About Scopes

8/2/2019 About Scopes

1/822 SkyWatch 2010

Telescopes are magical theres no other way to say it

You can enjoy years o happy stargazing with nothing but y

own eyes, but telescopes add a new dimension to the exper

ence. When Galileo rst turned his spyglass to the night skour centuries ago, he discovered marvels everywhere he lo

wonders that had been hiding in plain sight or all o his

unsuspected by any person beore him. And all telescope o

ever since participate in Galileos miracle. On any clear nig

can point our scopes upward and enter an enchanted world

completely alien to everyday experience.

There are innumerable dierent ways to enter this club.

browse the ads in this magazine, or visit the websites o ma

acturers and dealers, you will soon realize that telescopes c

in an overwhelming variety o sizes, shapes, and prices. To

sense o this embarrassment o riches, you need to ask you

ew basic questions.

How much are you willing to spend? How portable does

telescope need to be? Do you plan to do astrophotography?

above all, what do you hope or and expect rom astronomy

Great ExpectationsLets talk rst about what you can reasonably expect. A ew

objects (notably Saturn and the Moon) are guaranteed to lo

spectacular through any reputable telescope. But its only a

ter o time beore youll want to move on to subtler pleasure

you crave lots o quick gratication, you will probably want

see a lot o showpiece objects beore you progress to the sec

stage. I youre a more contemplative type the kind o pe

who enjoys identiying wildfowers or dierent species o s

rows then you may not need those jaw-dropping sights a

though youll surely be grateul when you nd them.

The main actors determining how impressive things lo

through your telescope are its aperture (the diameter o its

lens or mirror) and the observing conditions (the quality o

sky). Aperture is so important that its always the rst act s

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2/8SkyWatch 201

when describing a telescope. When I say that

I own a 4-inch scope, you know that its main

mirror or lens is 4 inches across and that

gives you a pretty good idea what youll see iyou look through it. To a crude rst approxi-

mation, all good 4-inch scopes are the same.

When youre observing the Moon and

the planets, larger apertures let you use higher magnications,

providing more detailed and impressive views. But this state-

ment comes with two caveats. Aperture wont help on the planets

unless its backed up by rst-rate optical and mechanical quality.

And on most nights, the amount o planetary detail thats visible

is limited by atmospheric conditions. When the seeingis poor

when youre viewing through roiling, boiling air theres no

benet to using magnications higher than 200, no matter how

big and good your telescope is. Hard-core planetary observers are

Above: Few consumer

commodities come in

such a wide variety o

sizes, shapes, and types as

telescopes. Here, the author

poses with his smallest, a

70-mm (.8-inch) reractor,

and his largest, a 1.5-inch

Dobsonian reector.

Photo: Carla Procaskey

These images were made by manipulating a Hubble Space Telescope photo to simulate Saturns appearance through dierent size tele-

scopes. The image at let shows how Saturn looks through a 1-inch telescope at 300 when atmospheric conditions are excellent. You

can use 300 on a 3-inch scope, but then the view is dim and uzzy (center). To get a comparably bright and crisp view through the

smaller scope, you would need to use much lower magnifcation, as shown at right. Photo: NASA / JPL

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3/824 SkyWatch 2010

Visual oBserVing:What to Know Before You Buy

happy to wait or that one night in ten when a 12-inch scopecan really strut its stu. But i your goal is to get pretty-darned-

good views o the planets on typical nights, youll be quite

happy with a high-quality 6-incher, and even a 3- or 4-inch

scope might be just ne.

Its a dierent story when youre observing star clusters,

nebulae, and galaxies, which are collectively know as deep-

Galaxies are too aint to stimulate color vision, so theyre always seen

in black and white. And they dont appear nearly as bright or crisp

through an eyepiece as they do in astrophotos. Nonethless, the

Andromeda Galaxy is very impressive when viewed through a 10-inch

telescope at a dark location, as shown at let. But even moderate

suburban light pollution overwhelms the galaxys aint outer sections

(right). Inside a major city, only the bright central oval is visible.

