The term “telescope” means, literally, “see-over-a-distance”… just as telephone means speak-over-a-distance, and television means VIEW over a distance. I s’pose that ogling retail goodies on internet websites would be “teleshopping“, using a credit-card to purchase them would be “telepaying“… and banks and congress use “telefee” to strip us of our disposable income so we can afford to do a lot less of the former. If you’re captain Kirk, you “teleport” down to the planet’s surface with an away-team… although that might be a somewhat nuanced definition, as they’re really moving the distance without actually, technically, traversing the distance… at least without wearing out their shoes in the process, or pestering Engineer Scott to pull a shuttlcraft out of the garage & warm it up.
Be that as it may, at its most basic, a telescope’s purpose is to gather as much light as possible- using a collector, also called a “primary“- a lens or mirror that’s of far larger aperture than the quarter-inch pupil of the human eye… and to then focus that light down into a point, where it’s magnified & viewed by an “eyepiece”, or “ocular”. If this collector is a lens, then the scope is a “refractor“; if a mirror, the scope is a “reflector“; if using both elements & a more complex configuration, it’s called a compound or “catadioptric” scope. Functional, respectable, & relatively affordable examples of each of these types are common & available.
So a telescope helps us see objects which are farther away- overcoming (to some degree) distance’s diminishing effects on both apparent size & brightness. In fact, with enough aperture (collector size) put to the task, an object’s brightness can be boosted enough to render large & nearby objects as visible, where they’d otherwise be invisibly dim to all but owls, and perhaps a few cats. It may be surprising to consider that there are several nearby galaxies in outer space that appear larger on-the-sky to us Earthlings than our own Moon, but are very difficult (if not impossible!) to see without optical aid! Yes, they’re large… just very, very dim.
This, then, is a telescope’s “power” to see stuff- its aperture; the more light it can collect & then feed into your eyes to boost their viewing capabilities, the more power it has! The magnification used is more like the gears on a ten-speed bike; if you’ve ever tried to pedal one up a grade, then you know that the gear doesn’t supply the “power”… YOU do! And if in too high of a gear, your legs feel it- big time! So it is with telescope magnification- the scope’s “legs” is its collector size; the larger the aperture, the more power available… and therefore the more magnification (“gearing”) range possible, and the more you can see.
The first telescopes to be developed (around 1608) were refractors, which are pretty basic scopes, often representing what most people think of when it comes to a telescope.
"Refractor", from "Wikipedia"
Optically, they have a lens group at the “top” of the tube, and a focusing mechanism for an eyepiece at the bottom end, modern versions commonly utilizing a 45-degree angled mirror at the focuser so as to present the image in a more ergonomic manner… otherwise you’d need to be a contortionist to enjoy using them whenever viewing up higher in the sky! Since a scope’s magnification factor is linked to its focal length (remember “FL”?), some early refractor versions were comically long & cumbersome rigs, supported by aerial wires & scaffolding!- just to get “power”. That same “power” lust is used to advertise many WalGetCo scopes, making outrageous claims for magnification in their ads & on the scope’s box. Not much has changed in 400 years…
Highly functional refracting telescopes can be found having primaries from 2.4 inches (60mm) up to about 5-inches. Larger refractors can be had, but their length & weight become expensive to produce, and to properly mount; an 8″ F7 refractor (remember FR, FL, & D!) has a tube about 5-feet long, and one that’s quite heavy- especially at the top end! I have a friend with a 6 inch F9 refractor of about that same length… it sits on a $2,000 mount, and takes TWO able-bodied guys to get it there safely!
Another aspect of refractors is their tendency to break visible light into some degree of spectrum while focusing it to a point. In other words, not ALL the wavelengths focus precisely to that same point. The result is some color “fringing” around focused images of brighter objects, also termed “chromaticism“- and the “faster” the scope’s optics, the more severe this will be! At F15 most refractors are very forgiving, producing nearly perfect images in this regard; F8 refractors still do fairly well, yet at F6 they’re starting to get a bit challenged at keeping the spectrum intact, and by F5 the brighter objects can get a bit messy, with pretty colors flaring off the edges. However, fainter objects can still fare decently in fast optics.
So the game is between FR & tube length, extremes in either creating complications in use, so optical designers are aiming for a happy balance- a point that can vary quite a bit depending on what the scope is designed to be viewing- Moon & planets at F10+, or else wide & starry vistas at F6-. Yet most newcomers to astronomy will be notably disappointed if their shiny new scope provides them soft, rainbowesque images of planets! And while refractors are often the choice for terrestrial viewing, the chromatic effects of the faster versions can be unsightly in this application, as well.
