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Buying Guide

Astro Binoculars Buying Guide

Last updated: September 1, 2017


This might come as a surprise but binoculars are actually one of the best ways to get started in astronomy. They are not simply for daytime viewing. Those bins you use for birding, hunting or watching baseball games could also be used for stargazing. As mentioned in our Telescope Buying Guide, if you’re looking to buy a telescope but your budget is $100 or less, consider binoculars instead. They are your best option and will perform far better than any telescope at that price point.

Like telescopes, binoculars come in a range of types, brands and sizes. This binoculars buying guide covers all of the essentials including descriptions of each specification, what features to look for, explains the advantages and disadvantages of different binoculars and teaches you how to test binoculars before you buy. Take some time to learn about the basics and let us help you find the perfect astro binoculars!

Table of Contents

  • Astro Binoculars
    • Advantages of binoculars over a telescope for astronomy
  • What can you expect to see with astro binoculars?
  • Binoculars Basics
    • Magnification
    • Aperture
    • Field of View
    • Exit Pupil
    • Eye Relief
    • Interpupillary Distance
    • Prisms
      • Porro Prisms
      • Roof Prisms
      • Types of Prism Glass
    • Coatings
    • Focus Systems
  • Comparing Binoculars
  • How to test binoculars before you buy
  • Image-Stabilized Binoculars
  • Mounts
  • What are the best astronomy binoculars for beginners?
  • Budget
  • Recommended Astro Binoculars

Astro Binoculars


While not as powerful as a telescope, astro binoculars provide a wide field of view which allows you to see more of the night sky and makes locating celestial objects easier. You can see more details using both eyes with binoculars than you would using a small telescope with the same aperture. Using both eyes also allows for more natural viewing. Binoculars are simple to use, lightweight, extremely portable and should last a lifetime. Paired with a planisphere or a good book on astronomy, you can definitely become highly skilled at navigating the night sky with binoculars.

You might already own a pair of binoculars but they may not necessarily be ideal for astronomy. Although not all binoculars are suitable for astronomy, astro binoculars can be used in the daytime for bird watching, viewing wildlife, landscapes, landmarks and sporting events.

If you eventually buy a telescope, your binoculars will still see a lot of use as they make a great companion to a telescope. Most experienced astronomers observing with a telescope will also keep astro binoculars close at hand for wide view scans of the night sky and to quickly locate celestial objects.

Advantages of binoculars over a telescope for astronomy

  • lightweight and portable – excellent for travel
  • provide a wide field of view
  • allow for more natural viewing as you can use both of your eyes
  • show more detail than a telescope with the same size objective lens or primary mirror
  • image is correctly oriented – right side up and not flipped
  • can be used for daytime viewing

What can you expect to see with astro binoculars?

It’s important to have realistic expectations. Binoculars are not a substitute for a telescope. There are hundreds of interesting celestial objects that can be viewed with binoculars both in the solar system and beyond. But only a telescope can show planetary details, greater lunar detail, and better views of deep-sky objects.

The moon and planets are bright enough to be unaffected by light pollution. Expect to see fantastic views of the moon including its craters, mountains and maria (large dark spots). Mars will appear as a small red dot. You should be able to see the golden color of Saturn. Unfortunately you will not be able to make out Saturn’s rings as only a proper telescope can provide that amount of detail. However, with high power binoculars on a mount you will see that the shape of Saturn is not round. Experienced users may observe Titan, Saturn’s largest moon. The Galilean moons of Jupiter – Io, Ganymede, Europa and Callisto – will be visible as tiny points of light surrounding Jupiter’s bright disk. You can observe that their relative positions change each night as they orbit the planet. The phases of Mercury and Venus will be visible. Neptune and Uranus will appear as stars. Binoculars are also terrific for viewing comets, asteroids and even man made objects like satellites and the International Space Station.

Viewing deep-sky objects with binoculars will greatly depend on dark skies. The darker the skies, the better the opportunity to see fainter deep-sky objects. In large cities with heavy light pollution, only a few dozen stars may be visible to the naked eye. The number of visible stars increases to a few hundred in the suburbs or a small city. Under dark skies you should be able to see around 2,500 stars with the naked eye. With 10×50 binoculars under dark skies, more than 100,000 stars will be visible. You can expect to see many double stars and most of the 110 Messier objects including open and globular star clusters, nebulae and galaxies like Andromeda.


