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How To Choose A Spotting Scope
How To Choose A Spotting Scope

Many people getting into nature study for the first time wonder whether they should start by purchasing a spotting scope or a binocular. Of course the answer to this depends on many things. Two of the most important factors concern the nature of the subjects to be observed and the distance between them and the observer.

In almost every instance in which the subject is near, or in situations in which only wide field views are desired, the binocular should be the first choice. They are convenient to use, offer a wider field of view, and, because of the ability to use both eyes, may yield as much as a 40% improvement in contrast.

Handheld binoculars, however, rarely come in magnifications higher than about 15 power. Thus for subjects which must be viewed at great distances, or on which greater magnifications are required, the spotting scope is the only logical choice.

What is a Spotting Scope?
Passenger planes, jet fighters, cargo planes and crop dusters all have one thing in common; they are all types of aircraft, each with a specific role to fill, and a specific name that aptly describes that role. Similarly, “spotting scopes” are in fact telescopes that have been specifically designed for daytime terrestrial viewing.

Considerably shorter and lighter than their astronomically dedicated cousins, spotting scopes with apertures between 50mm and 127mm (2 to 5 inches) are excellent for increasing our pleasure in any number of hobbies and research projects. They can also allow us to safely view (or photograph) targets that might otherwise place us in harms way. Suppose, for example, you wanted to observe hornets circling about their nest. A spotting scope set at 20-power would allow you to stand 50 feet from the nest and yet enjoy a view as if you were only 2 1/2 feet away-virtually close enough to touch it!

Kinds of Spotting Scopes
Spotting scopes come in a variety of configurations, typically: Schmidt-Cassegrains and Maksutov-Cassegrains, in which mirrors are used to fold the light back on itself making the instrument more portable and convenient to use, and the more common refracting type. In these instruments, light is gathered and brought to a focus by an “objective” lens.

When subjects are at great distances, or when high resolution-the ability to see fine detail-is required, spotting scopes with relatively large apertures are often employed. In order to make these instruments as portable as possible a “compound” system-such as the Schmidt or Maksutov described above-become the only practical option.

The majority of naturalists, however, find their needs met by using refracting spotting scopes of moderate aperture-typically 50mm to 102mm (2 to 4 inches)-and they are by far the most popular.

The Basic (Fixed Eyepiece) Spotting Scope
These spotting scopes represent the best value when thinking in terms of aperture vs. cost. This is because of the simplicity of their design and construction. They consist of an objective lens, a prism to offset the line of sight to a more convenient 45-degree angle and an eyepiece. Magnification is increased or decreased by using interchangeable eyepieces of various “powers.” It is this changing of eyepieces that leads many naturalists to choose the more popular and sophisticated zooming spotting scope.

The Advanced (Zoom) Spotting Scope
For individuals whose requirements are restricted to a limited number of targets at similar distances, or who will be making observations from their house or backyard, the spotting scope described above will perform admirably.

Bird watchers and other naturalists, however, are incessantly on the go and lean toward telescopes that can provide a variety of magnifications without having to locate and fumble about with a collection of eyepieces. For most of them, the “zoom” spotting scope is the instrument of choice. In general, they cost more than fixed-eyepiece scopes of equal aperture and quality, but make up for it in the convenience they offer.

Performance
Once you have decided which type of telescope will best suit your needs, and considerations concerning styling, weight and cost have been addressed, it is time to turn your attention to performance. The most important things to think about when considering a telescope’s performance involve: Aperture, Magnification and Anti-Reflective Coatings.

Aperture
Aperture is the measurement-usually stated in millimeters-of an instrument’s main lens or mirror. Every square inch of an objective lenses surface gathers as much light as 9 eyes wide open. This means that even a 60mm spotting scope is capable to gathering more light at any instant in time than 41 eyes! This explains why many people new to using optical instruments show great surprise with the ability to see things very well in low-light situations, or in shadows, that would otherwise be invisible. This is not a magical power of some high-tech marvel, just a simple optical system exploiting the laws of physics.

But how much aperture do you need? Everything we see either gives off or reflects a finite amount of light. Thus, to observe faint objects, or objects in poorly lighted areas, the greater the aperture, the brighter the image will be at a given magnification. But then we must look at the practical limits of size, weight, and cost. Those wanting to watch a group of goats a mile up the side of a mountain should consider spotting scopes with an aperture of 80mm or more, while those who demand little more than the ability to provide a good view of birds at a backyard feeder, a 40mm to 50mm instrument would perform quite well. For the needs expressed by most bird watchers and naturalists, telescopes with apertures between 60mm and 80mm will be a good compromise between portability and light grasp.