Photo: POSS-2 / Caltech / Palomar Observatory

sky objects. A handul o these are big and bright enough

look spectacular through small telescopes. But most dee

objects need at least 6 to 10 inches o aperture to look rea

impressive at least to untrained eyes. And or the mos

bigger is better, with hardly any limit. Some hard-core d

sky observers own telescopes with mirrors more than tw

across! However, theres more to deep-sky observing thasize o your scope.

As with planets, atmospheric conditions or deep-sky

observing vary rom one night to the next or even rom

hour to hour. In this case, transparency(clear, haze-ree a

more important than seeing(steady air thats ree o ther

currents). But light pollution is the biggest problem by a

Anywhere near a city, articial lights set the entire sk

aglow. Diuse objects like nebulae and galaxies are easil

overwhelmed by this skyglow. Telescopes help a little, bu

much all they can do is make things appear closer tha

actually are. No telescope can bring any galaxy closer tha

Milky Way, which is all around us. So i the sky is too brior you to see the Milky Way clearly with your unaided ey

youre going to get pretty poor views o any other galaxy

less o what instrument you use. No amount o aperture

compensate or light pollution when viewing diuse obj

So are big scopes useless in cities and suburbs? Not a

For one thing, they do show stunning views o the Moon

You will never see this

many stars in a globu-

lar cluster through theeyepiece o a telescope.

On the other hand, no

photograph can capture

the vibrancy o such a

star swarm when you

see its actual light with

your own eyes. So on

balance, this photograph

o Messier 13, the Great

Hercules Cluster, gives a

airly aithul impression

o the glory o a globu-

lar star cluster when

viewed through a large

amateur telescope under

pristine dark skies. Its

no wonder that globular

clusters are many star-

gazers avorite observ-

ing targets!


4/8SkyWatch 201

planets when the seeing allows. And though a galaxy may not

look great through a big scope rom a suburb, it looks better than

it does through a small scope at the same location. But more

to the point, aperture can compensate or light pollution when

youre viewing star clusters. The brightest o the globular star

clusters (see the picture on the opposite page) look just about

the same through a 10-inch scope in a city as they do through a4-incher at a dark site namely, incredibly beautiul!

Still, i you live in a city or suburb and love deep-sky observ-

ing, youd better select a telescope thats small enough to t in

your car, so that you can take it to dark locations or great galaxy

views. Even i you stay in your backyard, think twice beore buy-

ing an instrument thats going to be a hassle to move outside.

My 70-mm (2.8-inch) scope is nowhere near as powerul as my

7-incher, but I oten use it anyway simply because I can pick the

small scope up, tripod and all, and carry it easily with one hand.

Smaller apertures do have some intrinsic advantages, aside

rom cost and portability. One o them is eld o view. Its not an

ironclad relationship (well discuss the exceptions later), but as arule o thumb, the bigger the aperture, the less o the sky you can

view at one time. There arent many celestial objects too big to

view in a 12-inch scope, but there some, such as the Pleiades star

cluster. And they include some o the skys nest showpieces.

I you want to do astrophotography, its much easier to learn

on a small scope than a big one. And small aperture is no obsta-

cle at all to taking great astrophotos you just have to select

subjects that are appropriate or your telescope.

Finally, most stargazers end up owning two or more tele-

scopes, or dierent purposes and occasions. Theres a good argu-

ment or starting small and progressing to bigger scopes later.

That way, when you nally see your amiliar celestial sights with

lots o aperture, you will really appreciate them properly.

Bountiful BinocularsTaking this logic to its extreme, many experienced stargazers

recommend starting with binoculars. Im talking now about

conventional lightweight binoculars, not the exotic instruments

(some weighing more than 100 pounds) that are used or special-

ized tasks such as comet hunting.

