Reflector telescopes, often called Newtonian telescopes, utilize a concave mirror which focuses light at a single point, where the eyepiece then refocuses the light, forming an image. Reflector telescopes capture light through an open ended tube, where it travels to the bottom, where the mirror is located. Once the light reaches the bottom of the tube and bounces off of the mirror, it travels back up the tube, where it is captured by a small mirror that reflects the light to the eyepiece, which is located on the side of the telescope.

A large advantage to reflector telescopes is that they are relatively inexpensive, meaning that you can get a much larger telescope for your money, compared to a reflector telescope. Also, reflector telescope’s lack of a lens at the front element means that the telescope will not suffer from chromatic aberration.

A disadvantage to reflector telescopes is that the mirror at the end of the tube can become misaligned with transport or rough handling, requiring periodic collimation, or alignment.

Refractor telescopes are what the average person thinks of when they hear the word telescope. A refractor telescope is the type of telescope that has a lens on one end and an eyepiece on the other end. The light passes through the lens at the front of the telescope and travels down the barrel of the telescope, where it is focused by the eyepiece to form an image.

A large advantage to refractor telescopes is that there are no mirrors in the optical pathway, resulting in very sharp images.

A disadvantage to refractor telescopes is that they can suffer from chromatic aberration, which appears as color fringes at the edges of very bright objects. Apochramat refractor telescopes use lenses that are corrected for chromatic aberration; however, this greatly increases the cost of the telescope.

Catadioptric telescopes, two common types are known as Schmidt-Cassegrain telescopes (SCT) and Maksutov-Cassegrain telescopes (Maks), send the light along a folded optical path, essentially creating an optically long telescope in a physically short structure. Light passes through a lens at the front of the telescope, where it is focused on a mirror at the end of the tube, the light is reflected in the direction of the lens at the front of the telescope, where it is caught by a small curved mirror that focuses the light back toward the large mirror at the bottom of the telescope’s tube, where it passes through a hole in the center, once the light passes through the hole in the center of the tube, it is focused by the eyepiece to form an image.

The primary advantage to catadioptric telescopes is that their folded optical path allows them to be very short in length, which results in your being able to use lighter mounts, as the telescope is inherently more stable. Catadioptric telescopes are especially well suited for astrophotography, as they focus by moving the primary mirror, and not the eyepiece, meaning that they have a very long focal range, making catadioptric telescopes ideal for cameras, which require significant back focus room.

A disadvantage to a catadioptric telescope is that they suffer from central obstruction from the small secondary mirror, which causes a loss of light, which appears as an overall decrease in the amount of contrast in the scene when compared to a refractor telescope.

NASA has released a 4D live model of the earth’s ionosphere, where you are able to virtually fly through the ionized gas layer that encircles the border between the earth and space.

The ionosphere is the last layer of the earth’s atmosphere before entering space and stretches from 50-miles to 500-miles above the earth’s surface; it forms the inner edge of the magnetosphere and is located in the thermosphere. The ionosphere is created by solar radiation ionizing, or breaking apart, molecules and atoms in the earth’s upper atmosphere, resulting in a layer of electrons and ions.

The ionosphere is important to modern day communication, as radio signals are often bounced off of the lowest level of the ionosphere and reflected back to earth in order to reach greater distances; also, GPS singles from satellites must pass through the ionosphere before reaching receivers on
the ground or in planes, and atmospheric disturbances, caused by the sun, can cause errors up to 100-meters in location determination.

Download NASA’s 4D Ionosphere Model now.

Eyepieces are the part of the telescope that makes the image for your eye to see. The first half of the telescope is the lens or mirror, located at the front of back of the telescope’s tube, at the eyepiece is located at the side or back of the telescope’s tube.

On a refractor telescope, the eyepiece will sit in a focuser at the back of the telescope, which is a unit that is able to be moved in and out to focus the image that the telescope is looking at. The focuser unit comes in two standard sizes, 1.25 inches and 2 inches, before you buy your eyepieces, it is important to know which diameter eyepiece your focuser unit utilizes.

Calculating the power of the eyepiece that you will be using on your telescope requires some simple math. The primary lens or mirror in your telescope has a focal length, which is the distance that it takes for the light to be focused into a point; this is called FL. The eyepiece that you will look through also has a focal length, which is called fl. You need to concert the focal length of the primary lens or mirror and the focal length of the lens piece to millimeters, and then divide the focal length of the telescope by the focal length of the eyepiece (FL/fl). As an example, if your telescope had a 2000mm focal length (FL) and your eyepiece had a 20mm focal length (fl), the magnification would be 100x.

There are two main types of telescope eyepieces, Kellner eyepieces and Plossl eyepieces.

Kellner eyepieces utilize a three lens design, are fairly inexpensive and have good image quality from low to medium magnification.

Plossl eyepieces utilize at least a four lens design, where high quality eyepieces can become rather expensive. Plossl eyepieces are some of the most common on the market today and offer a relatively large field of view, however, also suffer from a very small amount of eye relief, which can lead to discomfort with excessive use.

Choosing the right telescope can be a daunting task, especially if it will be your first telescope. Your best bet is to avoid the temptation to go to a department store and play with the floor models, since you will probably walk away overpaying for a low quality refractor telescope that will frustrate you, especially when you try to use all of the poor quality included accessories.

Also, do not get caught up in the sales hype of 500x magnification telescopes. That will likely be of little value to you, especially since the majority of your viewing will likely take place in 100x-200x range.

You need to find a telescope that is optically high quality and mechanically solid.

Be sure that the mount that the telescope comes on is stable, when you touch the telescope or mount, the telescope should not wobble.

When you lock the telescope into a certain position, it should not move from that position.

The telescopes barrel should be made of metal or high quality cardboard; telescopes whose barrels are made out of plastic should be avoided.

Lenses and eyepieces should be made of glass and certainly not plastic, and there should be at least once eyepiece included as standard.

The telescope’s barrel should have a finder scope mounted on it, which is very useful for locating celestial objects before focusing on them with the telescope.

The aperture, or diameter, of the telescope’s primary lens or mirror is the most important factor in choosing a telescope; the larger the diameter of the primary lens or mirror of the telescope, the more light that the telescope will collect, resulting in a brighter and crisper image.

When purchasing your first telescope, a 70mm-90mm diameter is a good size for a refractor and a 6-8” mirror is a good size for a reflector.

You should buy the biggest telescope that you can afford and transport. Generally, you will store your telescope inside, and only bring it out when you want to use it, and if you purchase a reflector telescope that has over an 8” mirror, it is probably going to weigh more than 50lbs, so it is unlikely that you are going to bring it out very frequently.

You should expect to spend no less than $300 for your first telescope, and you might even want to budget a bit more