The greater the eye relief, generally, the more comfortable the binocular or telescope eyepiece usage will be, since your eye will not be pressed right up against the last piece of glass. Also, the greater the eye relief, the easier it is for people who wear glasses to use the eyepiece or binoculars, as the extra eye relief accommodates the space that glasses take up.

Ideally, you want what is known as the exit pupil of the eyepiece or binoculars and the entrance pupil of the eye to be the same size so that you are able to see a bright and crisp image. However, to have a large amount of eye relief, the exit pupil of the eyepiece or binoculars must be larger than the entrance pupil of the eye so that you have space to move back from the rear optical element; the result is that you are wasting some of the light that your telescope eyepiece or binoculars are gathering, but you are gaining a more comfortable viewing experience.

This post was written to answer a reader’s question, if you would like to have your questions answered, email questions@telescopes.org or comment on the site.

At any point during the free guided tour of space, you are able to temporarily pause the program for exploration, where you can seamlessly pan across the night sky and zoom into stars and planets for a better view. While using World Wide Telescope project software, you can view the night sky in visible light, x-ray and hydrogen alpha wavelengths. After exploring the sky on your own, you can resume your guided tour from where you left off.

Download World Wide Telescope now.

That is a very difficult question to answer, since so much of a telescope recommendation depends on what you want to look at, how far you need to transport your telescope to get to dark skies and how much you want to spend on the total system.

Orion manufactures one of the best beginner telescopes on the market. It is high quality, light weight enough to be considered semi-portable and is rather affordable. It is called the Orion SkyQuest XT8 Classic and is an 8” Dobsonian reflector telescope, and priced at $299.95, direct from Orion, is one of the best values on the market!

The Orion SkyQuest XT8 telescope focuses on the essential elements; ease of use and high quality optics, instead of trying to pack in every electronic gadget and tracking system possible (at the compromise of image quality), which so many similarly priced telescopes are guilty of.

When using the Orion SkyQuest XT8 telescope, you will need to make use of your knowledge of the constellations and sky charts to navigate the night sky, since the telescope must be manually adjusted for celestial exploration.

One of the Orion SkyQuest XT8 telescope’s greatest features is build quality. Unlike similar Dobsonian telescopes, the Orion SkyQuest XT8 actually uses a steel tube to encase the optics, instead of a cardboard tube. Also, the SkyQuest XT8 utilizes Teflon bearings with tensioning springs to balance and pull the telescope against the bearings, enabling the SkyQuest XT8 to be easily and securely positioned by hand without the worry of inaccuracy.

Dobsonian telescopes are often recommended for beginner astronomers, and one of the rarely mentioned benefits is that since they use such a simple mounting system, there is no need for tedious, and potentially frustrating, polar alignment (which is required for equatorial mount telescopes).

Buy the Orion SkyQuest XT8 Telescope now!

With binoculars, you can easily view objects in the night sky like, craters on the moon, the Milky Way galaxy, over 100,000 stars, faint nebulas and distant galaxies.

Binoculars are also very useful given the weight and size; they can be put into a back pack, set on the back seat of a car or carried around you neck without any inconvenience and with limited preparation, compared to a 6”, 10”, 12” or larger telescope.

Excellent quality binoculars can be purchased for a comparatively small amount of money. For instance, a few hundred dollars will buy you a fair quality entry level telescope, whereas, a few hundred dollars will buy you an excellent pair of binoculars, which you will never need to replace.

There are two types of binoculars, those that are handheld and those that are mounted. Generally, handheld binoculars will be lighter and will have less magnification power than mounted binoculars, but will provide a larger field of view of the sky.

When choosing binoculars for astronomy, realize that binoculars which are made for daytime terrestrial viewing won’t suffice for nighttime celestial viewing; however, binoculars made for nighttime celestial viewing will be fine for daytime terrestrial viewing.

Some good size binoculars for most people under 40 years old to consider include those that are 7×50 (7x magnification and 50mm aperture) or 8×56 (8x magnification and 56mm aperture) which both have an exit pupil of around 7mm (exit pupil = aperture / magnification). As people age, their pupils become less well able to dilate, meaning that using binoculars with an exit pupil bigger than the eye’s maximum pupil dilation diameter will cause the image to appear darker, since not all of the light collected by the binoculars is reaching the pupil of the eye. For people over 40-years old, a good size of binoculars to consider are the 7×35 or 8×40, which both have an exit pupil of 5mm.

Binoculars that utilize the porro-type prism are best for space use, as they produce a brighter image than roof-prism binoculars and tend to be a little less expensive, despite being slightly larger in size.

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.

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.