Telescope and Binocular Buying Guide

Whether you're a stargazer or birdwatcher, you'll need the right tools to see your subjects clearly. In this guide we break down the types of telescopes and binoculars, and share some important things to know before you make a purchase.

Types of Telescopes

Refractor

A refractor telescope uses a lens, which is a combination of two to four polished glass pieces, to bring light into focus, therefore producing images. There are two types of refractor lenses, achromat and apochromat. Achromat refractors use two pieces of glass to bring all colors of light into focus. Apochromat refractors are top of the line and they use four pieces of glass.

Good to know:* They require the least amount of maintenance of all telescopes because the lenses don't need recoating (like mirrors do) and they don't require realignment unless they take a fall.

• Small refractor telescopes are light enough to mount on a sturdy camera tripod. This makes them easy to grab and go.
• Refractor telescopes are best for observing bodies that need high contrast to see small details, such as planets and double stars.
• A refractor's closed tube requires time to adjust to temperature changes.

Reflector

A reflector telescope uses a system of mirrors to produce an image. The large, curved mirror situated at the bottom of the tube is the primary mirror, while the smaller, flat mirror situated near the top is the secondary mirror. Light travel down the tube, hits the primary mirror and reflects to the secondary mirror. The secondary mirror reflects the light, and the image, into the eye piece.

Good to know:* A reflecting telescope offers the best size-per-dollar ratio.

• The largest amateur telescopes are reflectors.
• They show no excess color, meaning you won't see color fringes around even the brightest objects.
• Because of how the mirror attaches to the tube, reflector telescopes are sensitive to jostling when they are transported. To ensure the best stargazing session, usually a reflector telescope will need to be collimated (adjust the mirrors) before each use.

Compound or Catadoptric

A compound telescope uses a combination of lenses and mirrors to produce an image. Light enters the tube through a corrector plate and hits the primary mirror at the tube's base. This mirror reflects light to a secondary mirror mounted on the corrector. The secondary mirror reflects light through a hole in the primary mirror to the eyepiece at the back of the telescope.

Good to know:* Most compact of all telescopes, compound models are easy to use on-the-go.

• Usually sold as complete systems
• The closed tube requires time to adjust to temperature changes.

• Any mirror or lens twice as large as another will capture four times as much light. So, a 6-inch mirror collects four times the light of a 3-inch mirror.

Telescope Mounts

​The mount is almost as important as the optical tube itself. An unstable mount will hinder even the best telescope from delivering a quality image. A mount that is too light will be jostled by the wind and cause your images to bounce. Below is a quick overview of the four types of telescope mounts.

Alt-azimuth Mount

The simplest type of mount, it moves up, down, left, and right.

Dobsonian Mount

The least expensive type of mount, a Dobsonian mount is often combined as a set with reflector telescopes. The tube sits loosely in the mount, making it easy to carry the two parts. Amateur telescopes that are more than 16 inches across sit in a Dobsonian mount.

Equatorial Mount

This mount allows the telescope to follow the stars as they move across the sky. Originally, one of the mount's axes was aligned parallel to the Earth's axis and moved with a weight-driven clock drive. Today, many equatorial mounts use a motor to move them.

Go-to Mount

Using motors and an onboard computer, this type of mount will find and track a celestial target. These highly accurate, and expensive, systems often have large databases full of thousands of objects.

• Tip!Telescopes flip the image you are viewing upside down, which isn't a big deal in space. However, if you're using a telescope to view earthbound objects, invest in an image erector. It will flip the image right-side up.

Binoculars

These versatile instruments are portable, relatively inexpensive, require little expertise to set up, and have a wide field of view. Unlike telescopes, what you see through binoculars is right-side up, making them easy to use. Binoculars contain prisms, which make the image appear the way you would view it with the naked eye. Also, many people find it more comfortable and natural to observe through both eyes, rather than one. Below are some important things to keep in mind before you purchase binoculars.

Prism Type

Roof Prism* This type of prism allows binoculars to have straight barrels and to be more compact.

• Tend to be more expensive than Porro-prism binoculars
• Produce dimmer images, which are not as desirable for astronomy

Porro Prism* Inside the barrel, Porro-prism binoculars have a zigzag shape.

• They are usually bigger and heavier than proof-prism models.​

​Measurements to Remember

If you will be using binoculars to stargaze, the front lens measurement is important. The larger the front lens is, the brighter the image will appear. Binocular measurements look like this 7x35 or 10x50. The second, larger number refers to the size of the front lens in millimeters. In 7x35 binoculars, the front lens is 35 millimeters, and in 10x50 binoculars, the front lens is 50 millimeters. A 50 millimeter lens will gather twice the amount of light as a 35 millimeter lens.

• Astronomy binoculars should have lenses at least 40 millimeters across. Smaller lenses will work in the daytime.
• Magnification on binoculars used for astronomy should be at least seven.
• Don't go any higher than 10x magnification for hand-held binoculars because you likely won't be able to hold them steady enough to see a sharp image. Use a tripod for binoculars over 10x magnification.