Binoculars Basics

All binoculars are described using two numbers, e.g. 7×50. The first number represents the magnification or power (7x) while the second number is the aperture measured in millimetres (50mm). These two numbers along with the field of view should be engraved or printed somewhere on the binoculars. Common binoculars for astronomy include 7×35, 7×50 and 8×40 with 10×50 being the most popular.

Source: Wikipedia

Magnification

As mentioned above, the first of two numbers used to identify a pair of binoculars indicates its magnification. Binoculars that are 10×50 have 10x magnification, meaning that an object will appear 10 times closer than with the naked eye. A higher magnification will darken the background of the night sky and improve contrast, allowing you to see fainter objects and finer details but reducing your field of view. Generally, the visibility of star clusters, nebulae and galaxies improves with increasing magnification.

For magnifications above 10x, the shakiness becomes amplified making it difficult to hold the binoculars steady by hand. There are some tricks to minimize shaking like sitting in a chair, resting your elbows on a solid surface or leaning against a wall or fence. Large aperture, high power binoculars will require a proper mount.

Avoid ‘zoom’ binoculars with variable magnification (e.g. 10-22×50). They do not provide the precision and optical quality required for astronomy purposes.

Aperture

For binoculars, the aperture is the diameter of each of the objective lenses measured in millimetres. It is the most important specification to look for when buying binoculars. An objective lens with a larger aperture can collect more light providing a brighter and sharper image. The larger the aperture, the more objects you can see including fainter ones.

Aperture doesn’t matter too much for daytime viewing and the front lenses can be smaller as there is plenty of natural light. For night sky viewing, astro binoculars should have an aperture of at least 35mm but preferably around 50mm. Keep in mind that the larger the aperture, the heavier the binoculars will be.

Field of View

The field of view is the width of the area of sky as seen through the binoculars. It is usually described in degrees, in feet at a distance of 1000 yards or in metres at a distance of 1000 metres.

One degree equals 52.5 feet at 1000 yards. If the field of view is given in feet and yards, you can calculate the equivalent field of view in degrees by dividing by 52.5. So a field of view of 420ft/1000yds is equal to 8 degrees (420ft/52.5 = 8). If the field of view is given in metres at a distance of 1000 metres (e.g. 140m/1000m), divide by 17.5 to calculate the field of view in degrees. A field of view of 140m/1000m is equivalent to 8 degrees (140m/17.5 = 8).

Exit Pupil


Light enters the binoculars through the objective lenses and exits through the ocular lenses in the eyepieces. If you hold the binoculars out in front of you while pointed towards a light source, you should see a small round disk that appears to be floating behind each eyepiece. The diameter of this disk of light is called the exit pupil. Ideally, the exit pupil should be no larger than the eye’s pupil. The size of the eye pupil can vary widely between individuals and also decreases with age. If the binoculars have an exit pupil larger than your eye’s pupil, that extra light is wasted and you could get away with using smaller aperture binoculars with a smaller exit pupil.

To calculate the exit pupil for a pair of binoculars, divide the aperture by the magnification. For 10×50 binoculars, the exit pupil is 5mm (50mm/10 = 5).

Eye Relief

Eye relief is the distance in millimetres between the eyepiece’s ocular lens and the observer’s eye while still being able to see the entire field of view. Binoculars with a long eye relief can be held further away from your face without reducing the field of view. This is especially important for those who wear eyeglasses as the glasses will force you to hold the binoculars away from your face. An eye relief of at least 15mm is typically necessary for those wearing glasses.

Eyecups that extend from the eyepieces serve two purposes. They block stray light and ensure that the proper eye relief is maintained while the binoculars are held to your face. They are typically made from rubber and either roll up and down, slide up and down or twist up and down. Most eyeglass wearers will use binoculars with the eyecups down.

Interpupillary Distance

Interpupillary distance is the distance between the center of the pupils of the observer’s eyes. The hinge allows binoculars to be adjusted to fit a person’s eye spacing by moving the barrels closer together or farther apart. When the eyepieces are in the correct position relative to your interpupillary distance, the separate images from each eyepiece will combine and the viewing area will appear as a single circle. Children and some women may have an interpupillary distance that is smaller than standard binoculars can accommodate and should try compact binoculars instead.

Prisms

Binoculars come in two styles based on the type of prisms used. Prisms correct the orientation of the image seen through binoculars so that it appears as it would with the naked eye. Without prisms, the image would appear upside down and laterally reversed. Prisms also fold the light path allowing binoculars to be much shorter.

