Astro Mini-Course Day 4

Telescopes and Binoculars: The Basics

Once you get a little practice reading a star map and finding your way around the sky, you’ll have many enjoyable nights of stargazing ahead of you using just your unaided eye.

For some, that’s all they’re after.

But many new stargazers are keen to get a telescope to see more.

And why not? Even a modest backyard telescope shows thousand of deep-sky objects and millions of stars… enough for a hundred lifetimes of careful observation and patient contemplation.

But many beginners rush into purchasing a big, complicated telescope and quickly become frustrated and disillusioned because they did not build enough skill and understanding to use such an instrument.

You’re not going to make that mistake… because today’s lesson will show you what you need to know and how to prepare yourself to move into the world of observing with optical instruments.

Binoculars

The easiest telescopes to use are a pair of binoculars. Binoculars are the ideal starter instrument because, compared to a full-sized telescope, they are simple and intuitive to use. You see the image right-side-up in front of you. And the large field of view of binoculars makes it easy to find what you’re looking for. Yet binoculars reveal many sights you learned about in Day One of this mini-course, including craters, mountains, and seas on the Moon, planets and their satellites, the occasional comet, countless double and variable stars, dozens of star clusters, and some of the brighter nebulae and galaxies.

All binoculars are marked with two key numbers, magnification and aperture. A pair markeed “7×50″, for example, magnifies 7 times and has objective lenses 50 mm in diameter. Bigger lenses let you see fainter objects.

Some binoculars are marked with the field of view, either in degrees or “feet at 1000 yards”. This tells you how wide a scene you’ll see. A typical pair lets you see 5 to 8 degrees, a little larger than the width of a golf ball held at arms length. In comparison, a telescope lets you see a field of view of 1 degree or less, which is like looking at the sky through a drinking straw.

For astronomy, more aperture is better. So a 10×80 pair lets you see fainter objects than a 10X50 pair. The trade-off? Bigger lenses means more weight, which makes them harder to hold for any length of time.

High power means more detail and a darker background sky. But it’s harder to keep a high-power pair of binoculars steady enough to see fine detail, since the shaking of your arms also gets magnified.

A good pair of new binoculars will cost $100-$300. A less expensive pair may suffer from poor optical coatings and materials. A more expensive pair gives you crisp images out to the edge of the field of view. Nice to have, but not a necessity.

You can learn more about choosing binoculars by clicking here…

A pair of 7×35 or 8×42 binoculars are give you an ideal blend of light weight, magnification, and bright images. You’ll use them for years.

A pair of Orion 8×42 binoculars… an ideal blend of weight, magnfication, and light-gathering ability

Don’t be afraid to invest in a good pair of binoculars. Most amateur astronomers, even those with a large telescope and years of experience, turn to their binoculars for quick and easy viewing and for sweeping the star fields of the Milky Way.

Once you’re set up with a pair of binoculars, head back outside with your star charts to sweep the constellations. You’ll be amazed at how much more you can see. Sights like the Pleiades, the Orion Nebula, the Andromeda Galaxy, and the star fields of the Milky Way are particularly spectacular sights with binoculars.

And if you can, find a guidebook like Steven James O’Meara’s book The Night Sky With Binoculars, or our own downloadable guidebook Stargazing For Beginners: A Binocular Tour of the Night Sky.

10 Things You Should Know/Do Before You Buy A Telescope

Most beginners who buy a telescope before learning the basics of what to see in the sky (and how to see it) usually get frustrated and give up astronomy before they barely get started.

So how do you know if you’re ready for a telescope? Here’s a subjective list of 10 things you need to know and do before you take the leap into telescopic observing:

• Know the main points on the celestial sphere: the horizon, zenith, meridian, location of the north (or south) pole, and the ecliptic (you got good start on this on Day 2)

• Learn a dozen or so bright stars and major constellations (the mini-tour and maps on Day 3 got you started on this)

• Learn to find and see things through binoculars, especially the Moon, Jupiter, and bright DSO’s like the Orion Nebula, Andromeda Galaxy, and the Pleiades.

• Look through a telescope and get a feel for how much (and how little) you can see. Many beginners are surprised to see only 0.5 to 1 degree of the sky at a time… it’s a little like looking at the sky through a drinking straw. You can try a friend’s telescope, or attend a star party held by a local astronomy club. If possible, have someone help you find and see a faint object in a telescope to get an idea of what’s involved

• Learn the main types of telescopes and their pros and cons (you’ll learn more about that today…)

• Learn the main features of a telescope… what they are, what they mean (again, a little more about that today). Knowledge is power.

• Determine where you will observe, and how you will get your telescope there. No use getting a big monster scope if you have to wrestled down the stairs of an apartment building every night.

• Figure out where you will store your telescope… it needs a clean dry place that’s conveniently located to let you move the scope out to your observing site.

• Think about what want to observe? Just the moon and planets? Faint fuzzies like nebula and galaxies? Birds and mountains? A little of everything? If you just want to see the moon and bright planets once in a while, you need far less telescope than if you’re bound and determine to look at faint galaxies, for example.

• Count your pennies… and decide how much you can spend on a telescope. Don’t get a new scope that costs less than US$300-$400. You will be disappointed with the quality. Save a little more, or stick with binoculars for now.

Selecting a Telescope

Like breakfast cereals, the number of telescopes on the market today is completely overwhelming. To help you make sense of what’s out there, let’s go over the essentials of telescope design and configuration to give you some idea of what to look for (and what to avoid) when shopping for a new scope. With these facts and suggestions, you’ll be ready to pore over telescope ads and understand which scopes are best for you.

In a nutshell… you want to get a telescope with good-quality optics and a smooth, steady mount. And all other things being equal, scopes with a larger objective lens or mirror shows you more.

