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Refractor Telescopes

The refractor telescope was invented over 400 years ago and is still going strong today.  Refractors use lenses to gather light and bring the resulting image to focus.  Refracting telescopes come in a variety of optical sizes, from the super portable inexpensive 50 mm to about 150 mm, or approximately 6". The main difference in refractor designs, and why you will notice such a wide range of prices when shopping for a refractor, has to do with the number and quality of lenses used.  An achromatic refractor is designed with two lenses that are therefore only capable of focusing two wavelengths of light, and are usually the most economical. These refractors are a good choice for beginners whose main interest includes traditional visual astronomy and/or daytime terrestrial use.

Explore Scientific 102 mm Air Spaced Doublet Refractor Telescope OTA

Refractors that are capable of focusing all three wavelengths of light (red, green & blue) at the same point are called apochromatic refractors.  Apochromats, or APO refractors, can be used for many pursuits in amateur astronomy, including terrestrial use, visual astronomy and astrophotography.  The APO design is more expensive than an achromat due to the increased number of lenses and/or the high-quality specialized glass that may be used to create all or some of the optics.  The body of an APO refractor, known as the OTA or optical tube assembly, is also more likely to be made from quality materials and have a precision focuser that is designed to bear the weight of a camera and other imaging equipment. 

Advantages of a Refractor Telescope:

  • Refractors require the lowest maintenance of any telescope design
  • The closed tube design of a refractor keeps out air currents that decrease image quality
  • By adding an image-erect prism diagonal, a refractor can be made to display images that are oriented correctly for daytime use
  • Longer focal length refractors achieve higher contrast images and darker background skies

Disadvantages of a Refractor Telescope:

  • Refractors have a higher cost per inch of aperture than reflectors
  • Refractors are expensive to make in apertures larger than 6", making them prohibitive for most people
  • Achromatic refractors suffer from chromatic aberration, which can be seen as purple glow around bright objects. APO refractors, however, can be nearly free of extra color.
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Ritchey-Chretien Telescopes

Since the Ritchey-Chretien telescope uses mirrors to gather light it is technically a reflector, but unlike the more common Newtonian, the RC - short for Ritchey-Chretien - has a Cassegrain configuration.  A Cassegrain also uses mirrors to gather the light, but has a folded optical path like a catadioptric telescope.  Because the optical path is folded, the tube of a RC telescope is much shorter, making larger apertures more portable.  Most large Ritchey-Chretien telescopes have an open truss tube design rather than a closed tube to decrease the weight of the optical tube as a whole.

Apertura 6in f/9 Ritchey-Chretien Steel Optical Tube

The Ritchey-Chretien telescope design uses two hyperbolic mirrors to correct for coma and spherical aberration.  Other aberrations, such as astigmatism, can be greatly reduced with an additional field flattener.  The majority of telescopes at professional observatories are a Ritchey-Chretien because their wide, flat fields are perfect for large CCD or CMOS sensors.  Smaller aperture RC telescopes are available for amateur astronomers who wish to work with an instrument that was designed for astro-imaging, including this inexpensive entry-level Ritchey-Chretien, the Apertura 6" f/9 Ritchey-Chretien OTA telescope.

Advantages of a Ritchey-Chretien Telescope:

  • The Ritchey-Chretien is a favorite of advanced astro-imagers due to it's wide, flat field of view and it's accommodation of large sensors
  • The folded optical path of the RC telescope shortens the tube length for more portability
  • Coma and spherical aberrations are corrected on the Ritchey-Chretien for better images
  • The Ritchey-Chretien has a flat focal plane for sharp focus across the field

Disadvantages of a Ritchey-Chretien Telescope:

  • The Ritchey-Chretien has the highest cost per inch of aperture
  • Most RC's are large and require high-end, big equatorial mounts
  • The Ritchey-Chretien is designed for imaging. While you can look through an RC telescope, it is trickier to use visually than other designs.
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Newtonian Reflector Telescopes

The most common reflector telescope we use today was invented by Isaac Newton, which is why you'll hear this type of telescope also referred to as a Newtonian reflector. Simply put, while a refractor telescope uses lenses to gather light, reflectors use mirrors. Light gathered from a primary mirror, located at the bottom of the telescope tube assembly, is reflected to a small secondary mirror that is angled perfectly so as to direct the beam of light into the observer's eye via an eyepiece.