There are two types of prisms used in binoculars:

  1. Porro prisms
  2. Roof prisms
Left: Porro prism binoculars; Right: Roof prism binoculars

Porro Prisms

Binoculars using Porro prisms have a stepped appearance. The eyepieces are close together while the objective lenses are offset and further apart. Porro prism binoculars provide the best value for money as they are cheaper to manufacture and less expensive than roof prism binoculars. The disadvantages are that Porro prism binoculars are larger and heavier than roof prism binoculars and are also more difficult to fully waterproof.

Roof Prisms

Binoculars using roof prisms have a sleek H-like appearance. The eyepieces and the objective lenses are inline with each other. They are smaller and lighter than Porro prism binoculars and also easier to waterproof. The downside is that roof prism binoculars are more expensive, requiring costly precision manufacturing, and special mirror and phase correction prism coatings.

There is no noticeable difference in performance between Porro prisms and roof prisms.

Types of Prism Glass

The prisms used in binoculars are made from one of two types of glass: BaK-4 (barium crown glass) or BK-7 (borosilicate crown glass). Look for prisms made from BaK-4 glass as it is of higher quality. BK-7 glass is cheaper and not as good.

To determine the type of prism glass used, hold the binoculars out in front of you pointed towards a light source and look at the exit pupil. If the exit pupil appears perfectly round then the glass is BaK-4. With BK-7 glass, the exit pupil will appear square with grey-blue edges.

Coatings

When light passes through a piece of glass, a percentage of light is reflected and lost. Binoculars have multiple air-to-glass surfaces which means multiple opportunities for light loss. Proper coatings increase the transmission of light through the lenses and reduce the amount of light that is reflected resulting in a brighter and sharper image. There are various terms used to describe binocular coatings including ‘coated’, ‘fully coated’, ‘multi-coated’ and ‘fully multi-coated’. Although there is no standard definition of these terms, they are commonly described as the following:

Coated – at least one air-to-glass surface has a layer of anti-reflective coating.

Fully coated – all air-to-glass surfaces have a layer of anti-reflective coating.

Multi-coated – at least one air-to-glass surface has more than one layer of anti-reflective coating while the other surfaces have one layer of coating or none at all.

Fully multi-coated – all air-to-glass surfaces have more than one layer of anti-reflective coating

The best way to determine the quality of the coatings is to physically inspect the binoculars. Fully multi-coated lenses should indicate higher quality and offer the best performance.

It’s highly unlikely that you will find new binoculars that are not coated. The type of lens coatings should not be a deal breaker. One exception is to avoid binoculars that advertise ‘ruby-coated’ lenses. They are typically found in cheap binoculars and are an indication of low-quality optics.

Focus Systems

There are two main types of focusing mechanisms for binoculars. The most common is the center-focus control which uses a wheel or knob to move both eyepieces back and forth to achieve focus. Most center-focus systems also have a separate diopter adjustment to adjust focus for one eyepiece, typically the right side, independently of the other to compensate for differences in vision between the observer’s two eyes.

The second type of focusing mechanism involves independent focusers for each eyepiece. Focusing each eyepiece is a slower process than using a center-focus control. Binoculars with an independent focus system tend to be more rugged and waterproof. The best quality binoculars will have independent focusing.

For astronomy purposes, celestial objects do not require continual refocusing as their distance does not change.


Comparing Binoculars

There have been several attempts to quantify the relative performance of binoculars with different combinations of magnification and aperture. In our opinion, the best and most realistic rating system used to compare binoculars is the Adler Index. Binoculars ranking higher on the Adler Index will have better performance, all other things remaining equal.

To calculate the Adler Index rating, multiply the magnification by the square root of the aperture. For example, 7×50 binoculars have an Adler Index rating of 49, while 10×42 binoculars have a rating of 65. This indicates that the 10×42 binoculars should outperform the 7x50s.

The table below compares the Adler Index rating of several astro binoculars:

BinocularsAdler Index Rating
7x3541
7x5049
8x4051
8x4252
10x4265
10x5071
15x70126
20x80179
25x70209

The Adler Index is a helpful tool but keep in mind that it’s just a simple calculation. It is no substitute for testing binoculars in person.


How to test binoculars before you buy

If you are buying binoculars in person or inspecting an existing pair of unknown specifications and origin, there are some steps you can follow to evaluate their quality and performance:

1. With a bright light behind you, look at the reflections in the objective lenses. Darker reflections are best and indicate quality lens coatings. Multi-coated lenses will have deep green or purple reflections while white reflections indicate a complete lack of lens coatings.