Probably the most popular binoculars or general-purpose

stargazing are 1050s. The numbers indicate that they magniy

10 times and have main lenses 50 mm (2 inches) wide. I youalready own small-aperture binoculars, like 821s, youll be

amazed how much they show when you point them at the night

sky. But i youre buying binoculars specically or astronomy, get

ones with at least 30- or 35-mm lenses, to gather as much light as

possible rom aint stars and deep-sky objects.

Selecting the best magnication is a tricky balancing act.

Higher powers show ner details and reveal ainter objects, but

they also magniy every vibration, making it very hard to get

a steady view when youre hand-holding the binoculars. Mo

people nd that 10 is about as high as they want to go, and

preer to stick to 7 or 8.

High powers also undermine one o binoculars biggest

advantages their tremendously wide elds o view. A tele

at 100 shows as much o the sky as you could see through

ticularly narrow drinking straw. By contrast, 7 binoculars

show a signicant chunk o a constellation. That makes it m

easier to nd things in the sky. And some binocular-sized

such as Orions Sword and Belt, are surpassingly beautiul

Are binoculars the right way or you to start? I youre th

o person or whom learning the constellations is a high pr

very likely yes. I youre eager to get the most glorious poss

views o specic objects, probably not. And binoculars don

enough magnication to show planets well, so i you crave

planetary views, by all means start instead with a telescope

Under dark skies, binoculars show breathtaking views o

Milky Way and many deep-sky objects. In a city, binoculars

be disappointing or deep-sky observing, but theyre great

learning the constellations. You can see ar more stars thro

10

50 binoculars in badly light-polluted surroundings thancould without them in the very best possible conditions.

You can get decent binoculars or very little money (less

$100 i you shop careully), so theyre a ne way to dip your

into astronomy without a huge nancial commitment. And

wont stop using them when you buy a telescope. On the co

telescopes and binoculars complement each other perectly

Moreover, binoculars are extremely useul during the dayti

or viewing anything rom baseball games to birds to boats

For astronomical

binoculars, bigger

main lenses are better

until the binoculars

become too expensive

or heavy.

Photo: iStock.com/kb


5/826 SkyWatch 2010


All About OpticsWeve already learned that the three things that matter most

about a telescope are aperture, aperture, and aperture. But

there is more to say. For instance, the ull description o my

second-smallest telescope is that its a 100-mm /6 achromatic

reractor. Lets see what those other words mean.

The terms are easier to grasp i you realize that theres no

undamental dierence between telescopes and camera lenses.

When you do prime-ocus astrophotography (see page 48), you

use your telescope as a camera lens. Conversely, you can buy

adapters to convert camera lenses into astronomical telescopes.

The number ollowing the / is theocal ratio exact

the same measurement that photographers call /stop.

dened as theocal length divided by the aperture. So my

100-mm /6 telescope has a ocal length o 6 100 = 600

A photographer would call it a 600-mm /6 telephoto len

because in daytime photography, ocal length is more im

tant than aperture.

When youre taking a photograph, the ocal length de

mines how big any given object appears in your photo. S

ocal lengths yield wide-angle views; long ones yield clos

shots. And the ocal ratio determines how long your exp

focal length, telescope ( mm)

aperture ( mm)Focal ratio = = f/.

Focal length, telescope mm ( inches)

Magnification = = focal length, telescope ( mm)

focal length, eyepiece ( mm)

Focal length,eyepiece mm

Apert

m( inch

Focal View

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6/8SkyWatch 201

needs to be. I you double the ocal ratio, you need to quadru-

ple the exposure time to get an equally bright photo. For this

reason, telescopes and camera lenses with small ocal ratios

are calledastand ones with long ocal ratios are called slow.

Focal ratio is much less important or visual observing,

because theres an additional element in the optical train: the

eyepiece. I you grew up in the days o lm photography, youknow that cameras make really small images. Photographic

negatives and slides need to be enlarged or you to see any

details in them. Likewise, a telescope orms a tiny image

inside the eyepieces barrel, and the eyepiece is basi-

cally just a magniying glass that lets you put your

eye close to that image so that you can see it better.