Celestron NexStar 130 SLT Computerized Telescope

To choose a reflector that is right for you, consider its aperture (diameter of the primary mirror), the focal length, which dictates how long the telescope tube will be, and what type of mount you require. While it is always true that a bigger mirror will gather more light than a smaller one, you will want to pick something that you can comfortably lift and use.  The mount you choose also dictates the total weight and heft of the telescope system.  A Dobsonian telescope for instance, is a Newtonian reflector on a simple rotating box-style mount.  Because the Dobsonian mount is not made of metal and does not have heavy counterweights to contend with, one can buy a fairly big 8" or 10" Dobsonian telescope without too much trouble moving it around.

However, if you want to do astrophotography, your Newtonian reflector needs an equatorial mount.  Many astro-imagers use a fairly fast Newtonian reflector on a computerized equatorial mount due to the lower cost as well as the wide-field views this type of telescope can achieve.

Advantages of a Newtonian Reflector: 

  • Newtonian reflectors are the least expensive telescope design per inch of aperture
  • Reflectors are offered in large apertures to see into deep space
  • A reflector does not suffer from chromatic aberration or false color
  • The long open tube of a Newtonian reflector keeps dew from forming on optics

Disadvantages of a Newtonian Reflector:

  • Reflectors require regular collimation for best results
  • Thermal air currents in a reflector's open tube assembly can distort the image
  • Reflectors have lower contrast than refractors of same aperture due to secondary mirror obstruction
  • The optics of a reflector are open to the air and so susceptible to dust & other particles
  • A parabolic primary mirror produces coma at edges of field, which is especially noticeable with faster f/3 -f/6 reflectors
  • Reflectors are not best for terrestrial use because they produce an upside-down image that cannot usually be corrected
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Catadioptric Telescopes

A catadioptric telescope uses a combination of mirrors and lenses to gather and focus light.  Common catadioptric designs include Schmidt-Cassegrain and Maksutov-Cassegrain telescopes as well as more exotic designs like the Celestron EdgeHD and Meade ACF series telescopes.  The folded optical path of all catadioptric telescopes make them very portable for the size of their optics.  For instance, a Celestron 8" f/10 NexStar SE Schmidt-Cassegrain OTA is just 17" long and weighs 12.5 pounds!   To bring home just how amazing that is, a Newtonian reflector or a refractor with the same aperture & focal ratio would be over 6 feet long and weigh much, much more.  Because a catadioptric telescope is so portable and lightweight for its aperture, the mount that supports it does not have to be Herculean and you will not need a ladder to reach the eyepiece when the telescope is pointed high overhead.  It's no wonder that Schmidt-Cassegrain telescopes are one of the most popular designs in the world! 

Celestron NexStar 8SE Computerized Telescope

All Catadioptric telescopes have a folded path optical system and make use of mirrors & lenses, but there are differences in their design that are worth noting.  The standard Schmidt-Cassegrain Telescope, or SCT, has a medium focal ratio of f/10, making it a jack-of-all-trades; good for terrestrial viewing, enjoying the planets, deep-sky observing, or astrophotography.  The SCT uses a flat-looking aspheric corrector lens on the front end of the telescope which is less expensive than other corrector plates, making the Schmidt-Cassegrain a good value as well.

The Maksutov-Cassegrain telescope has an interesting design, and if you love the views through a long refractor, the Mak-Cass might just win you over.  The corrector plate is a highly curved spherical lens called a meniscus.  Maksutov-Cassegrain are great for visual use or planetary imaging, the views will be full of contrast with a darker background sky due to their long focal lengths.  The downside of a long focal length, however, is longer exposure times, so Maksutov-Cassegrain telescopes are not the best choice for deep-space astro-imaging. 

