The Guide

Scope stats ex­plained.

Sky at Night Magazine - - CONTENTS - With Kev Lochun Kev Lochun is BBC Sky at Night Mag­a­zine’s pro­duc­tion editor

The lan­guage in which we talk about tele­scopes can be con­fus­ing at the best of times. Con­sider the num­ber of ways we can de­scribe a tele­scope’s de­sign – re­frac­tor, re­flec­tor, com­pound, cata­diop­tric, Cassegrain, Dob­so­nian and as­tro­graph, to name a few. Some tele­scopes can be con­sid­ered to be more than one of them. Two of those terms even mean the same thing.

What they all share, how­ever, is a hand­ful of core sta­tis­tics: four mea­sure­ments that will give you an idea of how any tele­scope will per­form, re­gard­less of its in­ner op­ti­cal ar­range­ment, all other things be­ing equal. These are aper­ture size, fo­cal length, fo­cal ra­tio and use­ful mag­ni­fi­ca­tion.

Know­ing what these num­bers mean isn’t vi­tal to us­ing a tele­scope, any more than un­der­stand­ing how your brain and eyes work to­gether is a pre­req­ui­site to be­ing able to en­joy the gift of sight, but in this case it’s worth the ef­fort. Once you know what these fig­ures mean, you’ll be in a good po­si­tion to work out what you can and can’t do with your scope, and what bud­get you might need to re­alise your longer-term goals.

1. Aper­ture

This fig­ure is the most straight­for­ward – the aper­ture is the di­am­e­ter of a tele­scope’s main lens or mir­ror, mea­sured in mil­lime­tres

and com­monly con­verted into inches. This num­ber de­scribes how much ‘light grasp’ a tele­scope has, by which we mean how many pho­tons it can col­lect. A big­ger aper­ture de­liv­ers brighter views, which leads to bet­ter con­trast and more de­tail. This is why aper­ture is of­ten de­scribed as be­ing the most im­por­tant fea­ture of a tele­scope; the more light you can gather, the fainter the ce­les­tial bod­ies you’ll be able to see.

The amount of light a tele­scope can gather is di­rectly pro­por­tional to the area of its aper­ture. The gains are rapid: based on area, a 6-inch aper­ture in­stru­ment will gather four times as much light as a 3-inch one, for in­stance.

2. Fo­cal length

Be­fore you can see your cho­sen tar­get, the rays of light pass­ing through the aper­ture have to be fo­cused to­gether, and the point where they con­verge is known as the fo­cal point. The dis­tance that the light has to travel be­tween the aper­ture and the fo­cal point forms our sec­ond core mea­sure­ment, the fo­cal length. This is recorded in mil­lime­tres. There is no fixed re­la­tion­ship be­tween an in­stru­ment’s aper­ture and its fo­cal length; it all de­pends how the lenses and mir­rors within the tube are ar­ranged.

Fo­cal length is use­ful for two rea­sons: it’s the ma­jor de­ter­mi­nant of use­ful mag­ni­fi­ca­tion (which we’ll get onto in a mo­ment) and it gives you a rough idea of what sort of field of view you can ex­pect. Smaller fo­cal lengths de­liver wider fields, so lean to­wards be­ing bet­ter suited to ob­serv­ing larger swathes of the night sky and for star hop­ping, while longer fo­cal lengths of­fer nar­rower fields – per­fect for plan­e­tary disc close-ups – and tend to al­low you to use eye­pieces with longer eye re­lief; that’s the ideal dis­tance your eye should be from the lens of an eyepiece, and it’s a par­tic­u­larly im­por­tant con­sid­er­a­tion if you wear glasses.

3. Fo­cal ra­tio

Our third core num­ber is the fo­cal ra­tio, also known as the f/num­ber, which de­scribes the re­la­tion­ship be­tween the fo­cal length and the aper­ture. You can work it out by di­vid­ing the fo­cal length by the aper­ture; both of these fig­ures should be in mil­lime­tres. Let’s say you have a 130mm aper­ture in­stru­ment with a fo­cal length of 900mm – its fo­cal ra­tio will be ‘f/6.92’.

Like fo­cal length, fo­cal ra­tio can tell you a lot about a tele­scope: larger f/num­bers im­ply higher mag­ni­fi­ca­tion with a given eyepiece and a nar­rower field of view, smaller f/num­bers the op­po­site. Ad­di­tion­ally, an f/num­ber can be de­scribed as ‘fast’ or ‘slow’, and this re­veals how a tele­scope will per­form when used for astropho­tog­ra­phy.

The terms fast and slow are a throw­back to the days of chem­i­cal cam­era film pro­cess­ing. Scopes with fast ra­tios (typ­i­cally f/5 or be­low) can cap­ture im­ages more quickly than their slow ra­tio (en­com­pass­ing f/9 and above) coun­ter­parts, but the trade-off comes in terms of depth of fo­cus; slow scopes are much more for­giv­ing in this re­gard.

4. Use­ful mag­ni­fi­ca­tion

Like the tele­scope it­self, ev­ery eyepiece has a fo­cal length, and it’s the re­la­tion­ship be­tween these two fo­cal lengths that gives you the mag­ni­fi­ca­tion (or ‘power’) of your setup. The cal­cu­la­tion is sim­ple: di­vide the fo­cal length of the scope by that of the eyepiece. So, if you have a scope with a 1,200mm fo­cal length and a 20mm eyepiece, your mag­ni­fi­ca­tion would be 60x. The smaller the fo­cal length of the eyepiece, the greater the re­sult­ing mag­ni­fi­ca­tion on any given tele­scope. Aper­ture is en­tirely ir­rel­e­vant in this case.

It’s worth be­ing able to work out how much mag­ni­fi­ca­tion you’re us­ing be­cause, un­like aper­ture, more isn’t nec­es­sar­ily bet­ter. The the­o­ret­i­cal use­ful limit is two times the aper­ture in mil­lime­tres; so for a 150mm aper­ture, that’s 300x mag­ni­fi­ca­tion. Push it beyond the use­ful mag­ni­fi­ca­tion and you’ll get a closer view of your cho­sen tar­get, but that view will be a fuzzy one, not to men­tion dim­mer.

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