2. Inspect the surface of each objective lens for scratches and look inside for dust, dirt or fungus.

3. Perform the same tests as above while inspecting the ocular lenses (eyepieces) instead.

4. Hold the binoculars out in front of you pointed towards a light source and look at the exit pupils. You want to see perfectly round and uniformly bright circles of light. Exit pupils with a diamond shape (edges that look cut off) indicate undersized prisms, a cost-cutting measure by the manufacture that will dim the image. Square exit pupils with grey-blue edges indicate full-sized prisms, however the prisms are made from inferior quality BK-7 glass.

5. Check the collimation of the binoculars to ensure that the two barrels are optically aligned. Poor collimation may strain your eyes and cause discomfort. You may see double images if the collimation is really bad. Good quality binoculars should maintain collimation for several decades while some low budget binoculars may lose collimation during shipping.

6. Look out a window or step outside and find a high contrast object like a tree, pole or building against a bright sky. Color fringing along the edges of the object indicates chromatic aberration. A little chromatic aberration is to be expected, however some binoculars will suffer more than others.

7. The image seen through binoculars with a wide field of view may lose sharpness and appear blurry along the edge of the view. This distortion may be worse in some binoculars than others so it’s best to compare multiple binoculars.

8. If you wear glasses, check that the eye relief is long enough for the entire field of view to be visible while wearing your glasses. Make sure the eyecups are not extended.

9. Test the focusing mechanism and diopter adjustment to ensure they are smooth throughout their entire range of motion.

10. Check the hinge by moving the barrels back and forth. Ensure it is smooth throughout its entire range of motion with consistent resistance.

11. Shake the binoculars and listen for rattling or internal noise. Binoculars should not have any loose internal parts.

12. How does the weight of the binoculars feel when held to your face? If your arms tired after a few minutes of holding the binoculars, they are too heavy. Try a lighter pair with a smaller aperture or plan to use a mount.

13. Inspect the rubber eyecups for cracking or splitting.

14. This step is not necessary unless you really want to be thorough. Some binoculars may actually have a smaller aperture than specified due to the internal optics and construction. Your 10×50 binoculars might actually perform as 10x40s. Shine a flashlight through one of the eyepieces and measure the diameter of the disk of light exiting the objective lens projected on a wall or piece of paper.

15. Looking at a bright star in the night sky is the ultimate test of optical performance for astro binoculars. Bring a star to focus in the center of the field of view. While focusing, the star should stay round without any rays extending from it. The star should appear sharp and pointlike. Move the binoculars so that the star is near the edge of the field of view. Is the star still sharp?


Image-Stabilized Binoculars

Canon 10×30 IS II Binoculars

The view through image-stabilized binoculars is seriously impressive, especially when experienced for the first time. Push a button and the battery-powered stabilizing mechanisms magically stop the image from shaking. With a perfectly steady image you can take advantage of higher magnifications and smaller apertures with better performance than traditional binoculars. Sounds too good to be true, right? Well, image-stabilized binoculars can be quite expensive. Expect to spend anywhere from $500 to $1500.


Mounts

Holding binoculars steady can be quite challenging. It’s incredibly frustrating to have your target object in view while it’s constantly shaking and jiggling around. You’re just not going to see much. The largest binoculars that most people can comfortably hold by hand while maintaining a steady view are 10×50.

Magnifications higher than 10x will only increase the shakiness and make observing more difficult. Binoculars with apertures larger than 50mm can be quite heavy and uncomfortable to hold longer than a few minutes. If the magnification is higher than 10x and/or the aperture larger than 50mm, consider mounting the binoculars to improve steadiness and give your arms a rest.

A monopod is a very portable and lightweight mount with adjustable height. It consists of a pole that extends from the ground to your eye level and a ball-head which allows the binoculars to be tilted in all directions. Monopods can be used while seated or standing. If the monopod will be used on an angle (not vertical) when seated or slightly reclined, a spiked foot at the base of the monopod can be stuck in the ground to keep it in position.

Using a tripod will provide better stability than a monopod. Binoculars with a tripod adapter can be mounted to most standard photo, video or telescope tripods via an L-bracket. However, some tripod heads may not permit viewing at or near the zenith. Tripods with an adjustable height center post should allow use while standing to view objects overhead. It’s important that the tripod is sturdy and the weight of the binoculars does not exceed the weight capacity of the tripod.

