The Guide

So, why are eye­pieces so im­por­tant?

Sky at Night Magazine - - CONTENTS - With Ni­cholas Joan­nou

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. And it’s the eyepiece that de­ter­mines the field of view (FOV) – how big a swathe of the sky you’re able to see. So, what you need is a se­lec­tion of eye­pieces to match each of your dif­fer­ent ob­serv­ing ob­jec­tives.

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 coun­ter­in­tu­itive, 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 is prefer­able.

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.

Eye­pieces de­ter­mine a tele­scope’s field of view and its mag­ni­fi­ca­tion

A tele­scope col­lects light; it’s the eyepiece that mag­ni­fies the im­age that reaches our reti­nas

…And leave a large im­age on our reti­nasA ba­sic un­der­stand­ing of how the hu­man eye works helps ex­plain the func­tion of eye­pieces

A Bar­low lens can be used to dou­ble or triple an eyepiece’s mag­ni­fi­ca­tion

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