Telescopes: A Close-up View(er)
If you haven’t got stars in your eyes, then this is the way to get them there: Telescopes! What an incredible and wonderful invention they were…
In the Beginning
Of course, telescopes did exist before Galileo Galilei decided to study the stars with one of his own design. They were just navigational telescopes, used by sea captains and navigators, to find landmarks on shore.
All telescopes were handcrafted—it couldn’t be any other way in the early 1600s C.E.—and consequently, they were restricted to magnifications of about 3x (pronounced “three power” or “three times”). This meant that they only had a very small magnification compared to the unaided eye.
Grinding a lens by hand is difficult, especially when they are very small, such as the eyepiece in the image above. Surprisingly, a large lens is much easier to grind, but getting glass slugs that were sufficiently clear to be useful as a lens was nearly impossible back then, so they did the best they could.
Galileo was particularly determined, and after much effort, created his master telescope that had a magnification of an incredible 7x! Mere peanuts to a modern-day hobbyist scope, of course, but a stunning achievement for over 400 years ago.
Optical telescopes all use lenses at some point, but in some cases, it is only in the final eyepiece. Let’s look at the three varieties of optical telescopes.
This is the earliest sort that we built and has passed down through history. The first lens to receive the light to make an image is the one closest to the object being viewed, and thus is called the Objective lens. It defines the aperture’s size, or how much light is allowed into the scope.
If the objective lens is 2.5 inches across (diameter), it has an aperture area of ~ five inches2. If it is 5 inches across, it has an aperture of ~20 inches2. By doubling the diameter of a lens, you increase its light-gathering capacity by four times. This is the Law of Squares, sometimes called the Inverse Square law, depending on the circumstance where it is being applied.
If the objective lens has an arbitrary diameter of “1”, it would gather “1” unit of light. If the objective lens was twice the diameter of the first lens, it would gather four units of light. At three times the original diameter, it would gather nine units of light.
The name “square” comes from applying powers, or “squares” to the lens size. For example, 12 is equal to one; 22 is equal to four; 32 is equal to nine, and so on.
You can calculate the light-gathering ability relative to another aperture by assigning a value to “1” to the smaller lens and then finding how much bigger the lens you want to compare is. Whatever the difference is, you square it.
Mathematically you would do it like this: If you want to compare a 5-inch lens to a 2-inch lens, you would divide 5” by 2” to see that it is 2.5 times larger; raise 2.5 to the power of 2, or 2.52, to discover that the 5” lens would be 6.25 times better at light gathering. It doesn’t matter if you use millimeters, feet, yards, or meters.
One of the faults of the Refractor telescope is how the light bends when it passes through glass. If the two sides of a piece of glass are not precisely parallel, then different frequencies (colors) of light emerge at slightly different angles. This is the principle at work in a prism that gives us that colorful spectrum.
If all of the lenses were made out of the same glass, each time it passed through a lens, the spread would get slightly worse, and each star would become surrounded by a rainbow halo, making it hard to see details. To compensate for this, we use different types of glass, such as silica glass or flint glass, where the rate of bending changes.
We can also use lenses ground in unique ways to alter the path of the light so that it comes out very nearly unaffected. Scopes with no chromatic aberration are incredibly costly ($1,000-$5,000), but surprisingly decent ones for hobbyists can often be had for $200.
In any case, with a refractor, you are generally faced with a very long instrument, which, unlike the “pirate” version, does not collapse into something that will fit into a pocket. They are generally fairly heavy since it is vitally important that the lenses stay a precise distance apart, and that they do not move. It is sealed at both ends, though, so this prevents dust intrusion.
Light passes through the objective lens and is focused on either the eyepiece or on a mirror that directs the image out the side to the eyepiece to make for easier viewing. Refractors are effective, sturdy, and reliable, if a bit on the heavy side, and harder to aim because of their length.
Also called Newtonian Reflectors, or Light Buckets, reflectors offer a comparatively huge aperture. The amateur variety might be just 15 cm (6 inches) up to 30 cm (12 inches) or more. Telescopes are all about light-gathering, and hobby-class reflectors outperform refractors, hands down. A 30 cm reflector lets you see objects that are 16 times fainter than a 7.5 cm (3 inch) refractor can render.
They do not use lenses, however. Instead, they use a highly polished and curved optical mirror at the base of the telescope (blue), opposite to the light opening, and protected from ambient light by a long, black-painted tube. This curved surface magnifies and redirects the image onto a flat (non-focusing) mirror, which redirects it to the focal plane where the eyepiece sits. There, the eyepiece can magnify the image even more, providing values like 200x, 300x, or more.
These mirrors are much thinner, lighter, and cheaper to manufacture than any collection of lenses. More importantly, mirrors do not introduce any chromatic aberration. They reflect the exact light they receive, without having to pass through any glass that can subtly alter its refraction.
If you look at the gold line, you can see that the focal length is longer than the barrel of the telescope—more telescope in less space. This big hollow tube is light, easy to move, short, and easy to aim.
Finally, we come to the ultimate mélange, where the previous two types are blended. The catadioptric telescope is shorter than a reflector, yet still has the long focal length.
The first difference you'll see is that it has a mild (low-powered) lens over the opening. This keeps the dust out and compensates for the curvature of the primary mirror. The mirror has a much deeper curve than a reflector telescope as if it were a slice of the outer skin peeled off a basketball; this shape causes spherical aberration because light will come into focus at different “depths” and make the whole image poorer. The corrector plate bends the light so it all “arrives” as if it were striking a flat surface.
Light passes through the corrector plate and travels down to the primary mirror. The light from the primary mirror is directed to another curved mirror. This secondary mirror is often mounted right on the corrector plate to extend the focal length of the whole telescope, but it can be mounted separately inside. This light is directed back to a hole in the primary mirror and passes through to the focal plane, where it finally gets to the eyepiece.
In other words, light travels the length of the tube three times, under increasing magnification. This is where the name “folded telescope" comes from because the whole scope is much smaller than the remarkable focal length.
These incredibly short (yet powerful) telescopes are light, easy to transport, set up, and enjoy. They're also more expensive, but if you have the budget, it represents an excellent purchase. Indeed, these telescopes incorporate some exciting technology for self-pointing and object-tracking, too. While such things can be done with refractors and reflectors, the compact size of the catadioptric lends itself to making the best use of this technology.
Big apertures are better. Magnification is less critical than the light-gathering ability. Reflectors are often the least expensive but give the best value for a newcomer on a budget. Catadioptrics are suitable for on-the-go viewing. Refractors are fine if you do all your viewing in one place, but they’re a little heavy and awkward to transport. They are excellent for Moon-watching.
Look for more telescope information soon! Meanwhile, join a local astronomy club and use other member’s telescopes for free. Learn which type you like the best before you invest in one of your own!