Parallelogram mounts offer the best viewing experience. They are versatile and can be used while sitting, reclined or standing. It’s incredibly easy to adjust the position of the binoculars with the silky smooth 360-degree motions. A counterweight balances the weight of the binoculars which can be raised or lowered while staying fixed on the target object. There are no restrictions to viewing directly overhead. A parallelogram mount can be purchased either with a tripod or separately and attached to an existing tripod.


What are the best astronomy binoculars for beginners?

For beginners wondering what astro binoculars to buy, we recommend 10×50 Porro prism binoculars. They offer a good balance of aperture size, magnification, weight and affordability. The 50mm objective lenses are relatively large and provide enough light gathering ability to see faint objects. Compared to 7×50 binoculars, the increased magnification of 10x50s will make objects appear larger and also darken the background of the night sky which allows easier detection of faint deep-sky objects. Most people should be able to comfortably hold 10×50 binoculars by hand and keep them steady. If you don’t think you will be able to, consider smaller binoculars like 7×50, 8×40 or 8×42.


Budget

A good budget for astro binoculars is between $100 and $300 with plenty of solid options at the lower end. There are functional low budget binoculars, however expect to find cheap build quality, BK-7 glass, poor coatings, collimation issues, and possibly a smaller aperture than advertised. On the other end of the spectrum, some giant binoculars and those with special features like image stabilization can cost upwards of several thousand dollars. For the recommended 10×50 Porro prism binoculars, there are several good quality choices for around $100.


Recommended Astro Binoculars

For Beginners

Low Budget Binoculars

Giant Binoculars

Telescope Buying Guide

Last updated: June 22, 2017


Amateur astronomy is a rewarding hobby filled with awe and wonder. There is nothing quite like looking through a telescope and seeing a planet, nebula or galaxy for the first time with your own eyes. Making the decision to buy your first telescope can be exciting. But how do you choose a telescope? What is the best beginner telescope? There are several factors to consider and unfortunately, the number of telescope types, brands, sizes, and wide ranging prices can be overwhelming and confusing.

This telescope buying guide provides some basic information on telescopes, the advantages and disadvantages of different types of telescopes, and addresses what features to look for and details to consider when selecting your first telescope. Our goal is to help you make an informed decision that satisfies your needs and meets your expectations. Let’s get started!

Table of Contents

  • Try Before You Buy
  • Portability
  • Start With Binoculars
  • What can you expect to see through a telescope?
  • Aperture, Focal Length and Magnification
    • Aperture
    • Focal Length
    • Magnification
  • Different Types of Telescopes
    • Refractor Telescopes
    • Reflector Telescopes
    • Catadioptric Telescopes
  • Telescope Mounts
    • Altazimuth Mounts
    • Equatorial Mounts
    • GoTo Mounts
  • Astrophotography
  • Budget
  • Summary
  • Recommended Telescopes
  • What's Next?

Try Before You Buy

Nothing beats hands on experience with a telescope. It would be a good idea to visit an astronomy club in your area and take part in a public viewing or star party. Club members are typically happy to show you their telescopes and you’ll be able to test out several types and sizes. Some astronomy clubs even offer loaner or rental telescopes. It’s always better to try before you buy in case you realize that you weren’t really that interested in astronomy.

Find a local astronomy club by visiting one of the following links:

  • Go-Astronomy.com
  • Sky & Telescope
  • NASA’s Night Sky Network
Source: National Park Service

Portability

A good rule is to buy the biggest telescope you can afford. However, the best telescope is the one you’re going to use most often so you need to consider portability.

If you only intend to use it in your backyard then a large telescope may not be an issue. If you need to drive away from the city to avoid light pollution, you’ll need to make sure the telescope can fit easily in your vehicle. You should also think about how comfortable you’ll be lugging around a large and heavy telescope. For example, an 8″ Dobsonian mounted reflector can weigh around 40lbs and measure almost 4 feet long.


Start With Binoculars

One of the best ways to introduce yourself to amateur astronomy is with binoculars – you might already own a pair. If your telescope budget is $100 or less, astro binoculars are the best option. With an extremely wide field of view, binoculars are a great way to learn how to navigate the night sky and the constellations especially if paired with a good astronomy book for beginners.

With binoculars and good viewing conditions, you will be pleasantly surprised at the number of night sky objects you can see. Expect to see more stars than with the naked eye, comets, planets, star clusters, nebulae and galaxies. The craters of the moon will also be visible as well as the moons of Jupiter. However, you will not be able to see any details of the planets and a telescope will reveal much greater detail of the moon.
For stargazing, the most popular astro binoculars are 10×50. The first number represents the magnification (10 times) while the second number is the diameter of the main lens measured in millimetres (50mm) also known as the aperture size. A higher magnification will allow you to see fainter objects and finer details but will reduce your field of view. It will also magnify the shakiness when holding your binoculars by hand. Resting your elbows on a solid surface like a table or using a tripod will minimize shaking especially if the magnification is greater than 10x.

