A spotting scope refers to an optical viewing device built specifically for viewing objects at high magnification. Its large lens enables you to view an extremely wide field of view, which is helpful when locating an object you are looking for in the sky. Spotting scopes are a great tool for outdoor adventures as they can magnify a clear image of things at great distances, such as birds, wildlife and even baseball games.
The lightweight nature of most spotting scopes makes them less expensive than telescopes, and they are easy to transport and use. Many are built for protection against harsh conditions, often waterproof and fog proof, making them great for regular outdoor activity.
The magnification of a spotting scope is often magnified by using its eyepiece; therefore, it can be used at much lower magnifications than a telescope and is useful in astronomy since binoculars or basic telescopes have low magnification capabilities. In addition, it can also be used for viewing objects in daylight.
To understand what goes on inside the spotting scope, it is important to first learn about optics and how they work. Once you know optic principles, you should be able to understand what goes on in the viewing tube of your spotting scope.
Firstly, let’s take a deep dive into some helpful terminology to bring us up to speed before we understand how spotting scopes work.
What Is a Diopter?
Diopters are units that measure the power of optical lenses (such as eyeglasses), which are used to correct nearsightedness, farsightedness, and astigmatism.
A diopter is equal to the reciprocal of the focal length in meters. The use of diopter compensation would be impossible if the spotting scope eyepiece was not movable to adjust for the different power of different eyes.
A rubber or plastic diopter adjustment ring controls the focusing (zoom) of the scope. This adjustment ring has markings for each power of zoom possible with that scope. A small lever below the adjustment ring can be pushed in; turning this lever adjusts the zoom power via an internal spring-loaded mechanism that changes the position of the correction lens within the viewing tube.
What Is an Exit Pupil?
The exit pupil is the circle of light that appears out from the eyepiece. The size of this circle is determined by dividing the objective lens by the focal length of an eyepiece. For example, if you use a 75 mm objective lens with a 20 mm eyepiece, your exit pupil will be 15.
A larger exit pupil is desirable because it gives a brighter target image.
So, How Does a Spotting Scope Work?
A sighting tube is where the image of your target comes into focus either because it is out to infinity or because it covers a large area known as an exit pupil. The light enters a convex lens, making its focal length shorter than that of the eyepiece’s focal length. The image of your target is then reflected by a mirror and comes into focus inside the viewing tube. The reflected image causes the same principles of reflection to cause an inverted image that must be corrected with a convex lens for the end result to be a right-side-up and non-inverted image.
Spotting scopes are essentially prism binoculars that use a prism to split the incoming light into two images. The two images are then focused onto a viewing tube. Objectives (also called objective lenses) range from 3x to 80x power, with the most common ones being 10×50 or 20×80. However, many higher-powered spotting scopes with over 100 times or more magnification may be available to amateur astronomers.
The prism slightly blurs the image and so causes the spotter to see through it differently than he would if he used his naked eye. This is known as ‘chromatic aberration’, making the spotter see a distorted image. This problem can be reduced by using an eyepiece with a large focal length, although these eyepieces have a smaller exit pupil.
Some spotting scopes incorporate an erecting prism that enables it to be used for terrestrial viewing without turning the scope. The prism is often located on the objective end of the scope. A small mirror on the side directs an image from the objective lens to a 90-degree prism, where it is flipped right side up and transmitted through a second mirror to another prism which then redirects it back to a small erecting lens at the viewing end. The erecting lens then flips the image (or views it while in a horizontal position) so that you can see it upright.
The main advantage of using an erecting prism in a spotting scope is that it enables you to view a wider field of view, making it easier to locate your target object. You will also be able to see more of the object within the field of view, so you may be able to identify fainter details and darker areas.
Focusing
The focusing mechanism of spotting scopes is either an internal or external system.
An internal system adjusts the dioptre compensation of the eyepiece by turning the focus wheel itself by rotating the entire body of the scope in relation to it.
An external system is when the viewer places his fingers on two knobs located outside the body of the spotting scope and pulls them in opposite directions. Pulling the knobs in opposite directions will move the eyepiece closer to or farther away from the objective lens to better focus an image.
Light Transmission
The light-gathering capabilities of a spotting scope are also critical. Spotting scopes are limited in their magnitude as a direct result of their scope’s design, which is why they are less expensive than telescopes.
A 50x50mm objective lens on a wide-field spotting scope will take in approximately one-third the light that an 80mm objective lens would gather. The advantage to this is easier imaging and observing under bright conditions.
Spotting scopes are not as useful in the daytime when the target objects are brighter and easier to see with binoculars or a simple telescope.
In addition, a large number of amateur astronomers prefer using telescopes for daytime observing due to their superior low and high brightness viewing capabilities.
Image Distortion
It is important to note that there are some disadvantages associated with using an eyepiece instead of a large objective lens when viewing deep-sky distant objects at low magnifications. The image quality is not as high, and can be distorted by chromatic aberration depending on which eyepiece you use.