Proprietary catadioptric telescope designs are worth a look, especially if you want to get into astro-imaging!  The Celestron EdgeHD aplanatic flat-field Schmidt optical system produces a flat field for stunning star images everywhere you look. In fact, the EdgeHD focal plane is 3 times flatter than a standard SCT!  Celestron EdgeHD telescopes are also compatible with the optional Fastar system, which turns the scope's f/10 system into a super-fast f/2 imaging powerhouse. 

Advantages of a Catadioptric Telescope:

  • Catadioptric telescopes have a folded optical light path for the best portability of all the designs
  • There is a huge selection of accessories available for Cat telescopes, allowing you to grow your system over time
  • Telescopes with a catadioptric design do not suffer from chromatic aberrations
  • Schmidt-Cassegrains and other catadioptric scopes are decently priced for the aperture and build quality
  • Catadioptric telescopes are available with or without an alt-azimuth or equatorial mount

Disadvantages of Catadioptric Telescope:

  • Collimation of optics is required occasionally, although not as often as reflectors
  • Catadioptric telescopes are more complex than reflectors or refractors, with more moving parts 
  • The secondary mirror obscures a percentage of light, similar to reflectors
  • Catadioptric scopes, especially larger Maksutov-Cassegrains, are more expensive than Newtonians of the same aperture
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Telescope Facts: Choosing The Right Telescope

Choosing the right telescopes online for you and your family can be a fun and enlightening experience if you take the time to answer a few key questions first.  Don't worry, this is not a test on your telescope knowledge!  We would just like you to ponder the following five questions, and by doing so, you will automatically narrow down your telescope choices to a more manageable level and have a greater chance of picking out the best telescope!  We will go into each of the questions in greater detail further down the page, so scroll down if you need more information.

How do you plan to use the telescope?

There are so many types of telescopes for sale, so asking yourself what you'd like to do with the telescope is very important!  Below is a list of activities you can do with a telescope to help you decide:

  1. You can use a telescope like you would a spotting scope for terrestrial use. This can include bird-watching and checking out wildlife, as well as people-watching or ship spotting from your deck or home with a view. 
  2. You can use a telescope to take pictures of birds, animals, landmarks, people and more during the day.
  3. You can use a telescope to look at the Moon and bright planets, including Venus, Jupiter, Saturn & Mars.
  4. You can use a telescope to look at galaxies, star clusters, nebulae and other deep-sky objects outside of our solar system.
  5. You can use a telescope to take pictures of celestial objects as close as the Moon, or faint, far-away galaxies.
  6. You can use a telescope to do some or ALL of the pursuits listed above!

Where do you plan to use your telescope?

Most people use their telescope from the comfort of their own backyards, and most set up their telescope each time they plan to use it.  If this is how you imagine using your new telescope, then you want to buy a telescope that is easy and fast to set up and break down.  Remember, the best telescope is the one you will use!  Some lucky amateur astronomers have a backyard or land available to install a personal observatory so they can leave the telescope set up yet protected from the elements. High Point can help you navigate that scenario if you plan to house your scope. 

What if you live in the city and plan to take your telescope to a dark-sky site for better views?  Most telescopes are built to travel but again, consider whether the telescope will fit in your vehicle and if you can set it up on your own.  Of course, if you want a telescope that can be set up on the roadside at a moment's notice, portability and ease of use are paramount.  It is also a good idea to buy a telescope case for those "have telescope, will travel" moments in order to protect your scope and make it easier to carry!

Are you interested in doing astrophotography with your telescope?

This question will help you decide what kind of telescope mount you need.  If your idea of astrophotography is using your smartphone and a telescope smartphone adapter to take quick photos of the Moon or bright planets, then congrats, you can still use most any type of telescope mount as long as it is sturdy enough to allow for shake-free images.  This is because your exposure time would be quick enough that the Earth's rotation would not come into play.  However, if you want to take pictures of celestial objects outside of the solar system, it will require a telescope on an equatorial mount.  An equatorial mount is designed to track the stars as they move across the sky due to the Earth's rotation.  We recommend a computerized equatorial mount to make it easier to align, find and track celestial objects.  The actual telescope you buy is also a consideration if you want to do deep-sky astro-imaging, but it will help narrow down your choices if you consider the 5th and last question first:

How much can you spend on a telescope?