Binoculars are the perfect companion to a telescope and they should last a lifetime. Being lightweight and extremely portable, binoculars are great for quickly and easily locating celestial objects in the sky.


What can you expect to see through a telescope?

This will depend on many factors including the type and size of telescope, skill-level, light pollution and local seeing (atmospheric) conditions.

Download The Evening Sky Map from Skymaps.com, a monthly 2-page guide to help you locate celestial objects. It's free and easily printable.

The Moon

With a constantly changing face, the moon offers excellent details even with a small telescope including craters and mountains.

The Planets

  • Jupiter provides the most detail of the planets offering views of its cloud bands, Great Red Spot, and 4 Galilean moons
  • Saturn’s rings and its largest moon, Titan
  • Mars and its polar ice caps (best every 2 years when closer to Earth)
  • the changing phases of Mercury and Venus
  • Neptune and Uranus will appear star-like

The Sun

The sun offers plenty of detail including multiple sunspots and surface granulation. You must use a proper solar filter when viewing the sun through a telescope. Failure to do so can result in severe eye damage.

Stars

Due to the extreme distance between the Earth and the stars, stars will only appear brighter through a telescope and not larger. You will observe many stars as pairs through your telescope. These binary stars or double stars consist of two stars, usually of different colors, orbiting around their common center of mass.

Deep-Sky Objects

The 110 Messier objects and the Herschel 400 objects including open and globular star clusters, nebulae, and galaxies.

You should be aware that the highly detailed and vibrantly colored photographs of planets, nebulae and galaxies you may have seen online or in books and magazines are NOT what you can expect to see with an amateur telescope. These images were either stacked and professionally manipulated, taken from large observatory telescopes with long exposures or from space with the Hubble Telescope.

Aperture, Focal Length, and Magnification

Aperture

The most important specification to look for when choosing a telescope is aperture. Aperture is the diameter of the telescope’s light-gathering lens or mirror (also called the objective). The larger the aperture, the more light will be collected resulting in brighter and sharper images. Aperture is typically measured in millimetres or inches. Doubling the aperture increases the surface area by a factor of four. For example, an 8″ diameter lens or mirror will collect four times as much light as a 4″ diameter lens or mirror.

Focal Length

A telescope’s focal length is the distance from the objective lens or mirror to the focal point. Longer focal lengths result in higher magnification, larger images and smaller fields of view.

The focal ratio (f/ratio) is equal to the focal length of the telescope divided by its aperture size. For example an 8″ (203mm) aperture telescope with a 1200mm focal length would have an f/ratio of f/5.9 (1200 divided by 203 equals 5.9). Smaller f/ratios result in lower magnification and wide fields of view. Higher f/ratios result in higher magnification and narrow fields of view.

Magnification

Perhaps the biggest misconception is that a telescope with high magnification (also called power) is required to see celestial objects. Magnification is one of the least important factors to consider. You can change the magnification of your telescope by switching eyepieces with different focal lengths.

Magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. For example, a telescope with a focal length of 1200mm and an eyepiece with a focal length of 25mm would provide a magnification of 48x (1200 divided by 25 equals 48).

As a rule of thumb, the maximum useful magnification for any telescope is 50 times the aperture in inches or two times the aperture in millimetres. For example, a 6” (150mm) reflector would have a maximum useful power of 300x. At higher powers, atmospheric conditions will distort and blur the image seen through a telescope. Therefore, the practical upper limit of magnification for any telescope is considered to be around 300x even if the theoretical upper limit is calculated to be higher. Most telescope viewing is done at low power anyways.

If you’re going to purchase a telescope, do not rush out and buy one from a toy store, department store or big box retailer. Some of them do carry brands and models recommended on this site but they also carry several junk telescopes. Proceed with caution and avoid telescopes advertised with ridiculously high magnification (400x to 600x).

Different Types of Telescopes

There are three main types of telescopes:

  1. Refractors
  2. Reflectors
  3. Catadioptrics

Refractor Telescopes


This is the image most people have when thinking of the word “telescope”. The first telescopes were refractors. A refractor telescope has a convex objective lens at the front end of a sealed tube and an eyepiece at the rear end. There are two basic types of refractors: achromatic and apochromatic.