The other solution to the refractor chromaticism gremlin is to produce primary optics that do a better job of focusing the entire visible spectrum to a true point, instead of a “range”. But to do so typically jacks the scope’s price up, and by no small degree! And the larger the aperture involved, the more severe the price bump. Here are a few links to illustrate this conundrum:
70mm F10 achro; 80mm F7.5 apo; 4-inch F10 achro; 4-inch F9 apo
These pricier refractor types are called “apochromats“, or “apo” for short, with some versions termed “ED”, and are not really recommended for folks just starting out. This would be particularly true where larger apertures are desired- there’s a better way to get into aperture! Lets go check out the “reflecting” telescope section…
In its simplest, and probably most common form, this scope type uses just two mirrors, and is called a “Newtonian Reflector”after its inventor- Sir Isaac Newton, in around 1668.
"Newtonian", from "Wikipedia"
Largely unchanged since then, optically simple, & wonderfully functional, a Newtonian has its primary mirror at the bottom end of an open tube, and a second, smaller mirror near the top- mounted at a 45-degree angle. This mirror, having a flat & aluminized surface, “picks-off” the focusing cone of light off the primary, & bounces it at 90-degrees through the side of the tube where a focuser & eyepiece provide the image.
The primary mirror in a reflecting scope is, basically, like a “vanity” mirror that magnifies one’s image for shaving or grooming- one polished into having a concave shaped, which is then aluminized. And since there are no LENSES in this scope’s arrangement, not even any glass for light to pass THROUGH– only aluminized reflecting surfaces- there is absolutely NO chromaticism happening! The resulting views display the benefits, too- sharp images without ANY “false color”… at least none from chromaticism.
Newtonian reflectors have an additional advantage over other scope types, in that they’re typically available at about half the price point for the same aperture, being commonly available from 3-4 inches up into the 10-12 inch aperture range. Reflectors of 4-8 inch aperture can even be found in the optically sweet F6-F8 FR range, and riding on equatorial mounts (recall from last time?), like these. Less costly versions come on simple, sturdy “lazy-susan” type swivel-base mounts working in alt-az (left-right & up-down) fashion; reflector types thusly mounted are termed “Dobsonians“, or “Dobs” for short- after the mount’s inventor- John Dobson, in the 1960’s. An 8-inch F6 or 6-inch F8 reflector, on either mounting type, makes for a killer scope- beginner or seasoned enthusiast!
Reflectors of 10-inch aperture & larger can be used on equatorial mounts, but like their larger refracting brethren, tube length & weight dictates heavy & expensive mounting… definitely material for astro veterans, folks heavily invested in their pursuits! However, these larger reflectors are also available in Dob format, utilizing solid-tube architecture up to 12-inch aperture, and also available in “strut” or “truss” structures for scopes 8 to 16-inches of aperture. Yet once you get into 12-inch “tuber” Dobs, they can be quite a handful- they’re lovingly referred to as “water-heater” Dobs, and for obvious reasons! The tubes on these will not fit in many smaller cars… 10-inchers seem to be about the largest size that’s commonly handled & transported by most folks & most cars, assuming a teen or adult of average size & strength.
“Truss” & “strut” Dobs, which by their design break down into smaller & lighter components, can generally be stored and/or transported by most users (& their vehicles), yet such a scope architecture introduces some complications & fuss-factor, requiring significant investment in time & patience on the part of their owners- perhaps not the best choice for a “first” scope, unless someone more experienced is on-hand to assist.
Newtonian reflectors are great all-around scopes, offering sharp views as well as the ability to see “wide”. The two main concerns that pertain uniquely to this design are maintaining their mirrors’ optical alignment, and when built faster than about F6 (typically the case with Newts 8-inches & larger), contending with their own optical bugaboo- an effect known as “coma“. In addition, Newtonians are not so well suited for viewing terrestrial sights.
Optical alignment of a reflector’s mirrors, a process called “collimation“, is a basic necessity for pretty much any of the species, a simple ritual every reflector owner will learn. Unlike a typical refracting telescope having fixed optics & a sealed tube, which are generally (although not always) simple “plug-‘n-play” affairs, a Newtonian reflector is an open tube, with adjustable holding-cells on both its mirrors. Yet for F6 & slower optics, it’s a quick & easy process, and fairly forgiving for results. An 8-inch F6 Newtonian of mine years ago was very easy to get (and keep!) collimated. However, larger scopes usually come F5 (& even faster!), where collimation can be a bit fussier & perfectionistic, and they really benefit from some good collimation tools- a small investment, really, for the optical benefits gained.
As for “coma”, an optical gremlin inherent in a Newtonian’s optical design, most folks aren’t bugged by it to the degree they might be by an inexpensive refractor’s chromatic effects… yet at F6 & faster its definitely there to be seen- turning stars in the outer edges of the view into varying degrees of fan-shaped blurs. At F5 more folks will see the effects of coma, as they’re encroaching farther into the field. Generally by around an FR of F4.5, most reflector owners will eventually break down, have a fit, and purchase themselves an optical coma-correcting widget & be done with the matter. Not necessarily a deal-breaker, yet as with collimating a faster Newt, investing in good tools & optical widgets is one more cost factor to consider in ownership.
