Why a Spotting Scope?

Sometimes, a binocular can’t get you close enough to a bird to show you the detail you need. Beyond a hundred feet or so, subtle field marks start becoming difficult to differentiate. You can tell it’s a Sandpiper, but what kind? A Western, a Semipalmated, a Baird’s? They look very similar at long distances.

A spotting scope gets you closer to the subject so you can tell what kind of Sandpiper it is, or whether that really is a deer lurking at the far end of your property. A spotting scope’s magnification starts where your binocular leaves off.

Spotting scopes are compact telescopes designed primarily for terrestrial observing and photography. In addition to birding, some other uses include:

1.- viewing distant sports events, such as boat races and mountain climbing;

2.- observing deer, mountain goats, and other easily-spooked animal life;

3.- use as a long distance microscope for safe close-up study of hornets’ nests, bee hives, etc.;

4.- surveillance of property and outbuildings for the isolated homeowner, rancher, or farmer;

5.- scanning ski resort, lake, or harbor activities from the home with a view;

6.- casual astronomy; and more.

There are two types of spotting scopes available: prismatic (a simple refractor, with a lens at one end and an eyepiece at the other) and catadioptric (a combination of lenses and mirrors).

Parts of prismatic spotting scope.gif (3983 bytes)An eyepiece placed directly in the back of an ordinary telescope shows you an image that’s upside down and backwards. Because of this, prismatic spotting scopes come with a built-in erecting prism system, such as the straight-through viewing porro prism in the prismatic scope below. Others, such as the catadioptric scope shown on the next page, come with a prism for observing at a 45° viewing angle (looking down at a 45° angle to see straight ahead). Both give you erect and right-reading images (correctly oriented from left to right) so you can read printing – distant boat names and license plates, for example.

Many spotting scopes come with a zoom eyepiece offering a range of magnifications. On some, you can replace the zoom eyepiece with other eyepieces, each with a single fixed magnification. A zoom is more flexible than a single power eyepiece, but generally has a narrower field of view and somewhat lower resolution and contrast because of the extra lenses in the zoom mechanism.

Low power eyepieces, or low power settings on a zoom, usually provide more satisfactory images than higher power eyepieces due to the wider fields of view and brighter images at low powers.

With many prismatic spotting scopes, the eyepiece is in a straight line with the main lens of the scope, such as in the scopes shown on this page. Many American birders prefer this straight-through viewing arrangement. It allows them to roughly center the scope on a distant bird by sighting over the scope barrel (or using the non-magnifying peep sight or bead and notch sight most scopes have as an aiming aid), before attempting to find the bird in the narrow field of the eyepiece. It also allows viewing over hedge tops with a minimum of the birder visible to disturb the birds. Straight-through viewing is also the most convenient for photography.

Scopes with a 45° viewing angle are the most popular in Europe, where observing is often done from a seated position in a "hide" or Prismatic zoom spotting scope.gif (3040 bytes) blind. A 45° viewing angle scope is the most comfortable type for tall observers to use without getting a crick in their neck, for observing while seated, as well as for examining tree tops. 45° viewing angle scopes are enjoying a rise in popularity in this country due to these comfort factors.

A spotting scope should always be used on a tripod, as hand-holding a high power scope is impractical. With straight-through scopes, a tall tripod is needed so that a tall observer can observe without uncomfortable crouching. A 45° viewing angle scope is handy when tall and short observers must share a spotting scope, as it eliminates the need to constantly raise or lower the tripod to accommodate their different eye levels.

Because of their limited magnification range, prismatic and refractor spotting scopes are generally the most useful for medium power birding at moderate distances from 150 feet or so, where binoculars start to leave off, out to perhaps 500 yards. (These distances are not limits, merely suggestions.)

Catadioptric (combination lens/mirror) spotting scopes are often powerful telephoto lenses equipped with an image-erecting prism system and an eyepiece for visual use. They are usually the best choice as a high power spotting scope if long distance nature photography is a major part of your birding plans.

Parts of catadioptric spotter.gif (4401 bytes)The combination of lenses and mirrors in a catadioptric spotting scope folds a long focal length into a short package, giving you high visual magnification and exceptional telephoto lens performance without excessive bulk.

The large aperture and long focal length of a catadioptric spotting scope make it very suitable for high power terrestrial views from a fixed location – such as from a patio, vacation cabin, or beach house. Its long focal length gives you a narrow field of view, however. This limits its usefulness for those activities requiring a wide "picture-window" view, such as scanning scenery, following fast-moving sports, close-in birding, etc. Catadiop-trics are generally most useful for high power observing at moderate to long distances of between 200 feet and one mile. (Again, these distance are not limits, merely suggestions.)

Because of its narrow field, a catadioptric spotting scope usually comes with a finderscope (a small low-power telescope, with crosshairs, mounted on the side of the main scope and used to help you center the scope on distant objects).

A few spotting scopes, most notably the catadioptric Questar and some versions of the TeleVue Pronto and Ranger refractors, have a 90° viewing angle eyepiece holder that positions the eyepiece at right angles to the line of sight. You look down into the eyepiece to see birds in front of you. This gives sharper images, but some people occasionally find the viewing position awkward. Also, the 90° eyepiece holder shows mirror images (printing is upright, but backwards). This means that a bird appearing to move left-to-right in the eyepiece requires that you move the scope right-to-left to track it properly, which some observers initially find confusing. It soon becomes second nature, however, just as it is no problem to comb your hair by looking at your reversed image in a mirror.

Spotting scope viewing angles.gif (2928 bytes)If straight-through viewing would be more comfortable, however, or if right-reading images are required for surveillance, reading the home ports of distant ships, etc., optional image erecting systems are available to give you both straight-through viewing and correctly oriented images with these 90° scopes.

Most catadioptric scopes come with a removable 45° viewing angle eyepiece holder for a viewing position intermediate between straight-through viewing prismatic scopes and 90° viewing Questar catadioptrics. These 45° eyepiece holders give correctly-oriented images (both erect and right-reading), without the need to buy a separate porro prism to make the target motion you see in the scope agree with what you see with your bare eyes. And the 45° viewing angle yields a comfortable viewing position for long birding sessions, particularly for tall observers with short tripods.

If you plan to use your automobile as a mobile blind, straight-through viewing – or a 45° viewing system that can be tilted from side to side – is virtually essential, as there’s usually not enough room to fit your head between your car’s headliner and the eyepiece of a 90° viewing angle scope on a car window mount.

The magnification of any catadioptric spotting scope can be changed by simply substituting a different eyepiece. Zoom eyepieces are available and provide considerable observing flexibility, although at the cost of somewhat lower contrast and a narrower field of view at low powers than single power eyepieces of comparable magnifications.

Due to a catadioptric’s folded optical path design, an image of its secondary mirror is sometimes visible as a grey blur in the center of the field during daylight observing at very low powers (lower than that provided by the eyepiece normally supplied with the scope). This secondary mirror image can be eliminated by switching to a higher power eyepiece.


. . . our 34th year