How to Choose Your First Telescope

Buying a first telescope is exciting, and it is also where many new stargazers go wrong.

Boxes promise huge magnifications and dazzling color images,

and too many telescopes end up in a closet after a few frustrating nights with a wobbly mount and a blurry view.

The good news is that choosing well comes down to a few clear principles.

Here is what really matters, how the main designs compare,

what to expect at the eyepiece and how to set yourself up for a great first night.

Aperture Is the Number That Matters Most

The aperture is the diameter of a telescope's main lens or mirror, usually given in millimeters or inches.

It determines how much light the telescope collects and how much fine detail it can show.

Light-gathering power grows with the area of the aperture,

so a 150-millimeter telescope collects about two and a quarter times as much light as a 100-millimeter one,

and several hundred times more than your dark-adapted eye.

Magnification, by contrast, is easy to change by swapping eyepieces, and it is often wildly exaggerated in advertising.

A useful rule of thumb is that the highest practical magnification is about twice the aperture in millimeters,

or about 50 times per inch, and even that only on nights with very steady air.

Most of your best views will come at modest powers.

Treat any box that leads with an enormous magnification figure as a warning sign.

The Main Telescope Designs

Refractors

Refractors use lenses at the front of a closed tube, the classic telescope shape.

They need little maintenance, cool down quickly and give crisp, high-contrast views, which makes them good for the Moon, planets and double stars.

Simpler designs show a faint colored fringe around bright objects, and refractors cost more per millimeter of aperture than other designs.

Reflectors

Reflectors, usually of the Newtonian design,

gather light with a curved mirror at the bottom of the tube and send it to an eyepiece near the top.

They offer the most aperture for the money and show no color fringing.

The open tube means the mirrors occasionally need realigning, a routine task called collimation.

Dobsonians

A Dobsonian is a Newtonian reflector on a simple, sturdy rocker base that sits on the ground.

You just push the tube to where you want to look.

This combination delivers large apertures, rock-steady views and very little setup,

which is why it is often recommended as a first serious telescope for visual observing.

Larger models are bulky, and the design is not suited to long-exposure photography.

Catadioptric Telescopes

Catadioptric designs, such as the Schmidt-Cassegrain and Maksutov-Cassegrain, combine mirrors with a corrector lens and fold the light path inside a short tube.

They are compact and portable for their aperture, and their long focal lengths suit the Moon, planets and double stars.

The trade-offs are a narrower field of view, a longer cool-down time and a front lens that easily collects dew.

The Mount Can Make or Break It

A telescope is only as good as the mount holding it.

If the view shakes every time you touch the focuser or a breeze blows, you will see little detail and give up quickly.

A modest telescope on a solid mount beats a bigger one on a flimsy tripod every time.

An altitude-azimuth mount moves up and down and side to side, which feels natural.

An equatorial mount tilts one axis to line up with Earth's axis,

so a single motion follows the stars across the sky;

it is excellent for tracking and photography but takes practice to set up and use.

Computerized mounts can find thousands of objects at the push of a button after a short alignment,

which helps under bright city skies, though they need power and add setup time and cost.

Some observers prefer to learn the sky first and add automation later.

Focal Length, Eyepieces and Magnification

Every telescope has a focal length, and every eyepiece has one too.

Divide the first by the second to get the magnification.

A telescope with a 1,200-millimeter focal length gives 48 times with a 25-millimeter eyepiece and 120 times with a 10-millimeter one.

A Barlow lens placed before the eyepiece typically doubles the magnification of each eyepiece you own.

Start every session with your longest-focal-length eyepiece, which gives the lowest power and the widest view,

and only increase the power once your target is centered. A small finder,

either a red-dot sight or a little finder scope, sits on the tube to help you aim;

aligning it with the main telescope is the single most useful setup step.

What You Will Really See

Realistic expectations prevent disappointment. Through a typical beginner's telescope under reasonable skies, you can expect:

• Sharp craters, mountains and valleys on the Moon, which never fails to impress.
• The rings of Saturn, even at fairly low magnification.
• Jupiter's cloud belts and its four largest moons, which change position from night to night.
• The changing phases of Venus.
• Colorful double stars and sparkling star clusters.
• Bright nebulae, such as the Orion Nebula, as soft gray glows.
• Galaxies as faint, fuzzy patches, not the colorful spirals of long-exposure photographs.

Mars is often disappointing, showing detail only around its closest approaches, and stars stay as points of light no matter how much you magnify them.

Questions to Ask Before You Choose

The best telescope is the one you will actually take outside. Before deciding, think honestly about a few things:

• How far will you carry it, and up or down how many stairs?
• Where will you store it, and will it fit in your car if you travel to dark skies?
• Are you more drawn to the Moon and planets, or to faint clusters and galaxies?
• Do you want to learn the sky by hand or have a computer find targets?
• Is photography a goal? Serious astrophotography usually calls for different equipment than visual observing.

If you are still unsure, a good pair of binoculars is an excellent first step,

and visiting a local astronomy club's public night lets you look through several designs before you commit.

Setting Up for Success

Take the telescope out well before you plan to observe, since mirrors and lenses need time,

sometimes an hour for larger instruments, to reach outdoor temperature. Until they do, the view shimmers.

Align your finder during the day on a distant treetop or pole, and practice focusing on it too.

Solar safety comes first.

Never point a telescope anywhere near the Sun unless a certified solar filter made for that telescope is securely fitted over the front of the tube, and keep the finder capped, removed or fitted with its own solar filter.

Never use a filter that screws into the eyepiece, and never let children use a telescope unsupervised in daylight.

Looking at the Sun through unfiltered optics can cause instant, permanent eye damage.

Making the Decision

Prioritize aperture you can comfortably carry, a mount that stays steady and a design that suits the objects you most want to see.

Ignore magnification claims, start with low-power eyepieces and make the Moon your first target.

Choose sensibly and your first telescope can become a companion for many years of clear nights.