So if you want to view exceptionally faint objects, you will want to use a set of good binoculars or a simple telescope instead. You will also see smaller and fainter objects, so you may need to focus more accurately on your target object.
Using Spotting Scopes For Astronomy
Spotting scopes are beneficial when observing “faint fuzzies” or faint deep-sky objects. Many astronomers prefer a spotting scope because it enables them to pan and follow an entity across the night’s sky and precisely pinpoint the object’s movements.
In astronomy, astronomers use telescopes to study distant objects in space; however, sometimes, an astronomer may only wish to observe something for a few minutes. Many astronomy observing sessions also occur at night; a spotting scope can be used to observe faint objects of interest at much lower power than a telescope.
A spotting scope also allows greater general observation flexibility, being more convenient than binoculars or a budget telescope. It may be easier to use a spotting scope in low light conditions such as twilight, where you may experience difficulty with a telescope or binoculars with bright light sources nearby, such as another person at home, street lights or advertisements.
Because a spotting scope is generally lighter in weight than a telescope, many astronomers prefer spotting scopes as they allow them to observe more target objects comfortably and in a short time span.
Also, spotting scopes can be much more compact in size than telescopes, which is important given their size limitation imposed by the size of their magnifying eyepiece lenses.
Lastly, a spotting scope is particularly useful for observing deep-sky objects like galaxies, star clusters, and nebulae since it can also easily be used with a camera or video recorder to capture photos of these targets.
Shopping For a Spotting Scope
When shopping for a spotting scope, it is important to assess the features you want. Some come with additional accessories and components that certain people might not require. In addition, you need to know if your spotting scope will be used for astronomy or terrestrial viewing.
Straight or Angled?
First, before you start looking at features, you’ll need to decide whether you want a straight scope or an angled scope. This can help you eliminate and narrow down your choice. The same levels of high magnifications and image quality are mostly the same between the two. It mainly boils down to comfort and positioning.
A straight spotting scope is great for easy transport and use; you can carry it easily since it is more compact. You can even get straight spotting scopes that collapse into itself when not being used. This type of collapsible spotting scope makes it easy to transport while getting up close and personal with your target object without any trouble. However, a straight spotting scope can be detrimental when hunting as you’ll need to make your tripod taller and may become more visible. A taller tripod also means a less stable base which could affect image quality, even if you’re using a good tripod.
On the other hand, an angled spotting scope allows the tripod to sit lower down, making it less prone to shake. The angled eyepiece also allows for more comfortable and discreet positioning when hunting and less neck strain when viewing objects high in the sky.
Accessories
Modern good quality scopes also include important features such as interchangeable eyepieces, focusing variable zoom, a sunshade and/or lens caps, a tripod mounting thread and rubberized ends to improve grip.
Glass Type and Lens Coating
Try to look for high-quality glass marked as ED or HD glass.
Also, bear in mind that different types of lens coating regarding the optics can affect costs and magnification. For example, they can be coated, fully coated, multi-coated, and even fully multi-coated for more detail and better image quality, as each type has its own reflection prevention properties.
Adapters and Tripods
Some models also have a mounting ring that attaches to the adaptor, allowing you to place the spotting scope on a camera tripod, giving you greater stability. The adaptor provides greater flexibility and can be easily removed when not needed.
Some spotting scopes come with an included tripod adaptor, which allows for greater convenience in viewing objects.
Eye Relief
Eye relief is the distance from the tip of the ocular lens on your spotting scope to the tip of your eye.
Many spotting scopes have an eye-relief feature that prevents the user’s eye from touching the lens. This helps prevent damage to your eye if you accidentally bump into the spotting scope while it is being used.
Also, eye relief is important for people who wear glasses, especially those with thick glass lenses. Wearing glasses can hinder good magnification for the user as it prevents them from getting close enough to the lens for the best field of view. If you wear glasses, look for a scope that offers long eye relief, or states a figure of 14mm and above.
The Final Word
So, now you know how spotting scopes work, and how a good quality device can provide higher magnification in certain settings. Also, we discussed briefly how to use a spotting scope and the main features that you should look for when shopping around for one.
So, if you’re after clear and detailed image quality of the entire field ahead, a good spotting scope may be just what you need to help you get maximum magnification of your daily surroundings.
You can use it on your next trip for various long-distance viewing applications, such as bird watching, hunting and target practice. They’re definitely a great accomplice for anyone who enjoys outdoor activities.
Additional Resources
Sources and References
- How To Understand Spotting Scope Features – opticsplanet.com
- Spotting Scopes vs. Telescopes for Beginner Astronomers – bhphotovideo.com
Sam loves to learn about animals and their habitats. He has been a nature lover from a very young age, and has been writing papers and articles about wildlife for as long as he can remember.