Refractors are easy to use, reliable, require little to no maintenance or adjustment, and accumulate very little dust. They are excellent for lunar, planetary, and binary star observing, however the smaller apertures make it difficult to view dim deep-sky objects.

Refractors have the highest cost per inch of aperture. Larger aperture refractors can be very long and heavy compared to equivalent aperture reflectors or catadioptrics. They require a tall and bulky mount as the eyepiece is located at the rear.

Apochromatic refractor telescopes are quite expensive due to need for additional lenses made from special types of glass to correct an effect known as chromatic aberration. When light passes through the objective lens, each wavelength of light is refracted by a different amount and the colors fail to converge at the same focal point. Think of the rainbow effect of light passing through a glass prism. Chromatic aberration is typically seen as color fringing around bright objects, like a purple halo around planets and stars.

One added bonus with refractors is that they can also double as a terrestrial telescope for daytime distance viewing of landscapes, landmarks, boats or wildlife.

Pros

  • easy to use
  • reliable
  • little to no maintenance or dust with sealed tube
  • excellent for observing moon & planets
  • can use for daytime terrestrial viewing

Cons

  • highest cost per inch of aperture
  • difficulty viewing dim deep-sky objects
  • larger apertures are long & heavy
  • chromatic aberration
  • require tall & bulky mount
  • uncomfortable viewing angle

Reflector Telescopes


Reflector telescopes, also known as Newtonian telescopes, are not sealed tubes like refractors but are open at one end. Instead of an objective lens, a concave primary mirror collects and focuses incoming light onto a flat diagonal secondary mirror which reflects the image to an eyepiece on the side of the tube.

They offer the best value per inch of aperture of the three types of telescopes due to the cheaper manufacturing costs for mirrors than for lenses. This relatively low cost results in much more affordable large apertures which are perfect for viewing deep-sky objects.

A Dobsonian telescope is a large aperture Newtonian reflector on a simplified altazimuth mount that sits on the ground. Dobsonians are very popular with beginners. They are easy to setup, simple to operate and offer the best bang for your buck.

Occasional cleaning may be required as the open tube allows dust, pollen, and dirt to accumulate on the mirrors. The mirrors can also get out of alignment and require periodic manual adjustment called collimation. Collimation is not a time-consuming or difficult process although it may take some practice. Instructions should be included with your telescope.

Reflectors do not suffer from chromatic aberration but they do have a different type of optical aberration known as coma. Coma gives stars a wedge-shaped appearance near the edge of the field of view, however stars in the center of the field of view are unaffected. Coma is more noticeable in reflectors with smaller f/ratios.

Pros

  • best value – cheapest per inch of aperture
  • perfect for viewing deep-sky objects
  • large apertures – light-buckets
  • no chromatic aberration
  • Dobsonians are extremely easy to use
  • comfortable viewing angle

Cons

  • mirrors collect dust, pollen, dirt with open tube
  • requires occasional collimation
  • coma
  • Dobsonians are large and heavy – portability & storage issues
  • not good for astrophotography

Catadioptric Telescopes


Catadioptric (or compound) telescopes use a hybrid design with a combination of lenses and mirrors in a sealed tube which fold the optical path to form an image. This provides a focal length much longer than the length of the compact optical tube. They are more commonly known by the two popular designs: Schmidt-Cassegrain and Maksutov-Cassegrain.

Catadioptric telescopes offer the most compact and lightweight design resulting in high portability and easy mounting. Cost per inch of aperture is midway between reflectors and refractors.

They are the most versatile of the three telescope types with great viewing of the moon, planets, and deep-sky objects, as well as the ability for daytime terrestrial viewing and astrophotography. They are very easy to use.

The optical elements of Maksutov-Cassegrain telescopes are typically fixed in alignment and do not require collimation. Schmidt-Cassegrain telescopes may require occasional collimation but it is a much easier process than collimating a reflector telescope. The sealed design eliminates dust and dirt, and little if any maintenance is required. Catadioptrics have the longest cooldown time (thermal stabilization) of any design which means you’ll have to wait longer for optimal viewing.