The shorter the eyepieces ocal length is, the closer

you can get, the more the image is magnied, and the

ner the visible details are.

The net eect is that the view through a 100-mm

/12 telescope using an eyepiece with a 20-mm ocal

length is identical to the view through a 100-mm /6telescope o equal optical quality when you use a 10-

mm eyepiece. You can compensate completely or the

lower magnication inherent to shorter ocal ratios

by using eyepieces with shorter ocal lengths.

However, the opposite is not true. I you take a

photo o a persons whole body, you can crop and enlarge the

ace, but i you take a close-up o the ace, you cant turn that

into a wide-angle view o the whole body. Likewise, you cant

get a true wide-angle view out o an /12 telescope though

you can come close by using eyepieces with 2-inch barrels

instead o the normal 1-inch eyepieces, as shown at let.

So or visual observing, a 100-mm /6 can do everything

that a 100-mm /12 can, but not vice versa. In addition, the

/6 is probably physically smaller, because or most telescope

designs, the ocal length is roughly equal to the length o the

tube. In that case, why make /12 scopes at all? It turns out th

its harder to provide high optical quality in ast mirrors and

lenses than in slow ones. Thats partly due to practical manu-

acturing obstacles that can be overcome at a cost. But som

o the problems o ast optics are inherent and unxable.

How about achromatic reractor, the last part o my tele-

scopes description? A reractorgathers light with a big lens inthe ront, while a reectorgathers light with a big mirror in th

back. There are also compound designs that use both lenses

and mirrors, the most popular being the Schmidt-Cassegrain

and Maksutov-Cassegrain. The advantages and disadvantages

these designs are summarized on the acing page.

The original reractors used by Galileo and his contempo-

raries had a single lens up ront. They showed gaudy colored

ringes around all objects, because glass bends dierent color

A telescope orms an internal image that is then magnifed by the eye-piece. The size o the image is restricted by the eyepieces barrel. To

ft more in, you either have to use an eyepiece with a larger barrel (

inches instead o 1 inches) or use a scope with a shorter ocal length.

Dobsonian mounts archeap, simple, and e

so many stargazers re

mend 6- to 10-inch Do

beginners on a budget

can also be equipped w

push-to electronics tha

you which way to pus

scope to reach your ta

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7/828 SkyWatch 2010


o light by dierent amounts. This so-calledalse coloris

inherent in all reractors, but it was greatly reduced in the

18th century by using two lenses made o dierent kinds

o glass. These achromatic reractors reigned supreme until

the 20th century, when apochromatic reractors reduced alse

color even more by using three lenses and/or exotic glasses.

Apochromats provide the best possible perormance per unit

o aperture, and are also ideal or astrophotography. And not

surprisingly, theyre also the most expensive telescopes per

unit o aperture.

In act, reractors in general even achromats are

prohibitively expensive in genuinely large apertures.Amateur astronomers love to argue whether reractors or

refectors are better, but this is really mostly a debate about

the relative merits o small and big scopes. Reractors reign

supreme at the small end o the scale, while refectors are lit-

erally the only choice or genuinely large apertures. (Thats

why almost all telescopes used by proessional astronomers

are refectors.) Theres only serious competition between the

two designs in the 4- to 6-inch range.

Compound telescopes such as the Schmidt-Cassegrai

(SCT) and Maksutov (Mak) are attractive to amateur astr

mers mainly because theyre small and light. An 8-inch

Newtonian refector is a monster, with a tube more than

long. But the optics or an 8-inch /10 SCT squeeze into a

less than 2 eet long. In addition to making SCTs highly

table, that places much less strain on their mounts a sthat well cover in the next section.

The All-Important MountTelescope optics are glamorous and un to talk about. Bu

hunk o metal or wood that supports the scope the mo

is every bit as important as the optical tube. In act, as

photographers sometimes spend considerably more mon

their mounts than on their scopes.