Most people have no idea how much they will need to spend to buy a decent telescope that will let them observe most everything they are interested in.  After all, a telescope is often a one-time purchase, and unless we do some targeted research, we are not apt to see advertisements for telescopes like we do cars or washing machines.  It probably will not help you to know that the price of telescopes runs the gamut from around $100 to over $10,000, because really, it all comes down to what you are willing to spend, not what's available.  If the telescope in question is for a child and you are not sure how long their interest in astronomy will last, it is perfectly reasonable to choose a kid's telescope on the lower end of the price scale.  We do recommend you choose a telescope, however, that has a sturdy, easy to use mount and optics that are big enough to see an array of objects.  For more suggestions, here is a quick article on the best kid's telescopes.

Similarly, a telescope for an adult should be sturdy, move easily, and have quality optics that are big enough to see their items of interest.  If someone dreams of looking at galaxies, shoot for at least a 4-5" telescope  Bigger is better and will increase the detail as well as the number of deep sky objects one can see, but of course more aperture increases the price and the size/weight of the telescope system.  Prices will also increase for computerized telescopes, but if you can swing it, a GoTo computerized telescope is worth it!  These telescopes can almost align themselves and then find thousands of objects with the push of a few buttons.  Some even tell you about the object while you are looking at it.

You will spend the most on telescopes for astrophotography.  This makes sense if you think about all of the components of a good astro-imaging telescope:  optical quality, optical size, mount precision, and electronics that will support precise tracking of the object while the photograph is being taken.  That said, astro-imagers today are achieving results that only professionals would have been capable of a decade ago.  It is an exciting time to be in the hobby of astronomy!

What is the Best Telescope to Buy?


There is no such thing as a "best telescope", only the best telescope for you at this point in your astronomy adventure.  Today your best telescope might be an 8" Dobsonian, but a year or two from now you may be looking for a Celestron EdgeHD telescope on an equatorial mount.  It all depends on what interests you most and which compromises you are willing to make (or NOT). 

To get the most out of the information from this point on, take a look at the five questions we asked in the How to Choose a Telescope section and think about how they apply to you.  There are several characteristics you will want to pay attention to when buying a telescope, including...

Aperture:  An old adage about choosing the best telescope goes like this:  "Buy the biggest telescope you can afford that you can set up by yourself and will use..." While there is obviously a bit more to it than that, this is wise advice because it is a fact that the bigger the diameter of the primary optics, the more you will see. So it would be a no-brainer to get the biggest telescope you can afford, right?

Here is where the second and third parts of the adage come into play:  "...that you can set up by yourself and will use."  The bigger the optics, the bigger and heavier the telescope.  If you buy a telescope that is heavier than you can handle alone, you will tend to use it less.  You'll look at that big beautiful telescope and say, "maybe tomorrow night".  If that keeps happening, it doesn't matter how big your telescope is because it is not being used.  Sure, it looks impressive to friends, but it will have a tendency to gather dust.

Get a Dobsonian telescope if you want a lot of aperture for less money and don't care about motors and electronics!  Dobs make a perfect first telescope, and many, many amateur astronomers never outgrow it.  Even if they decide to get into astrophotography and buy another telescope on an equatorial mount, most Dobsonian owners love their telescope enough to keep it and pull it out whenever they want to experience the cosmos visually.

Schmidt-Cassegrain telescopes use a folded light path design to keep the tube assembly portable for their aperture.  For example, a 9.25" SCT weighs just 20 pounds and is only 22" long, making it fairly easy to hoist onto an equatorial mount if you so choose.  The point is, if you want aperture, portability AND electronics, look into a Schmidt-Cassegrain or EdgeHD telescope in the size you'd like, and then choose whether to have it ride on an equatorial mount and tripod or a computerized alt-azimuth mount and tripod.  Either works great for visual and short exposure photography, but for true astro-imaging, choose an equatorial mount.