Pros

  • compact & lightweight – best portability
  • easy mounting
  • easy to use
  • good for viewing planets & deep-sky objects
  • daytime terrestrial viewing
  • excellent for astrophotography

Cons

  • more expensive than reflectors
  • occasional collimation for Schmidt-Cassegrain
  • longest cooldown time
  • narrowest field of view

Telescope Mounts

The optical tube is only one half of a telescope – the other half being the mount. It is just as important as the optical tube. The mount keeps the telescope steady by minimizing shaking. Vibrations are magnified at high power and a shaky view will render even the best telescope useless. As the Earth rotates, celestial objects will appear to move across the sky. The telescope will need to be frequently moved in small increments to keep on object centered in view. The mount should allow these movements to be done as smoothly as possible.

The two types of telescope mounts are altazimuth and equatorial.

Altazimuth Mounts


Altazimuth (alt-az) mounts are the most common type of telescope mount. They move up and down (altitude), and side to side (azimuth) similar to a camera tripod. Tracking celestial objects across the sky is done by moving the telescope with your hand or by using slow motion controls, depending on the model. The drawback to an altazimuth mount is that it requires simultaneously adjusting both axes.

Example of a Dobsonian mount

Dobsonians are an extremely popular type of altazimuth mount for large reflector telescopes due to their simplicity, ease of use, and low cost. A good way to picture a Dobsonian is a see-saw on a lazy susan.

Equatorial Mounts


Equatorial (EQ) mounts are more precise than altazimuth mounts. Aligning one axis with Polaris (the North Star) will allow you to adjust only the polar axis to track an object as the Earth rotates. There is a learning curve to understand how to operate an EQ mount but once you’ve figured it out, tracking celestial objects becomes much easier as you only adjust a single knob.

Many equatorial mounts are heavier than altazimuths because they require a counterweight to balance the telescope. An equatorial mount is the best option for astrophotography.

GoTo Mounts

Computerized or GoTo mounts are becoming increasingly popular. They come with a small motor drive, built-in computer and hand-held controller which allows you to look up celestial objects in a database. The GoTo will automatically locate the specific object and track its movement across the sky. All you have to do is look through the eyepiece.

If you want to learn how to navigate the night sky, go with a manual mount. GoTo mounts remove the required practice and skill for locating celestial objects. There is also a trade-off with optical quality as the electronics typically eat a few hundred dollars of your budget, leaving you with a smaller aperture. For the same price as a GoTo model, you could purchase a non-computerized telescope with a much larger aperture and better viewing capabilities.


Astrophotography

This guide focuses on the basics of buying a telescope. Astrophotography can be quite complicated and expensive, and we suggest beginners first learn the basics of operating a telescope before diving into astrophotography. Although astrophotography can require specialized equipment and accessories, most telescopes can be combined with a camera.


Budget

As mentioned above, if your budget is $100 or less, serious consideration should be given to purchasing binoculars instead of a telescope. There really aren’t any telescopes worth buying in this price range. Telescopes are precision optical instruments and the lenses and mirrors can’t be manufactured cheaply without cutting corners. A cheap telescope is essentially a toy and you’ll quickly be disappointed and left wondering why you wasted your money.

There aren’t many good options for under $200 either. Another reason sub-$200 telescopes are not recommended is due to the low quality mounts which are simply not stable enough to be usable. Many of the mounts included with telescopes in this price are actually camera tripods. There certainly are a few exceptions including some tabletop telescopes. If you can wait, it might be best to save a little bit more money for a better quality telescope.

Serious beginners should plan for a budget of $300 to $400. This would allow you to buy a 6” Dobsonian mounted reflector telescope, a starter book on astronomy, and an extra eyepiece. This budget would allow you to purchase an 8″ Dobsonian, however there wouldn’t be any money left over for a book or accessories. There are other options with smaller apertures in this price range including refractors, non-Dobsonian reflectors and Maksutov-Cassegrain telescopes but you’ll get the best value with a Dobsonian.

If your budget is higher than $400, there will be plenty of quality options available including larger aperture Dobsonians and GoTo mount catadioptrics. Expect to spend a minimum of $700 for a decent GoTo telescope with a 5″ aperture. There are cheaper GoTo alternatives, however the apertures will be smaller and the optics will be of lesser quality.


Summary

  • try to get hands-on experience with a telescope before you buy
  • consider size, weight, and portability
  • start with binoculars
  • aperture is the most important factor to look for when buying a telescope
  • ignore magnification – can change by switching eyepieces
  • learn the basics before trying GoTo Mounts or astrophotography
  • Dobsonian reflectors offer best value

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What’s Next?

If there are any astronomy words or terms that you’re still unclear about, check out our Glossary of Astronomy Terms.

If you’re looking for recommendations, check out our Best Telescope for Beginners and Best Budget Telescope.

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