The rst job o the mount is to keep the telescope rom

shaking, which is much harder than it sounds. At high p

every little wobble or vibration is magnied so much tha

whole image becomes unviewable.A mount also has to allow the telescope to move extre

smoothly. Celestial objects dont stand still or you to loo

Or, more precisely, they do stand still, but they appearto

across the sky as Earth rotates underneath you. At 200

takes about a minute or an object to cross rom one edg

the eyepiece to the other. To keep an object centered, you

to move the scope requently and in very tiny increments

The very simplest kind o mount is the Dobsonian des

shown on the preceding page. Dobsonian mounts do onl

two tasks outlined above keep the scope steady and let

move smoothly and they do those two jobs extremely

and at very low weight and cost. That makes them exceed

popular or large refectors, which would be impractical t

mount any other way.

Many mounts have motor drives that track celestial ob

automatically. This is essential or astrophotography and

great convenience or high-power planetary viewing. But

sky objects stay in view relatively long at the modest mag

tions typically used to view them, so motor drive is less c

or deep-sky observing.

Many mounts are also equipped with computers that

locate celestial objects and point your telescope at them

well as continuing to track them once theyre located. ThGo To mounts are extremely popular, because locating a

objects manually takes considerable skill and practice. (B

or some people, thats part o the joy o stargazing.) Five

ten years ago, only the most expensive Go To mounts wo

really well, but high-quality Go To is now becoming reas

inexpensive.

Dobsonian mounts can be equipped with Go To drive

custom add-on, but there are also now mass-market Dob

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8/8SkyWatch 201

push to capability. This involves no motors, but an electronic

device tells you which way to push your scope so that it ends up

aimed at your intended target.

Traditionally, serious stargazers always used equatorial

mounts, which have one axis parallel to Earths axis o rotation.

These are the only kind that can track celestial objects automati-

cally without a computer. But with the advent o the Dobsoniandesign and computer-controlled tracking, altazimuth mounts

have gained greatly in popularity or strictly visual observing.

As you can see in the photos on the opposite page, altazimuth

mounts are simple and stable, while equatorial mounts are inher-

ently o-balance (except at Earths poles, where the two systems

are identical). Thats why all big proessional scopes built in

recent years have altazimuth mounts.

But equatorial mounts are still the best choice or serious

amateur astrophotographers. Thats because celestial objects

rotate (or appear to do so) as they move across the sky. Proes-

sionals (and a ew amateurs) use special derotators to compensate

or this. But equatorial mounts automatically keep your cameraoriented the same way as the object that its shooting.

Smart ShoppingOkay, so now you know most o the terminology used to describe

telescopes. How do you actually go about selecting one to buy?

Reading about scopes is all very well, but its no substitute

or actually looking through one. And thats even more true or

binoculars and eyepieces, where individual preerences are

extremely important. I there are any reputable telescope deal

ers in your area, its well worth paying them a visit so that you

can see a wide range o astronomical equipment rsthand.

In addition, get in touch with your local astronomy club.

Theres a airly comprehensive list at SkyandTelescope.com/

clubs, and an internet search may yield additional results. Moclubs have public viewing sessions, where the members will b

all too eager to show o their toys to you.

As or individual makes and models, Sky & Telescope has

reviewed many o them. You can download individual reports

or a modest ee rom SkyandTelescope.com/sttr.

The internet is chock-ull o telescope reviews some o

the highest possible quality and others worse than useless. Yo

can probably learn to separate the wheat rom the cha with a

little practice. One happening place or internet astronomy

these days is Cloudy Nights (www.cloudynights.com). Cloudy

Nights Beginners Forum is particularly helpul; their motto

theres no such thing as a stupid question.Whatever you do, dont orget to have un. Buying your rs

telescope is a serious commitment o time and money. But

its also a great opportunity to explore the wild and wonderul

world o astronomical equipment.

Sky & Telescope associate editorTony Flandersowns fve tele-

scopes and fve pairs o binoculars.

A star party oers an unparalled

opportunity to try out a wide assort-ment o dierent telescopes.

About Scopes

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