Focal Length:  The focal length is the distance, given in millimeters, from the primary mirror to the point where the light rays come together in focus.  For a refractor or a Newtonian reflector, the focal length gives a good indication of how long the tube is.  For catadioptric telescopes like the Schmidt-Cassegrain, knowing the focal length is not a clue to the tube length since the optical path is folded to create a more portable telescope. 

The focal length also affects the field of view.  Longer focal length telescopes have a narrower field of view but more magnification, while a shorter focal length telescope has a wider field of view that works great for big, faint deep sky objects viewed visually or for astrophotography.  You can buy both reflectors and refractors in shorter or longer focal lengths.

Lots of times the focal ratio is displayed rather than the focal length when shopping for a telescope.  The focal ratio can be calculated if you have the focal length and aperture.  Just divide the aperture (convert to millimeters first) into the focal length to get the focal ratio.  Once you get used to it, seeing the focal ratio gives you a good indication of the field of view.  The smaller the f number, the wider the field and more optimized for astrophotography.

Schmidt-Cassegrain and EdgeHD telescopes have a focal ratio of f/10.  This is a good combination of field of view and magnification for people who want to do a bit of everything the astronomy hobby has to offer.  There are also focal reducers available to shorten a longer focal length and create a wider field that is better for astro-imaging, especially with today's larger sensors.

Choice of Telescope Mount:  You can buy telescopes on either an equatorial mount or an alt-azimuth mount, or you can buy a telescope OTA, or optical tube assembly, without a mount.  Choosing which mount type is best for you is fairly straightforward, so let's take them one at a time:

Telescopes on Alt-Azimuth Mounts:  Telescopes that come with an alt-azimuth mount are a popular choice for beginners or as a kid's telescope due to their ease of use and relative portability.  An alt-azimuth mount is designed to move north, south, east or west, either manually or electronically via a hand controller.  These simple movements make an alt-azimuth mount a great choice if you want to use your refractor or catadioptric telescope for observing in the daytime as well as at night.  That is because an alt-azimuth mount mimics the way objects move on the earth: up, down, back and forth.  Alt-azimuth telescopes are also a good choice for visual astronomy.  It is simple enough to make adjustments in both axes (north/south, east/west) as celestial objects shift position due to the earth's rotation.  A Dobsonian telescope is a good example of a simple alt-azimuth design.  Simply push, pull or rotate the telescope to find and follow the celestial object of your choice!  Telescopes on computerized alt-azimuth mounts, like the Celestron NexStar SE do an amazing job of finding and tracking celestial objects for visual astronomy, and a user could even take quick pictures of the Moon and bright planets, but if you want to take astro-images of objects out of our solar system, it will require an equatorial mount.

Telescopes on Equatorial Mounts:  Equatorial mounts can be polar-aligned to compensate for the Earth's rotation, allowing them to manually or electronically track celestial objects.  We recommend you buy an equatorially mounted telescope if you are intrigued by the idea of astrophotography.  You can buy a manual equatorial mount (no motors) and use it just fine for visual astronomy, but if astro-imaging is in your future, we recommend you buy a telescope on a computerized equatorial mount.  While there are still equatorial mounts on the market that have motors available for both axes (a minimum requirement for astrophotography), computerized mounts are more plentiful than ever and can be an economical choice since you won't have to worry about adding motorization to your manual mount later on.  If you already have a telescope optical tube assembly and plan to buy an equatorial mount separately, make sure it has enough capacity to carry the weight of your telescope, camera, or any other equipment you plan to ride on your scope.

Telescope Optical Tube Assemblies or OTA:  You can buy an OTA if you already have a good mount and want to upgrade your telescope system.  Make sure your mount can handle the weight of the telescope you plan to buy. Amateur astronomers also buy telescope optical tubes so they can build a custom telescope system, with an equatorial mount and either a traditional tripod or placed on a pier in their personal observatory.  Most of us won't be able to realize the dream of an observatory, but for those who have the means and the land available, it is a wonderful way to start observing at a moment's notice and never have to set up your telescope again.

