2017 August 11
In this short article we will discuss eyepieces, what they do and how they work. Every telescope needs an eyepiece for visual use. Eyepieces are small lenses or combinations of lenses mounted in metal barrels that fit into the telescope’s focusing mount.
A telescope creates an image of whatever it is aimed at. If you point your telescope, without an eyepiece fitted, at the moon or a field of bright stars you will see an image floating in space. The job of the eyepiece is to take this image, magnify it and feed it into your eye.
If you have just bought a new telescope it will usually come with two eyepieces giving different magnifications. Although these eyepieces may not be of the finest quality they will certainly do to get you started. As you progress through the hobby of astronomy you will undoubtedly want to increase your stock of eyepieces. The good thing is that you can use them across different telescopes so an eyepiece can be for life not just for Christmas! As you might expect, using poor quality eyepieces on a good quality telescope will spoil that telescope’s performance. Conversely even the finest eyepiece cannot turn a bad telescope into a new Hubble.
Every eyepiece you buy is an investment in your astronomical future so it is worth buying the best you can afford, but be warned some of the very best eyepieces can cost as much as a reasonably sized telescope. The good news is that unless you want the widest fields of view and observe under the darkest skies you can still purchase suitable, good quality eyepieces at sensible prices.
Eyepieces consist of a number of glass lenses (often referred to as elements) housed inside a casing which itself is attached to a barrel that slips into the focuser of the telescope. The “size” of an eyepiece is a measure of the diameter of the barrel which is measured in inches; the commonest sizes for modern amateurs are 1.25 inch and 2 inch. Occasionally you may come across smaller sized barrels (commonly 0.965″), often found on older refracting telescopes, and 3″ diameter barrel eyepieces may be found in professional observatories.
The size you need will be dictated by your telescope’s focuser. Obviously you cannot fit a 2″ eyepiece into a 1.25″ hole. However 2″ focusing mounts generally come with an adapter that allows you to use 1.25″ eyepieces as well. Adapters can be commercially sourced, or home-made, to allow the larger diameter barrel sizes to be used in smaller focuser but this usually leads to a truncated view and is not recommended. The 1.25″ size is perfectly acceptable for normal amateur use. The larger sized eyepieces come into their own at low powers with extra wide fields of view.
So what makes a good eyepiece?
Firstly it should give a sharp pinpoint image across the entire field of view. This is the ideal but is not always achievable. In particular, depending on the type of telescope and the quality of the eyepiece, there may be some distortion around the edges (such as coma aberration). It is worth making the point here that telescopes with fast focal ratios, f/5 or faster, are much more demanding of eyepiece quality than are slower telescopes (>f/10).
The next thing to look for is a lack of ‘ghosts’. Ghosts are caused by internal reflections in the eyepiece that create a false image of a bright object just outside the field of view. These can be very convincing and have even been mistaken for a new comet. Most modern eyepieces have anti-reflection coatings to the elements to combat this; these coatings may give a purplish tint to the lenses when looked at obliquely.
As stated earlier, the purpose of the eyepiece is to magnify the image. The obvious question is how much does any particular eyepiece actually magnify? The key piece of information to know is the eyepiece’s focal length; this is usually stamped on the eyepiece itself and is generally given in millimeters.
To calculate the magnification any eyepiece will offer the observer, divide the telescope’s focal length (in millimeters) by the eyepiece focal length (also in millimeters).
So using a 10mm eyepiece on a telescope with a focal length of 1000mm gives a magnification of 1000 / 10 = 100. Similarly a 5mm eyepiece would give 1000 / 5 = 200. From this it is clear that the shorter the focal length of the eyepiece, the higher the magnification. Equally the same eyepiece will give different magnifications on different focal length telescopes. Generally magnifications are written with an ‘x’ such as 100x and referred to as 100 times or 100 power.
So what is the best magnification?
The answer is that it depends. Low power is ideal for locating things in the sky in the first place and for large objects like the Pleiades star cluster that require a wide field of view. For seeing the finest detail, for example when observing the planets, a much higher magnification is desirable.
In practical terms the maximum magnification is approximately double the telescope’s aperture in millimeters. So a 100mm aperture telescope has a maximum magnification of about 200x, and for a 250mm aperture it is about 500x. In reality you would only be able to use powers this high very infrequently.
There are two reasons for this, the first is that for non-point sources of light, like a planet, the higher the magnification the dimmer the image becomes. The second reason is the Earth’s atmosphere. The air above us is in a state of constant motion, and through a telescope this can look like the distant target is shimmering , as though looking through boiling water. This shimmering becomes more visible with increasing powers. Only in the rare occasions of what is called very good ‘seeing’ will it be possible to use the highest magnification.
A more useful limit is perhaps half to two thirds of the maximum magnification. If you are ever using a telescope and the image looks like a boiling jellyfish try reducing the magnification. A small sharp image is always better than a large blurred one.
If you point a telescope at a bright sky and look into the eyepiece from a distance of a few centimeters you will see a small bright disk of light. This is known as the ‘exit pupil’ and is the cone of light rays leaving the eyepiece which then enter the eye. If the diameter of the exit pupil is larger than that of the eye’s pupil then not all the light leaving the telescope is entering your eye. This will have effectively converted an expensive telescope into a much smaller one!
