrations in eyepieces with a smaller AFOV. Therefore cheap and good plössls etc. are much more common. Getting the same level of correction over a 82° field is much harder and for this you will pay a lot more.

A few worthwhile things to know about optical design:

Internal blackening – this is to reduce internal reflections and ghosting from bright objects, thereby increasing contrast. Most eyepieces have this.

Coating – eyepieces will often be described as “coated, multi-coated, fully coated, or fully multi-coated”. Coatings are designed to increase light transmission (hence brightness and contrast), by reducing reflection off the lens elements. Multi coatings are more efficient. Fully means that all glass-air surfaces are coated. Uncoated optics are rare these days, and best avoided.

Scope focal ratio
A “fast” scope with a small focal ratio, like our Newtonian at f/5, has a light cone steeper than a “slow” scope - say an f/10. This means the eyepiece has to work harder to make the light parallel and give a sharp image.

Generally speaking, owners of slow scopes do not need to spend as much on eyepieces that will perform well as owners of fast scopes.

With our example scope (f/5) you should pick carefully. Decent plössls and orthoscopics are good performers in fast scopes and are generally safe bets. However, if you want a large, well-corrected AFOV, you need to research into particular eyepieces and how they perform – there’s no certain way of telling.

Eye Relief (ER)
This is the distance from the eye lens that your eye needs to be in order to see the full AFOV and is usually stated by the manufacturer. Generally, shorter f/l eyepieces have less ER. Of course there are exceptions.

This can be a problem for observers who wear glasses while observing, which limits how close you can get. However, if you do not have severe astigmatism, you can easily observe without your glasses, as the scope will correct for your short/long sightedness.

So the more eye relief the better? Not quite – too long eye relief can lead to:

Kidney Beaning
This is a property specific to some eyepieces, and is more prone to eyepieces with loads of eye relief. It causes irritating blackening of some of the FOV (often in the shape of a kidney bean) when your eye is not placed centrally in the exit pupil.

This affects some people more than others, and it depends on specific models. Personally I am very prone to be bothered by the beaning on a particular eyepiece, while others may not even notice it.

Additional Features
Eye guards – these are for practical purpose. Their job is to reduce stray light coming in between your eye and the eyepiece, and can assist with correct eye placement. There are various different designs of these and you have to decide which you find most comfortable.

Safety groove – this is a small notch near the base of the barrel, which is designed to reduce the risk of the eyepiece slipping out of the focuser. Most modern eyepieces have this.

Threaded for filters – this means that you can screw filters onto the end of the eyepiece barrel.

Parfocality – eyepieces from a single series will sometimes need little or no refocusing when switching from one to another. The eyepieces are said to be parfocal with one another. This is of convenience, but the same effect can be obtained by using parfocalising rings, which is a ring that slides onto the barrel and sets the focus point.

Weight – Some eyepieces are very heavy. This means that switching from a small light eyepiece to a heavy one can disturb the balance of your scope. It can also be uncomfortable handling such an eyepiece.