Telescope FAQs

What is the most popular telescope?

Telescope popularity changes over time, and so the most popular telescope in 2021 may not be the most popular telescope in 2022.  That said, there are some telescopes that remain popular over time due to amazing technology, ease of use or price point.  Here are a few of our favorite popular telescopes for sale:  Dobsonian telescopes have remained popular over several decades.  They are priced low compared to other telescopes of the same size, are easy to use, and help you learn the night sky.  We recommend the Apertura AD 8" Dobsonian for its quality and included accessories.   Another amazingly popular telescope is the Celestron NexStar 8SE.  The NexStar 8SE is a portable Schmidt-Cassegrain computerized telescope that is easy to use and has 8 inches of aperture to see deep into space.  Smaller NexStar SE telescopes are also available if you want something even more portable that costs less.

How do you know a telescope is good?

Keeping in mind that the word "good" is subjective, there are a few simple ways to know if you have a decent telescope.  First and foremost is the quality of the image it delivers.  A good telescope will be able to produce sharp images of the Moon and brighter planets on a clear night of steady seeing.  Also, a good telescope should not wobble on its mount or tripod.  Even if the telescope itself focuses and has good optics, a cheap or defective telescope mount or tripod (or both) will make it almost impossible to find, follow and view objects.  Thirdly, the problem may not be whether the telescope works, but if it is big enough to see the objects you want to see.  If you are pleased with the images your telescope produces and it makes you happy, then you, personally, have a "good" telescope.  Others may want something bigger, more computerized or better for astrophotography, and you may want that too someday, but for now, assuming you can achieve pleasing views and the mount is steady, enjoy your telescope!

What type of telescope is best?

The best telescope is one that you can afford, that you can set up on your own, and that you will use!  After that, it all depends on what you'd like to do with your telescope, because there is no one telescope that is "best".  Folks who want to look at the Moon, Saturn and Jupiter occasionally but who are not interested in photography would do perfectly well with a medium-sized refractor on a simple quality alt-azimuth mount, whereas those who want to do photography would require an equatorial mount with either computerization or dual axis motor drive.  If you want to see as deeply into space as possible, you will want to buy the biggest telescope you can comfortably use.  Remember, the bigger the telescope optics, the brighter deep space images will be.  As you can see, there really is no one best telescope!  For help in choosing the "best" telescope for YOU, which is what really matters, contact a member of our product advisor team and we will guide you through the world of telescopes and buying telescopes online!

Can you look through a telescope with glasses?

Yes, you can keep your glasses on when viewing through a telescope if you like, and in fact you will definitely need to leave them on if you have astigmatism.  There are a few things to keep in mind for a good experience, however.  Wearing glasses keeps your eye further away from the telescope eyepiece, and if the eyepiece does not have good eye relief - the distance from the lens to where the image comes to focus - you may not be able to see the object very well, if at all.  When looking through someone else's telescope, the only thing you can do when this happens is remove your glasses and focus the telescope as best you can.  An alternative is to try folding down the rubber eyecup of the eyepiece when viewing with glasses.  This brings you a little closer, and depending on the eyepiece, you may have some luck.  However, when it comes to your own equipment, if you want or need to keep your glasses on, we recommend all of your eyepieces have a minimum of 15 mm of eye relief, 18-20 mm is even better.  This puts the focus point right about where your glasses meet the eyepiece, and will make a huge difference in your viewing comfort.

What is the best telescope mount?