Magnification
Few things in optics are understood less than magnification. Owing to the endless stream of articles and sales brochures extolling “high powered telescopes,” the unwary consumer is often left to believe that magnification is the single most important feature to consider when planning the purchase of a spotting scope. This is simply not true; one should not use more magnification than is necessary to do the particular job for which it was selected. Why? Because when you increase magnification you:

1) decrease image brightness by spreading available light over a greater area

2) decrease the field of view; making objects harder to find and keep centered

3) introduce more image degrading vibrations

4) accentuate atmospheric disturbances

For years, the most popular spotting scopes have had magnifications ranging from 15 to 60 power, topping out at about 25% of the “theoretical” magnification for a good 60mm telescope of any type. However, pushing mathematical modeling aside and sticking to the practical limitations of the instrument, an observer will be assured of having nothing but good viewing experiences. Unfortunately, many observers fall into the “bigger is better” trap, and push the limits of their spotting scopes to the point that they are left with fuzzy, jittery images.

Optical Coatings
Good anti-reflection coatings are crucial to the optimal performance of any optical instrument-especially binoculars and spotting scopes. At the beginning of the Second World War, only about 50% of the light that was captured by a 7x50 binocular’s objective lens managed to get to the observer’s eyes. The rest was absorbed into the glass or scattered about in the system. With the development of anti-reflection coatings light transmission was increased to 85 percent. Today, magnesium fluoride coatings can increase light transmission to about 89 percent with multi-coated lenses transmitting up to an incredible 98% per surface!

A Word About Tripods
Most naturalists mount binoculars with magnifications above about 10 power on a tripod, and since spotting scopes have magnification ranges that usually start at 15 or more, it stands to reason that all spotting scopes will perform better if mounted on a good tripod.

Tripods come with a variety of features, and prices vary accordingly. Unlike spotting scopes, however, most good models remain the same for several years. Thus, wise shoppers should select a model with the quality and features necessary to make their purchase a lasting value.

Some Points to Consider

Rigidity:
Select a tripod specific to its application and the size and weight of the load it will be expected to carry, keeping in mind that rigidity is more a product of design than weight. Birders tend to be on the go and usually want a tripod that is lightweight-just large enough to carry a light duty binocular or spotting scope. Amateur astronomers, on the other hand, tend to use their tripods in only one or two locations in the course of an evening’s viewing session and tend to use heavier equipment. Placing a rickety or poorly designed tripod under a $1,000 spotting scope is tantamount to hitching an expensive sports car to a team of goats-you could save on gasoline, but, overall, you would probably notice a few frustrating shortcomings in the decision to do so.

The Legs:
Photo tripods have legs with three sections that “telescope” in two places, allowing a tripod that might stand more than 5-feet tall while in use to be collapsed to just over 2-feet for storage or travel. Locking mechanisms may be of the clamp- or screw-type. If properly made, these styles will perform equally well. However, the clamp-type provides for much faster set-up and adjustment times.

The Head:
A tripod’s “head” is the part onto which the spotting scope is attached. Older, or less expensive, tripods often have heads that are adjustable through the loosening and tightening of two bolts-for up and down and side to side motion. A refinement to this is seen on heads on which these bolts are operated by long handles that can be used not only for tensioning, but aiming the instrument as well.

The best tripods have “fluid” heads-“fluid” referring to the smoothness of motion and not a liquid component-which operate on a user-adjusted clutch system that can be easily adjusted to match the tension needed to support the weight of a given instrument.

The Shoe:
Some spotting scopes attach to tripods through the use of a 1/4-20 screw which passes through a metal plate on top of the tripod head and into a mounting bracket on the bottom of the scope. While this is a very efficient way to attach the tripod, it leaves much to be desired when speed is of the essence-as it usually is.

In many of the best tripods, the metal plate (or “Shoe”) mentioned above is removable and may be left attached to the scope at all times. The scope may then be placed onto the head by snapping the shoe into the top of the tripod head, and removed with a “quick-release” latch or finger lever. This not only makes it possible to have several instruments ready for use at a moment‘s notice (additional shoes may be purchased separately), but lessens the chances of the scope being dropped while trying to attach it to the tripod.

Final Pre-Flight Instructions
1) Consider how you intend to use your new spotting scope. If it is just to be used around the house in daylight, a 50mm or 60mm instrument might be just the ticket. If you are an avid birder in search of getting a better look at those high-nesting raptors, you will want to consider an 80mm to 127mm telescope.

2) Decide which style will best suit your needs. A fixed-eyepiece will give you the largest aperture for the dollar; a zooming telescope will be the most convenient to use.

3) If you do not already own a good tripod, you will want to purchase one with your telescopes. The higher magnifications of a spotting scope will make holding it in your hands impractical.

4) Always use your spotting scope at realistic magnifications-15 to 60 power; slightly higher in instruments with aperture greater than 80 millimeters.

5) Spotting scopes may range in price from $100 to more than $2,000. Don’t be fooled into thinking that your enjoyment must be directly proportional to its cost. It is true that in most cases quality comes with price. However, optical experts agree that-once a certain level of quality has been reached-It often takes a 100% increase in cost to acquire a 10% increase in performance.

6) Have fun!

Details
Date Taken: 03/15/2011
Author: Orion Staff
Category: Telescopes

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