The diameter of the exit pupil is calculated by dividing the aperture of the telescope by the magnification achieved with that particular eyepiece. This means a 100mm aperture telescope at 50x magnification has an exit pupil of 2mm. The maximum diameter of a fully dark adapted eye pupil is 7-8mm, but this decreases fairly linearly with age from teenage years; figure 2 demonstrates this and is adapted from research on healthy volunteers eyes (Bradley et al, 2011).
This suggests the minimum magnification is given by multiplying the aperture in millimeters by approximately 0.12, though this value needs to increase with age and pupil size.
It is now possible to obtain zoom eyepieces that can give a variety of magnifications just by twisting the barrel. In the past these often gave a performance inferior to individual eyepieces but with the advent of new types of glass and modern designs they can give good results albeit at a price; commonly used zoom eyepieces offer the user a range from 8mm to 24mm though others are available.
Barlow lenses and Powermates
A very useful addition to any set of eyepieces is what is known as a Barlow lens. (The optics manufacturer TeleVue produce Barlow lenses but also manufacture a reportedly superior lens called a Powermate which is used in the same way as a Barlow lens.) Barlow lenses consist of a series of lens elements in a tube into which the eyepiece plugs and the whole assembly then fits into the focuser. These lenses modify the light cone emerging from the telescope in such a way that they effectively increase the focal length of the telescope and the magnification the observer experiences for any given eyepiece.
A 2x Barlow increases the magnification an eyepiece achieves by a factor of 2; a 100x magnification view would become 200x magnification view with a 2x Barlow. These lenses come in a range of magnifications, but commonly 2x, 2.5x, 3x, 4x and 5x; they also come as 1.25″ and 2″ barrel diameters (see figure 3). Effectively for the price of a Barlow you have doubled the number of eyepieces in your set.
The same rules apply for maximum magnification even when using a Barlow lens. Do no exceed the advice earlier, keep the overall magnification under two times the aperture in millimetres.
A more in depth treatment of the Barlow lens is available here.
When you use a telescope your eye naturally takes up a position just the right distance from the top of the eyepiece. This distance, from eye to eyepiece, is known as the ‘eye relief’ and varies from eyepiece to eyepiece. The general rule is that the shorter the focal length the shorter the eye relief; too short an eye relief can be uncomfortable as the eye is almost up against the lens. Modern designs of eyepieces tend to have better eye relief than older ones. Another advantage of using a Barlow lens is that you can get the higher power but retain the eye relief of the original lower power eyepiece.
If I normally wear glasses…
A common question is should I wear my eyeglasses when using a telescope? If at all possible the answer should be no. If your prescription shows you are simply short or long sighted then a simple twist of the focusing knob is all that is required to bring the image back to sharpness. If you have astigmatism however the answer is more nuanced.
Slight astigmatism should not be a problem. Focus on a star in the centre of the field at high power. If the star appears as a point then it is fine. If however it appears as an oval or a straight line then astigmatism is present. Try twisting your head round while looking through the eyepiece. If the oval/straight line turns with you then the problem is in your eye and you will likely need to wear your glasses but try your other eye first. If the line does not rotate with your head then the issue is in your telescope not your eye.
The problem with wearing glasses is the eye relief and whether you can get your eye close enough while still wearing them. If you have to wear your glasses try to find eyepieces with long eye relief, quite often they will have a rubber eyecup that can be folded down to accommodate this.
Field of view
Every eyepiece will have a defined apparent field of view; this may be marked on the eyepiece and is measured in degrees and confusingly the value given doesn’t denote the true field of view you see when using the eyepiece.
Typical eyepieces nowadays have apparent fields of view of 40-60 degrees. In recent years however new designs have arrived on the market with apparent fields of view of 70, 80 or even more than 100 degrees. These wide angled apparent field of view eyepieces are often in 2″ format and can be very expensive especially the ones with an ultra-wide field of view.
To translate these eyepiece apparent fields of view into what you actually see through the telescope (the true field of view) you need to divide the apparent field of view by the magnification. So an eyepiece with an apparent field of view of 50 degrees giving a magnification of 100x would have a true field of view of 50 / 100 = 0.5 degrees, just about the size of the full moon.
Nowadays there are many different types of eyepieces from many different manufacturers and the choices can be confusing.
Good general types of eyepiece include Orthoscopic or Plossl designs which can be used comfortably with most telescopes. More modern eyepieces with many more internal elements, such as Nagler and Ethos, offer astonishingly widefield views, though these often carry a similarly impressive price tag and need to be paired with a suitably widefield telescope.
A good basic set of eyepieces might be ones that give magnifications of approximately 0.2x, 0.5x, and 1.0x the aperture of your telescope in millimeters, offering low, medium and high magnification views.
As noted above, if your telescope came with a set of eyepieces then start with them and maybe add a Barlow lens or Powermate. Alternatively two or three Plossls or Orthoscopic eyepieces will be fine.
As you progress in this hobby you will likely want to expand your set and maybe improve the quality as well. By this time you may well have decided to specialise in a particular branch of observational astronomy and this may influence your choice of eyepiece. The BAA Section directors and other officers will be happy to advise you and inform you of the eyepieces they most commonly use.
Whatever you decide, your eyepieces will become a key part of every telescope you own and looked after carefully will give a lifetime of pleasure.