To determine the best telescope mount for you, it is important to think about what you plan to do with the telescope. In general, the "best" telescope mount is a solidly built equatorial mount and tripod that has a large enough weight capacity to easily carry the telescope and any other accessories you plan to use. However, if you want to use your telescope as a spotting scope during the daytime and look at the Moon and planets at night, we would recommend a solid alt-azimuth mount instead.  Alt-azimuth mounts, with their simple up, down, right and left movements work best for daytime use or nighttime use when looking at objects that are easy to find in the sky.  Telescopes on computerized Alt-Azimuth mounts, like those included with the Celestron NexStar SE line of telescopes, do a great job for both day time and night time observing, however it would not be considered the "best" mount if you plan to do astrophotography.  This takes us back to the solid equatorial mount, which is required for long exposures of galaxies, nebulae and other deep sky objects.  While you can buy a manual equatorial mount and add dual-axis motors and a hand controller for astrophotography, these days most people prefer the computerized equatorial mount, like the Celestron Advanced VX Mount.  Smart electronics help you with alignment and are very accurate when tracking, which makes it way easier to get good results, especially when doing longer exposures.

What telescope accessories do I need?

While the telescope accessories you need depend on the telescope and the kind of observing or photography you plan to do, all telescopes could benefit from a Moon Filter!  This inexpensive telescope accessory threads onto a standard eyepiece and blocks some of the light emitted from the Moon.  Using a moon filter is similar to wearing sunglasses to help you see better and protect your eyes from becoming overwhelmed with too much light.  Here are some other popular telescope accessories:  A dew shield for Schmidt-Cassegrain and other catadioptric telescope users, a star chart or planisphere and red flashlight (useful even if your telescope is computerized), extra eyepieces for a change of magnification, and a portable battery and DC or AC adapter to power electronic telescopes.  That's just the beginning!  Our product advisors can help you choose your telescope accessories, so when you're ready, give us a call!

Is it safe to look at the Sun with a telescope?

No, it is not safe to look at the Sun through your telescope UNLESS you are using a dedicated solar telescope with built-in filtering or you have a quality solar filter properly installed over the front aperture a regular telescope!  Have you watched a piece of paper catch fire when light was shined on it through a magnifying glass?  A telescope is a powerful magnifying glass!  Protect your eyes at ALL times.  Beware - even people who do the right thing and use a properly installed solar filter make the mistake of looking through their finder scope to locate the Sun in their eyepiece!  To keep from accidentally doing that, remove the finder scope all together and either replace it with a solar finder, which projects an image telling you when the Sun is in the field of view, or use the telescope's shadow to ascertain when the sun is in the proper location.  Finally, if you have a little solar filter that screws onto an eyepiece, throw it away. These old-time "solar filters" have been known to fall apart in mid-use because the sun actually melted the glue or other adhesive holding them together! 

What does seeing conditions mean?

When amateur astronomers are talking about seeing conditions in astronomy, they are discussing (or complaining about!) current atmospheric conditions. I say "current" because the sky's atmosphere is always changing and will be different not only from night to night, but sometimes from hour to hour! Things like dust, pollen, air currents, temperature, dew point and weather conditions all affect what you see through your telescope. Objects that were in sharp focus at a particular magnification on Saturday night may not come to focus with the same eyepiece on Sunday. This is one reason why amateur astronomers have several eyepieces (3-5 is most common). They not only allow you to increase magnification on nights of good seeing, but decrease magnification when the air is less steady. By the way, if you have a catadioptric telescope with a corrector plate at the very front of your optical tube, a dew shield or dew heater will help keep dew from forming on your optics on a damp night.

Telescope Terms To Know


Aperture is the diameter of a telescope's primary mirror or lens listed in millimeters or inches.  The bigger the aperture of a telescope, the more light it will gather, allowing the observer to see more detail on celestial objects and ascertain finer details that a telescope of lesser aperture may not see.

Dobsonian Telescope

The Dobsonian telescope consists of a Newtonian reflector optical tube assembly mounted on a very simple alt-azimuth box-style mount with a lazy susan base. This base was invented by John Dobson to encourage people to make their own telescopes from start to finish. Up until that time, telescopes required a heavy equatorial mount that produced a financial or weight barrier for some otherwise enthusiastic budding astronomers. John Dobson and his base changed all that, and even today, Dobsonian telescopes still provide the most light-gathering dollar for dollar.

