Eyepieces determine a telescope’s field of view and its magnification. Image Credit: iStock
With a telescope and a suitable mount, the single accessory that
will do the most
to enhance your observing experience – no matter what size of telescope you’re using – is a good quality eyepiece.
After all, it’s the eyepiece and
not the telescope that actually produces the magnified image.
A telescope’s job is to gather light; as much light as possible in order to make distant, faint objects look brighter.
They do this with an objective lens (in a refractor) or an objective mirror (in a reflector) and the aperture of the objective is the key – the wider it is, the more light the scope can gather.
The objective then concentrates that light at the focal point inside the scope.
You can buy telescopes that make claims for high magnification or zoom functions, but these are gimmicks that, for the most part, you can ignore.
It’s the eyepiece that takes the gathered light and turns it into the sharp image that reaches your retina.
It’s the eyepiece that controls how large (how magnified) that image appears.
A basic understanding of how the human eye works helps explain the function of eyepieces So, what you need is a selection of eyepieces to match each of your different observing objectives.
And it’s the eyepiece that determines the field of view (FOV) – how big a swathe of the sky you’re able to see.
All eyepiece barrel sizes are standardised to either 1.25-inch or 2-inch.
There is no inherent advantage in one over the other; the difference is only to accommodate the physical limitations of lens sizes as they become larger or variations in design, nothing more.
You choose which eyepiece to use according to how you want to ‘frame’
your chosen target in the field of view
(see ‘Field of view’, below right) at the desired magnification.
It’s a little counter-intuitive, but you will generally need higher magnifications for objects that are relatively close to Earth, such as the planets, and lower magnifications for larger, more distant objects, such as galaxies.
This is because deep-space objects usually take up a larger area in the sky than the points of light formed by the stars and planets.
You also need to take into account that the more you magnify
an image, the fainter it will appear, so high magnification of distant, faint objects has its own problems.
Choosing what you need
To calculate the magnification an eyepiece will achieve with any given telescope, simply divide the focal length of the telescope (usually written on the scope’s body near the focuser or the front lens)
by the focal length of the eyepiece (found on its collar).
Another important factor in choosing eyepieces is eye relief, or how far away from the eyepiece’s lens your eye needs
to be to see the entire the field of view.
It’s a matter of viewing comfort and for anyone wearing glasses, a long eye relief
A telescope collects light; it’s the eyepiece that magnifies the image that reaches our retinas
Of all the different types of eyepieces available, the Plössl eyepiece is perhaps the most common, usually with an FOV of 40-50° and a short eye relief.
It’s an effective all-round option that can vary
in individual quality and price.
Not as common these days, but still in use are orthoscopic eyepieces, which have similar attributes to the equivalent Plössls, but
are usually not quite as good – although they are useful for Moon and planet observations.
Then there are wide- and ultra-wide-angle eyepieces, that offer very large fields of views up to 110°.
These are good for clusters of stars, deep-space objects and close-up details on the Moon.
A worthy addition to any eyepiece collection is a Barlow lens, which is not so much an eyepiece, as an eyepiece’s friend.
A Barlow lens intercepts the light from the telescope before giving it to an eyepiece, doubling or tripling the magnification you would get from an eyepiece alone.
A single well-made Barlow effectively doubles the number of eyepieces you have.
A good general collection of eyepieces for beginners would include a high magnification eyepiece; a wide/ultra-wide angle eyepiece; a medium power, general purpose eyepiece; and/or a Barlow lens.
This will give you a good selection of magnifications and FOVs, ready for use on many different objects.
Nicholas Joannou is an elected fellow of the Royal Astronomical Society