So here lies the line of distinction on Newtonian reflecting telescopes: An 8-inch F6 & smaller/slower scope provides the most bang-for-the-buck in optical capabilities, yet remains simple to maintain & easy enough to handle & transport for almost any user, young or older. 10-inch & larger/faster scopes become somewhat costlier, more cumbersome to manage, and a bit fussier to deal with optically… yet, ultimately, will see more due to their larger aperture. This, or something very similar to this, is the conclusion reached by many seasoned veterans who address the matter, particularly in regards to someone just starting out in astronomy.
There remains one more basic telescope type, which is a kind of “hybrid” between a refractor & a Newtonian reflector. These are called “compound”, or “catadioptric” telescopes, and they employ a full-aperture lens up front, called a “corrector”,
"Mak", from "Wikipedia"
a concave mirror inside the tube & mounted at the rear (much like a Newtonian), and a smaller secondary mirror attached to the center of the corrector. So all together, there are THREE optical elements- and a sealed tube. They’re probably more akin to a reflector than a refractor, yet are different enough from either to warrant a distinct category!
Smaller versions of these, usually from 3-1/2 inches aperture up to 6-7 inches are of a specific optical design called “Maksutov”, and are affectionately known as “Maks“… whereas larger variants, apertures from 5-inches up to 14-inches, are of a design variation called “Schmidt-Cassegrain”, or “SCT” for short.
"SCT", from "Wikipedia"
While made in longer FRs, typically F10 to F15, their “folded” optical design allows for a very short, stubby tube, and makes them fairly lightweight in smaller apertures, and therefore quite portable & easy to mount. Some versions are used as telephoto lenses on cameras, for terrestrial & wildlife shots, and also sporting events. Larger versions, though still short & stubby, get to weighing quite a bit, so require beefy mounts for adequate support. I used a 10-inch SCT for over a decade, and lifting its weight out of storage & onto its mount was enough to make one a bit sore of muscles the next morning!
Maks and SCTs up to maybe 5-inches are usually more “affordable”, as good telescopes go, and while this design is slightly more complicated optically, are usually pretty well built, and can be fine performers. The Mak design is particularly well regarded for viewing the Moon & planets! The two gremlins that pester these scopes are, primarily, dewing of the exposed corrector lens up front, since its so exposed to the elements, and that their long FR design makes them relatively narrow for view. Yet “dewshields” are fairly easy to purchase or make, and in smaller apertures, the narrower field is really not all that significant.
In the final analysis, a 3-inch F10 refractor, this 4-1/3 inch F9 Mak, and a 5-inch F7 Newtonian reflector will be far more similar than different in use, and in the views they provide- assuming they’re each of comparable build & optical quality. Yet refracting scopes beyond 4-inch & catadioptric scopes beyond 5-inch aperture can start getting more expensive, whereas Newtonians (especially of Dob mounting) remain relatively affordable into the 6-8 inch aperture range. This factoid may or may not enter into one’s scope shopping equation- but it’s worth mentioning.
I certainly hope this information has been helpful, and not too confusing or otherwise off-putting for any trying to make sense of these matters. After quite a bit of time, having used & studied samples of each of these scope types over the years, including those of the WalGetCo variety, i’ve come to the conclusion that telescopes have personalities, in addition to FLs & FRs & other technical specs; this aspect can have a strong effect on how people & scopes interact with one another. So along with the info, opinion, & advice given to this point, it should also be stressed that getting some hands-on experience with the hardware is ALWAYS a good idea- whether that be at a friend’s house using his/her scope, at a public “starparty” where several scopes are set up for the public to view with, or even if merely spending time at a good astronomy retailer- fiddling with their demos on the showroom floor!
Update: Sky & Telescope magazine, an excellent & time-proven observing companion for many of us astroheads, has posted on their website a similar article- one with very nice graphics. Perhaps this stalwart resource can compliment the search, pointing readers toward the right scope?
While it’s true that a telescope’s purpose is to gather as much light as possible, and to help us see objects which are farther away, too faint… or both- those are both rather cold & clinical descriptions of its function. Yet there’s something very significant still missing from this idea… so please allow me another run at it:
A telescope offers us the privilege of seeing that which most folks never get to see, things that mankind has only recently even been able to see, things which have been up there and “out there” since the very beginning of time… things which have been seen, experienced, & enjoyed by only One– He who created them, in the beginning, for His own enjoyment.
Like a little child who’s daddy has lovingly hoisted him up so he can see over the fence into the world beyond, this wonderful use of glass & metal, skillfully combined & crafted, allows us to share in something way beyond ourselves, our grasp, understanding, & wildest imaginations! Could it be true, as stated by the famous Astronomer, Johannes Kepler, that a telescope helps us “think God’s thoughts after Him“?