EdgeHD Telescope

EdgeHD is Celestron's top of the line flat field aplanatic Schmidt-Cassegrain telescope series. You can buy a variety of apertures, from 8 to 14 inches, as an OTA or as part of a telescope/mount package. EdgeHD telescopes are great for visual use but they really shine for astrophotography. With a wide field of view that is three times flatter than a standard SCT, you will see beautiful, pinpoint stars across the entire field, even if you have a full frame (35 mm) CMOS or CCD camera sensor to satisfy.


An eyepiece is a group of lenses housed in a small package that is closest to the eye when used with a telescope, microscope, or spotting scope. The eyepiece provides a particular magnification when paired with a telescope, therefore most amateur astronomers use a variety of eyepieces to change magnification for different types of objects. The eyepiece nomenclature is expressed in its focal length in millimeters. To figure out the magnification, simply divide the focal length of the eyepiece into the focal length of the telescope. The result is the magnification provided in your particular telescope or one with the same focal length.

Finder Scope

A finder scope fits on top of the main telescope and is used to help you find and center objects in your eyepiece. A finder can be as simple as a red dot finder or it can be a high quality small telescope in its own right.

Focal Length

The focal length is the distance, usually measured in millimeters, between the primary mirror or lens and the point at which the image comes to focus.  Generally, classic refractors have a longer focal length, Newtonian reflectors tend to have a focal length that is shorter, and Schmidt-Cassegrain fall somewhere in the middle.

Focal Ratio

The focal ratio is calculated by dividing the aperture (mm) of the primary mirror or lens into the focal length. Example: 2500 mm divided by 254 mm (10") equals an f/ratio of 9.84, which is usually rounded off, in this case to f/10. The focal ratio signifies how quickly a telescope gathers light and tells us something about the telescope's field of view, how long exposures will take during astrophotography sessions, and how much magnification the eyepiece will produce for that telescope.

Highest Useful Magnification

The term Highest Useful Magnification is used by telescope manufacturers to describe the most magnification you can typically be expected to use on a normal night and still bring an image to sharp focus. A basic rule of thumb for maximum magnification is 40X-50X per inch of aperture, with max magnifications generally topping out at 500X or so regardless of the aperture. You will find that maximums vary depending on the night. Observing conditions change constantly and will cause a once sharp view to become blurry or allow a blurry view to clear up in seconds. It is the nature of telescope observing on a planet with an atmosphere.

Imaging Newtonian

An Imaging Newtonian is a fast reflector telescope that is normally optimized for astrophotography or astro-imaging. Most Imaging Newtonian telescopes have an f/5 focal ratio or less, and some are not meant for visual use at all but rather, were designed to be dedicated imaging telescopes. It is common to find fast imaging Newtonian reflectors for sale as optical tube assemblies only, allowing the astro-imager to use his choice of equatorial mount.


The acronym OTA stands for Optical Tube Assembly. An OTA is simply the telescope portion of a telescope/mount/tripod package. Some telescope users prefer to buy the OTA separately so they can create a custom astrophotography set-up or use a mount they already own.

Reflector Telescope

A reflector is a telescope design in which mirrors are used to gather and focus light. Reflector telescopes are commonly called Newtonian Reflectors, or simply a Newtonian in deference to their inventor, Sir Isaac Newton.

Refractor Telescope

A refractor is a telescope design that uses lenses to gather and focus light. While there are some exceptions to the rule, achromatic refractors use two lenses in their design, and while they are economical, achromats are only capable of focusing two out of three wavelengths of light. An APO telescope, or apochromatic refractor, uses 3 lenses to bring all three wavelengths of light to a single focus to produce an image virtually free of extraneous color.


The acronym SCT stands for Schmidt-Cassegrain Telescope, one of the most popular telescope designs in amateur astronomy today. A Schmidt-Cassegrain, which belongs more broadly to the Catadioptric telescope type, uses a folded optical design incorporating both mirrors and lenses to gather and bring the light to focus. The folded light path allows for a short tube assembly even with relatively large apertures of 8" or more. A shorter tube length makes the SCT far more portable than a classic Newtonian or refractor of the same aperture.