A lot of people looking for their first telescope can be under the impression that they will be frequently using it for terrestrial viewing, or get sucked into setups that offer easy use in the daytime in exchange for so-so astronomical views. A lot of literature often seems to confuse even further, and with that in mind, we’d like to explain the differences between spotting scopes and telescopes and when each makes sense.
What is a spotting scope?
A spotting scope is simply a telescope that is purpose-built for daytime use. The majority of spotting scopes have built-in zoom eyepieces that can not be removed, provide a range of magnifications, and are usually made to be used on photo tripods of some kind. Most spotting scopes are refracting telescopes, using a curved lens, usually with 2 or 3 separate pieces of glass, to focus light and provide an image. A few are Maksutov-Cassegrain or Schmidt-Cassegrain telescopes, which use multiple curved mirrors and lenses to focus the image and tend to be more compact for a given focal length or aperture than a refractor. You’re unlikely to see a Newtonian reflector used as a spotting scope, as it would prove to be quite an impractical arrangement.
Spotting scopes also almost always incorporate prisms, usually permanently built into the back of the telescope, to provide a correct image both left-right and up-down so that distant objects are in the correct orientation and text can be read with them from afar. Some high-quality spotting scopes may be waterproof or at least sealed on the inside to make them more rugged and less vulnerable to harsh conditions like rain and sea spray.
Optical Quality Differences
One of the things that people often do not realize about spotting scopes is that they don’t really tend to have good optics, nor do they need to. For one thing, most spotters are refractors and most of the lower-priced ones are achromats – using two lenses at the front, and with short focal ratios to keep them compact. For an astronomical telescope, a fast and cheap achromat provides images that are rarely sharp at high magnifications and bright objects suffer from chromatic aberration, causing ugly purple halos to appear around them. Chromatic aberration is much less obvious in the daytime, and most spotting scopes are used at relatively low magnifications, so there’s a little more tolerance here. The multiple prisms in a spotting scope also can add chromatic aberration, and cheap ones will also decrease the sharpness of the views somewhat – but for viewing things in the daytime, one is much less likely to notice.
Point a typical spotting scope at the Moon and you’ll probably get an okay view, but the planets are a different story. Venus’ phases are visible as are Jupiter’s moons, but you’ll probably struggle to make out Saturn’s rings or get a clear view of Jupiter’s cloud belts. Spotting scopes are typically the worst corrected towards the red end of the visible spectrum, so Mars is nearly guaranteed to be little more than a blob.
Since spotters are made for daytime use, the brightness of their images is also not too important. The prisms and multiple lens elements in the eyepiece absorb a fair bit of light in cheaper spotting scopes, which means the views of deep-sky objects will be dimmer than in a similarly sized astronomical telescope.
Most spotting scopes have a 45-degree angled or straight through the back eyepiece, which is because they are primarily aimed around or below the horizon, and usually not that far upwards. As a result, when aimed high in the sky, the eyepiece will be pointing downwards, towards the ground, which will require a really uncomfortable posture to get your eyeball below the bottom of the telescope.
Spotting scopes seldom have a finder of any kind attached, which makes aiming them at astronomical targets even more difficult and unpleasant. Aiming at a terrestrial object without a finder is fairly easy as the surrounding area provides context, which is rather lacking in the night sky.
Is a spotting scope or a telescope right for me?
If you’re primarily interested in watching distant things on land – ships, birds, and so forth – with the occasional peek at the moon or other night sky objects, then a spotting scope is probably right for you. But if you want sharp, bright, and detailed views of the Moon, planets, stars, nebulae, and galaxies, a true astronomical telescope is a far better option that will lead to long-lasting enjoyment and less frustration.
Avoid or at least be skeptical of manufacturers selling “astronomical/terrestrial” telescopes, particularly if they have “erect image” eyepieces included and/or are Newtonian reflectors. These are usually low quality instruments designed to fit a given price point and a set of marketing claims rather than provide quality views.