Telescope and Astronomy Glossary

The glossary is pretty small, so it is quite possible that what you are looking for is not here yet. We will be adding to it though, and including more pictures.

Aperture

The diameter of the main mirror or lens. Aperture is the most important thing to know about a telescope. It determines what you can see. Take a look at the difference aperture makes.

Dark Nebula

Dark nebulae are clouds of dust that have no stars near them to make them glow. Instead, they block the light of the stars behind them making a "hole" in the sky. In star-rich areas of the sky, dark nebulae can show off their often interesting and intricate shapes. Image Credit European Southern Observatory (ESO).

Dobsonian

Dobsonians are simple telescopes that just move up and down, and left and right (or, more correctly, they move in altitude and azimuth), and have their base right on the ground. Dobsonians are named after John Dobson (who prefers to call them "sidewalk telescopes"). He popularized the idea of spending most of your money or effort on the optics of your scope and little or nothing on the mount, tube, focuser, and all the rest. Mirrors are commonly 8 to 20 inches wide, which was unheard of a few decades ago, before the "Dobsonian revolution". Lately, "Dobsonian" has come to mean any telescope built in this style even though many of them are not at all inexpensive, and some are practically works of art.

Emission Nebula

An emission nebula is made out of gas and glows because it is being ionized by the radiation of nearby stars. Emission nebulae are usually red or teal depending on the gasses that are being ionized. The picture shows the Tarantula Nebula. The cluster in the center is making most of the gas cloud glow. Image Credit European Southern Observatory (ESO).

Eye Relief

This is a measure of how far from an eyepiece you need to hold your eye in order to see the whole image. Cheap eyepieces tend to have less eye relief causing you to have to press your eye against them. If you wear glasses while observing, you will need to purchase eyepieces that have lots of eye relief (15 or 20 millimeters).

Field of View

There are two kinds of fields of view: apparent field of view, and real field of view. In a discussion of eyepieces, the most important one is the apparent field of view. It is a measure of how much of your visual field is taken up by the image you see. The picture below shows the difference between an eyepiece with a 50 degree FOV, and one with an 80 degree FOV. Both are giving the same magnification, but one shows a lot more sky.

The real field of view is a measure of how much of the sky you are seeing through a telescope. It is usually from less than a tenth of a degree to two degrees of the sky. (For comparision, the full moon is half a degree wide.) You can calculate the real field of view of an eyepiece by dividing its apparent FOV by the magnification it gives you.

Focal Length

The focal length of a telescope is the distance from the mirror or lens at which it forms an image. The focal length will determine how long the telescope is. You need to know the focal length of the scope to find the magnification of an eyepiece, but unless you are building a scope, it doesn't matter much more than that.

You can find the focal length in several ways. The focal length of a Schmidt-Cassegrain or Maksotov telescope is usually printed near the front lens. This is also sometimes true of refractors. If you have a manual, it will always list the focal length somewhere. Often you can find it on the Internet by doing a search for the model number of the scope. If all else fails, you can hold a tape measure up to the tube: in a reflector or refractor, the length of the tube will be pretty close to the focal length.

The focal length can be printed in a couple ways. It may simply say, "FL=1200mm." It also may list the focal ratio. That is an "f", a slash, and a number: "f/6" for example (pronounced "f six"). To find the focal length, multiply that number by the aperture of the scope. Focal lengths are usually listed in millimeters or inches.

The focal length of an eyepiece is always measured in millimeters, and is usually printed on the eyepiece. Dividing that number into the focal length of a telescope will tell you what magnification the eyepiece gives in that scope. (When you divide, remember to use the telescope's focal length in millimeters.)

Galaxy

A galaxy is a collection of billions of stars as well as massive clouds of dust and gas. Galaxies come in many shapes and sizes. There are spheres, spirals, and irregular blobs. The image of the spiral galaxy is from the European Southern Observatory (ESO).

The Milky Way is the galaxy we live in. It is a spiral, but in the night sky it looks like an irregular, milky stripe because we are seeing it edge-on from the inside. Just about everything you can see in the night sky is part of the Milky Way.

There are billions of other galaxies in the universe. You can see many with a telescope, but the light of those billions of stars and nebulae is reduced to a feeble glow by the tremendous distance. Nearby galaxies show detail in a large telescope, but the rest are simply dim, fuzzy spots. Still, many astronomers like the challenge of tracking them down anyway.

Galaxies are sometimes confused with solar systems. Take a look at that entry to understand the difference.

Globular Cluster

Globular clusters are huge balls of ancient stars. We aren't sure exactly when or how they formed, but every galaxy has some of them. The Milky Way has around 200 orbiting within and around it. Some globular clusters are estimated to hold a million stars! They can be seen as diffuse glows or giant, glimmering spheres depending on how close they are and what kind of telescope you use. If you use the Hubble, you get an image like this one from the Hubble Heritage projet.

Nebula

There are several types of nebulae, but all are either clouds of dust or gas. (The word nebula means cloud.) See: Dark Nebula, Emission Nebula, Planetary Nebula, Reflection Nebula

Open Cluster

Open clusters contain up to a few hundred stars formed near each other from the same cloud of gas. Open clusters are not as compact as globular clusters, but that means you can see the individual stars much better—even with a small telescope.

Planetary Nebula

Planetary nebulae are formed when a medium sized star gets old. These stars puff off their outer layers leaving behind a white dwarf star and an expanding cloud of gas. Many planetary nebulae are round, which is how they got their name, but they come in other shapes too. There are hourglasses, helices, and cylinders. Some have more complex shapes that so far defy explanation. They come in a wide range of brightness and size, and some can be even be seen in binoculars. All three planetary nebula images are from the Hubble Heritage project.

Reflection Nebula

Reflection Nebulae are dust clouds reflecting the light of nearby stars. They are usually bluish in color. The image is from the Hubble Heritage project.

Solar System

The solar system consists of the sun and everything orbiting it. In orbit are nine planets, a bunch of minor planets and asteroids, comets, and cold, icy kuiper belt objects out at the edge. Technically, ours is the only solar system, but the term is being used to describe other star systems with planets too these days.