Dobsonian vs Newtonian telescopes
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Dobsonian and Newtonian telescopes are both reflector telescopes, utilizing a mirror and not lenses to see into space.
The term Newtonian refers to the actual telescope itself; whereas, the term Dobsonian refers to the telescope with its altazimuth mount.
An Altazimuth mount, also known as an alt-azimuth mount, is a very simple dual-axis telescope mount.
Dobsonian telescopes use a mechanically operated alt-azimuth mount, meaning that to move the telescope when navigating through space you literally push it and pull it up and down and left and right until you are pointing at what you want to see. The telescope moves smoothly due to the mount’s bearings and stays in place due to the mount’s friction and there are no parts to breakdown or wear out over time.
The only real disadvantage with a Dobsonian telescope is that it is tricky to track objects as they move through space, making it an impractical type of telescope to use for astrophotography.
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Why choose a reflector telescope?
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Reflector telescopes are a good choice for beginner astronomers.
Reflector telescopes, sometimes called Dobsonian or Newtonian telescopes, are great telescopes for those just getting started in astronomy for a number of reasons.
Generally, reflector telescopes are relatively inexpensive to produce, when compared to similar refractor telescopes and similar catadioptric telescopes.
The relatively low cost of reflector telescopes provides tremendous value, where you are not sacrificing optical quality or build quality for price.
You will be able to clearly see distant and faint objects that other similarly priced refractor telescopes and catadioptric telescopes would prevent you from seeing.
It is important to note that reflector telescopes use a mirror and not lenses, and that the top of the telescope tube is open, meaning that dust and debris can fall onto the mirror. Also, on occasion, you will need to clean the telescope’s mirror and possibly even collimate it (realign it).
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Telescope upgrading
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What do you actually get when you upgrade from a child’s telescope to a beginner’s telescope or from a beginner’s telescope to an intermediate telescope?
Almost always, when you upgrade your telescope, you are paying for three elements; portability, optical quality and build quality.
For portability, you should expect your upgrade to lead you to a telescope that can easily be moved by one person while carrying it in one or two pieces, each weighing around 20lbs or less.
For optical quality, you should expect your upgrade to lead you to a telescope that has an aperture of f-8 or faster (meaning a number that is 8 or smaller), that has limited chromatic aberration (fringes of color around the image), no vignetting (dark corners of the image), no spherical aberrations (halos around the image) and no other distortions (dark or blurry images, etc).
For build quality, you should expect your upgrade to lead you to a telescope that is built with strong and lightweight materials that feel durable during use and the telescope should come attached to a very sturdy mount which offers smooth repositioning adjustments.
You may also find upgrades like on-board tracking computers, heaters to evaporate dew, GPS location systems and CCD imagers on higher priced telescopes.
Two telescope upgrades that we recommend are the Orion SkyQuest XT6 and the Orion SkyQuest XT8.
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Planisphere
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Before you even purchase your first telescope, the very first thing that you need to buy is a plastic planisphere, or a star map, so that you can learn the constellations to be able to navigate your way through the night sky.
You need to purchase a planisphere for the correct hemisphere that you will be observing from, and also for the correct latitude that you will be observing from. For the northern United States, the 40-50 degree latitude star map will be sufficient, for other regions, determine the latitude that you will be observing the night sky from, and purchase the appropriate planisphere.
The number one reason for owning a planisphere is to be able to know what you are looking at, and to be able to navigate your way from object to object and from constellation to constellation without having to guess or rely on the auto-tracking computer system on your telescope.
Planispheres, or star maps, are around $10 and should not need to be replaced for many years, if ever.
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Eye Relief and Why it Matters
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When buying a telescope eyepiece or binoculars, an important consideration is eye relief, which is the distance between the exit pupil of the optical system and the entrance pupil of the eye; in other words, how much space there is between last element (the farthest back piece) of the the eyepiece or binoculars and the place where your eye goes to clearly see the image.
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, click the “Ask Our Astronomy Experts a Question!” link above or leave a comment on the site.
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World Wide Telescope Project
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The World Wide Telescope project is software that allows your computer to act as a telescope, presenting you with the best images from ground based and space based telescopes from around the world, where you are able to take guided tours from astronomers and educators, who happen to work out of the best observatories and planetariums in the nation.
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.
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One of the most frequently asked questions in astronomy is, “What should my first quality telescope be?”
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 under $400, 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).
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Binoculars have their place in astronomy, and three of their biggest advantages come down to portability, field of view and price.
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.
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Reflector, Refractor and Catadioptric Telescopes
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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.
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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.
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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.
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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.
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Telescope eyepieces
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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 convert 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.
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Advice on buying your first telescope
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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 
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GPredict Satellite tracking software
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GPredict Satellite tracking software is great free software that tracks satellites and predicts their orbit in real-time. GPredict is able to track an unlimited number of satellites and displays their positions and additional information in lists, tables, maps and as polar plots. Beyond real-time tracking, GPredict is able to predict the time of future satellite passes and display information about upcoming passes.
A great feature of GPredict is that it allows the user to group satellites into modules, where each module can be configured independently from the rest. GPredict also allows the user to track satellites from different observation locations.
Notable features of the GPredict software include:
- Accurate real-time satellite tracking using the NORAD SGP4 / SDP4 algorithms
- Unlimited number of satellites or ground stations
- Graphical user interface utilizing maps, tables and polar plots
- Customizable satellite modules
- Future satellite path predictions
- Automatic Keplerian elements updates
Space Weather
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SpaceWeather.com is a great resource for planning your celestial observing with free information about solar wind, solar flares, asteroid flyby alerts, sunspot numbers, coronal holes, geomagnetic storms, near earth asteroids and more.
Stay current with space weather updates at SpaceWeather.com!
Me & Isaac Newton
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Presented by billionaire Paul Allen and featuring chemist Gertrude Elion, environmental physicist Ashok Gadgil, theoretical physicist Michio Kaku, computer scientist Maja Mataric, cognitive scientist Steven Pinker, professor of cancer medicine Karol Sikora and primatologist Patricia Wright, the 110-minute documentary, Me & Isaac Newton, humorously explores the life, accolades and accomplishments of seven leading scientists.
Particularly suited for high school students, Me & Isaac Newton discusses how the seven featured leading scientists became interested in science, and what remarkable experiences their education and profession have led to.
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