Astronomical telescopes (optical, not radio) are divided broadly into two categories, refractors and reflectors. A refractor consists of a relatively long tube with a lens called the objective lens semi-permanently mounted at the far end and an eyepiece mounted at the end nearest the observer. The telescope will work if the eyepiece, like the objective, consists of a single convex lens, but better optical performance results when it is made up of two convex lenses. The eyepiece lenses mount at a fixed distance from one another in the eyepiece barrel. A rack and pinion adjustment moves the barrel closer or farther from the objective.
The key metrics in telescopes are aperture and focal ratio. The aperture (applicable to the objective lens) is the diameter. Amateur telescopes generally range between 1.75 and 6.5 in. Almost all large telescopes currently in use are a type referred to as a reflector. For a refractor, as may be expected, in larger sizes the cost escalates, in part because of the expense in grinding and polishing larger lenses and in part because it takes a more rigid mount to control vibration with more weight and higher magnifications.
One of the reasons for the upper limit on refractor aperture size is that a larger lens will tend to sag under its own weight, throwing it out of true and making for a distorted image. This is especially so when the tube is moved to a near vertical position. A reflector has an objective mirror, in contrast, that can be supported from behind so as to be less prone to sagging.
A large aperture is desirable because there is more light-gathering capability. Regardless of the magnification or aperture of a telescope, the image of a star will never appear as a disc with an apparent diameter, except in the case of a few of the largest stars such as Betelgeuse.
Focal ratio, other than aperture, is the other parameter of interest. It is also called f-ratio. It is similar to the f-ratio of a camera, but the number is higher. The f-ratio is a measure of the focal length of the objective lens divided by its aperture. Because the size of the objective is less than its distance to the eyepiece, the f-ratio will be a number greater than one.
An objective lens with a smaller f-ratio will yield a larger field of view, which in most cases is highly desirable. In refracting telescopes, this configuration is more optically demanding for the manufacturer in terms of getting a quality image that does not vary from center-to-edge of the field-of-view.
Refracting telescopes are more elegant and upscale, but reflectors have advantages that let them dominate the field. Reflectors use a single or combination of curved mirrors that reflect light and form an image. Because the objective mirrors, unlike lenses, can be supported from the backside, they do not sag and distort the image in the manner of refractor lenses that are over six or so inches. Also, for some reflectors, the tube is short so the telescope is easy to move about.
There are several types of reflectors, classified by the mirror configuration and optical path. No matter how you look at, there must be a mechanism for getting the light to the objective mirror and from there to the secondary mirror and eyepiece. This typically involves placing a secondary mirror in such a position that it reflects the light beam out through an opening in the side of the optical tube (Newtonian reflector) or back through the objective mirror (Schmidt-Cassegrain reflector) so the image can appear in the outside world.
Either way, the light path is interrupted. But contrary to what one might think, the image is not greatly compromised by a shadow or blurring in the field. A small percentage of light is lost relative to a refractor but because the secondary mirror is not at the focal point, the image survives with essentially the same fidelity to the object that is viewed.
Outside professional observatories, Newtonian reflectors are available with objective mirrors as large as 24 in. The 8 to 12-in. range is considered large, and those are serious telescopes, well-suited for astrophotography.
The distinguishing quality of a Newtonian reflector is that the eyepiece mounts at the side and perpendicular to the telescope tube. This makes for easy viewing, although a ladder is needed to access the eyepiece on larger versions.
In a Newtonian reflector, the light enters the tube and strikes the concave objective mirror mounted at the far end of the tube. The light travels back to the secondary mirror, which mounts at a 45° angle to the centerline of the tube, so the light travels to the eyepiece off to the side.
Because the secondary mirror along with its mounting hardware blocks a portion of the light path, the effective aperture is reduced a slight amount, which does not happen in a refractor. Consequently, the image has less contrast. In view of the large aperture that is available in a reflector, that is not a serious drawback.
A Schmidt-Cassegrain telescope has a convex secondary mirror, which makes possible a physically shorter tube despite the long focal length. For this reason, a Schmidt-Cassegrain has a distinctive look (short and fat) and is easier to move around than a Newtonian telescope of the same focal length.
The objective mirror has a round opening in its center, and that is where the eyepiece resides. The observer looks in through the end of the telescope, as in a refractor. At the far end of the tube is a transparent corrector plate. Because there are no side supports, there is no loss of contrast as in a Newtonian type. Still, the opening in the center of the primary mirror reduces the effective aperture.
The secondary mirror is supported by the corrector plate at the input end of the tube. The light path is in through the corrector plate, down to the objective mirror, all the way back to the secondary mirror at the corrector plate, then back through the opening in the center of the objective mirror, and finally out through the eyepiece. The fact that light travels through the tube twice is what accounts for its shorter length. The shorter tube length reduces high-magnification vibration, and that is a definite plus.
A drawback in the Schmidt-Cassegrain is that the corrector plate makes it more prone to condensation, though less a victim of dust accumulation. Newtonian and Schmidt-Cassegrain reflectors each have advantages. For a given cost, a larger aperture is possible in the Newtonian.
Astronomical telescopes have large apertures, long focal lengths and various add-ons such as cameras, finder scopes and Barlow lenses. Cumulatively, they contribute weight, making a heavy tube that due to its length and high magnification is prone to vibration. So the telescope mount and support, tripod or pier, must be heavy, built to exacting specifications and resting firmly or attached securely to the ground.
If the telescope is tripod mounted, better results are obtained by placing the three legs on a grassy area as opposed to hard surfaces. A plywood shelf attached to all three legs some distance below where they meet at the top and bungee cords near the bottom greatly strengthens the tripod, reducing vibration. Believe it or not, a medium-weight chain, about 16 inches long, hanging from the optical tube near the input end will dampen harmful motion.
Attached to the pier or tripod is the telescope mount, and there are several types currently in use. The most basic type is the altazimuth mount. It lets the telescope move along two independent axes, perpendicular to one another. These axes are fixed with respect to the earth’s surface and do not permit tracking of stellar objects.
Until recently, all but the most low-end amateur telescopes had equatorial mounts that with a single-axis could follow the apparent motion of stars that resulted from the earth’s rotation. With one simple clock drive, sky objects could be tracked for time-exposure photography and sustained observing sessions. With today’s digital sophistication, altazimuth mounts can be equipped with separate drives that will let the telescope simultaneously track along both axes. This is the current method used in large professional observatories where the electronics and software requirements are less costly than building a large dual-axis mount.
Amateur astronomers continue to favor the equatorial mount. It has been refined over the years and works well for astrophotography and go-to technology. The basic idea is that one axis of rotation is parallel to the earth’s axis of rotation. With respect to the earth’s surface, it is tilted at an angle that varies with the observer’s latitude.
Other configurations also allow tracking of the stars. One possibility is a tilting platform that supports a conventional altazimuth mount. The angle built into the platform corresponds to the latitude. A German equatorial mount provides superior tracking with excellent stability.