But portability and convenience matter, too— because the best telescope is the one you actually use most frequently.

Aperture

The most important characteristic of a telescope is its aperture — the diameter of its light-gathering lens or mirror, often called the objective. As a rule of thumb, your telescope should have at least 2.8 inches (70 mm) aperture — and preferably more.

Large-aperture scopes let you see fainter objects and finer detail. But don’t discount a small scope… it can still show you a lot… especially if you have good dark sky. You can, for example, see dozens of galaxies beyond our own Milky Way through a scope with 80 mm aperture from a dark location. But you need a 6- or 8-inch telescope to see such galaxies from a suburban backyard. But aside from dark skies, the view through a large-aperture telescope is almost always more impressive than the view of the same object through a smaller scope.

Remember… aperture is critical. Forget about telescopes you see advertised by their magnification — especially crazy-high powers like 500-600×. A telescope’s maximum useful magnification is 50 times its aperture in inches, or twice its aperture in millimeters. You’d need a huge 12-inch scope to get a decent image at 600×. And even that requires a night when observing conditions are ideal.

The objective’s focal length is a key specification. That’s the distance it takes the lens or mirror to bring star light to a sharp focusl. To determine a telescope’s magnification, simply divide the objective’s focal length by the focal length of the eyepiece. Each is marked clearly on the tube of the scope and barrel of the eyepiece. If, for example, a telescope has a focal length of 500 mm and you use it with a 25-mm eyepiece, the magnification is 500/25, or 20x. Most telescopes come with one or two eyepieces; you change magnification by switching eyepieces with different focal lengths.

Another key specification of a telescope is the focal ratio, which is the focal length divided by the objective diameter. A long focal ratio implies higher magnification and narrower field of view with a given eyepiece.

Type of Telescopes

For amateur astronomers, there are three main telescope configurations:

The optics and light path of refractors, reflectors, and compound telescopes.

• Refractors collect light with a large lens at the top of the tube. A refractor is the type of telescope you may most recognize. While generally low maintenance, refractors are expensive compared to their aperture, or light-gathering capacity. Because lenses refract blue light more than red light, refractors also tend to show false color, especially on brighter objects like the moon and planets. But refractors also produce the highest contrast of any telescope, which makes them superb for astrophotography and visual observation of fine detail.

• Reflectors (also called Newtonian reflectors) collect light using a mirror at the rear of the main tube. For a given aperture, these are generally the least expensive telescope. The simple Dobsonian configuration of a Newtonian reflector offers especially superb value. Reflectors occasionally require adjustment of their optical alignment– especially if the scope gets bumped around. Reflectors are good all around telescopes, but they tend to be the bulkiest design, with tube lengths of 4-5 feet.

• Compound (or catadioptric) telescopes use a combination of lenses and mirrors to pack the optics into a short, light-weight tube; the two most common designs are the Schmidt-Cassegrain and Maksutov-Cassegrain. Compound scopes are also excellent all-around performers, though they tend to have larger focal ratios and smaller fields of view than Newtonian reflectors. Because they have more optics than reflectors, they are also more expensive.

Telescope Mounts

Even the finest telescope isn’t of much value if it doesn’t have a sturdy mount to point it around and damp out vibrations. Some telescopes come packaged with a mount and a tripod; smaller scopes may just have mounting rings or plates which allows them to be attached to a mount which you buy separately.

All telescope mounts are one of two types. An alt-azimuth mount lets the scope to move up-down (altitude) and left-right (azimuth). It’s quick to set up and intuitive to use. Many Newtonian reflectors come on a simple wooden platform known as a Dobsonian mount, which is a variation of the alt-az mount.

A more involved mount, designed to track the motion of the stars by turning on a single axis, is called an equatorial mount. Because only one axis needs to be moved to track celestial objects, equatorial mounts can be more easily motorized. To work properly, one axis of the mount must be aligned with the north celestial pole (or the south celestial pole, if you observe in the southern hemisphere). There are many variations of equatorial mounts; they all tend to be larger and heavier than alt-az designs.

A variation of an altazimuth mount (left) and equatorial mount (right).

Some telescopes come with mounts that have small motors to move them around the sky with the push of a button. Often called “Go To” telescopes, a small computer is built into a hand control. Once you’ve entered the current date, time, and location, the computer and mount can point and track thousands of celestial objects. Some “Go To”s even let you choose a guided tour of the best celestial sights, complete with a digital readout describing information about each object.

Go To telescopes are a great convenience, and help you spend more time looking at objects and less time finding them. But they are no substitute for learning your way around the sky.

Where to learn more….
Finder scopes and eyepieces are critical accessories for a telescope. A finder and one or two eyepieces are supplied with most scopes. You might consider getting one or two more. Here is a short guide to eyepiece terminology and selection. And here’s a little about finder scopes…

Scopes To Avoid
Whatever you do, please, please don’t buy cheap department store telescopes that advertise based on magnification only. They cost a little less than the telescopes you see in the astronomy magazines, but they are invariably junk. Stay away!

Just Tell Me What Scope To Get…!
If you’re reading this web page and you just want a quick straight-up recommendation of a telescope to get a beginner… a good 8-inch Dobsonian reflector is a good bet. You’ll pay $400-$500 for such a scope, some of which have a computer for helping find objects. If compactness and convenience is more important, try the compound ETX125 telescope from Meade. It goes for about $1,000 and collects less light than an 8-inch Dob, but it has full Go-To capability and is easy to haul around and store.

An 8-inch Dobsonian-style reflector… a fine beginner telescope.

And remember …don’t rush out and get a telescope before you’re ready. Start with learning to read a star chart, and with finding your way around the sky with your unaided eye. Move to binoculars shortly after. And have a little patience before you get your telescope… it will be worth the wait.

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