The refracting telescopes, in older days, suffered from severe chromatic aberration, which is now being dealt with in a variety of ways including better refractor designs.However, back in the 17th century, a new design arose that acted as an alternative to the refractor design, not suffering from chromatic aberration, owing to the fact that only lenses suffer from this kind of aberration and not mirrors. Telescopes built on that design, the Reflecting telescopes, primarily use a single or combination of curved mirrors that reflect light and form an image. We can see why they are called reflecting, as opposed to refracting.
Although reflecting telescopes are afflicted with other types of optical aberrations, it is a design that allows for very large diameter objectives. Large diameters, and in turn large apertures, as discussed before, allow for brighter and clearer images. Almost all of the major telescopes used in astronomy research today are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a catoptric (i.e. dealing with the phenomena of reflected light and image-forming optical systems using mirrors) telescope.
The idea that curved mirrors behave like lenses dates back at least to the 11th century. Curved mirrors can be concave (bulging inwards) or convex (bulging outwards), as discussed in the post “Optical Elements“. Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are sometimes used in optical devices. The most common non-spherical type are parabolic reflectors. The curved mirrors used in the reflecting telescopes today are usually parabolic. Parabolic mirrors don’t suffer from spherical aberration, either, which spherical mirrors do.
A parabolic (or paraboloid or paraboloidal) reflector (or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis. The parabolic reflector transforms an incoming plane wave traveling along the axis into a spherical wave converging toward the focus.
Parabolic reflectors are used to collect energy from a distant source (for example, incoming star light) and bring it to a common focal point, thus correcting spherical aberration found in simpler spherical reflectors. An interesting thing to know about the parabolic mirrors is that since the principles of reflection are reversible, parabolic reflectors can also be used to project energy of a source at its focus outward in a parallel beam, used in devices such as spotlights and car headlights.
Isaac Newton has been generally credited with building the first reflecting telescope in 1668. It used a spherically ground metal primary mirror and a small diagonal mirror in an optical configuration that has come to be known as the Newtonian telescope (discussed later). Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor performance of the alloy mirrors being used at the time meant it took over 100 years for them to become popular. Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in the 18th century, silver coated glass mirrors in the 19th century, long-lasting aluminum coatings in the 20th century, segmented mirrors (an array of smaller mirrors designed to act as segments of a single large curved mirror) to allow larger diameters, and active optics which is a technology used to actively shapes a telescope’s mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress – essential for large diameters and segmented mirrors.
The late 20th century has seen the development of adaptive optics (discussed below) and lucky imaging (discussed below) to overcome the problems of seeing.
Astronomical seeing (or just seeing) is an interesting phenomenon. It refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth’s atmosphere varying the optical refractive index. The astronomical seeing conditions on a given night at a given location describe how much the Earth’s atmosphere perturbs the images of stars as seen through a telescope.
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions: it aims at correcting the deformations of an incoming wavefront by deforming a mirror in order to compensate for the distortion.
Lucky imaging (also called lucky exposures) is a form of speckle imaging (in simple terms, a resolution-improving technique) used for astrophotography. Speckle imaging techniques use a high-speed camera with exposure times short enough (100 ms or less) so that the changes in the Earth’s atmosphere during the exposure are minimal.
A curved primary mirror is the reflector telescope’s basic optical element that creates an image at the focal plane. Well, that’s easier said than understood. You must be wondering that a mirror is not transparent so how does it gather light from the source and project the image for the observer to see while looking from the other side. Well, generally a secondary mirror is added to modify the optical characteristics and/or redirect the light to the eyepiece, or film or digital sensors for visual observation.
The primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to a spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective first surface mirror (with the reflective surface above a backing, as opposed to the conventional mirror where the reflective surface is covered by glass or acrylic).
Some telescopes use primary mirrors which are made differently. Molten glass is rotated to make its surface paraboloidal, and is kept rotating while it cools and solidifies. The resulting mirror shape approximates a desired paraboloid shape that requires minimal grinding and polishing to reach the exact figure needed.
Problems with the reflector design
No telescope is perfect, and while reflective telescopes are free from many aberrations like chromatic, they are still prone to some other aberrations and errors.
The need to image objects at distances up to infinity, view them at different wavelengths of light, along with the requirement to have some way to view the image the primary mirror produces, means there is always some compromise in a reflecting telescope’s optical design.
As mentioned above, because the primary mirror focuses light to a common point in front of its own reflecting surface almost all reflecting telescope designs have a secondary mirror, film holder, or detector near that focal point. This secondary mirror partially obstructs the light from reaching the primary mirror. Not only does this cause some reduction in the amount of light the system collects, it also causes a loss in contrast in the image due to diffraction effects of the obstruction as well as diffraction spikes caused by most secondary support structures.
A simple spherical mirror cannot bring light from a distant object to a common focus since the reflection of light rays striking the mirror near its edge do not converge with those that reflect from nearer the center of the mirror (a defect called spherical aberration, if you remember). To avoid this problem most reflecting telescopes, as mentioned before, use parabolic mirrors, a shape that can focus all the light to a common focus. Parabolic mirrors work well with objects near the center of the image they produce, (light traveling parallel to the mirror’s optical axis), but towards the edge of that same field of view they suffer from the following off-axis aberrations (discussed in the post “Optical Elements” but summarized here again):
Coma – An aberration where point sources (stars) at the center of the image are focused to a point but typically appears as “comet-like” () radial smudges that get worse towards the edges of the image.
Field curvature – The best image plane is, in general, curved, which may not correspond to the detector’s shape and leads to a focus error across the field. It is sometimes corrected by a field flattening lens.
Astigmatism – A variation of focus around the aperture causing point source images off-axis to appear elliptical. Astigmatism is not usually a problem in a narrow field of view, but in a wide field image it gets rapidly worse and varies quadratically with field angle (squared).
Distortion – Distortion does not affect image quality (sharpness) but does affect object shapes. It is sometimes corrected by image processing.
One problem that reflectors don’t suffer from is lens sagging (refractors suffer from this because of the use of lenses, as discussed in the post “Refracting Telescopes“). However, reflecting telescopes do suffer from mirror sagging, but that is not as severe as lens sagging, reason being that mirrors can be effectively supported by the entire opposite face, making mirror sag much less of a problem.
There are reflecting telescope designs that use modified mirror surfaces (such as the Ritchey–Chrétien telescope) or some form of correcting lens (such as catadioptric telescopes, discussed later) that correct some of these aberrations.
Now, let’s take a look at the different design variations of reflecting telescopes.
The Gregorian telescope consists of two concave mirrors; the primary mirror (a concave paraboloid) collects the light and brings it to a focus before the secondary mirror (a concave ellipsoid, a 3D analogue of an ellipse) where it is reflected back through a hole in the centre of the primary, and thence out the bottom end of the instrument where it can be viewed with the aid of the eyepiece. This produces an upright image, useful for terrestrial observations. Some small spotting scopes are still built this way. There are several large modern telescopes that use a Gregorian configuration such as the Magellan telescopes.
The Newtonian telescope was designed by Isaac Newton in 1668. It usually has a paraboloid primary mirror (although, at focal ratios of f/8 or longer, a spherical primary mirror can be sufficient for high visual resolution). A flat secondary mirror reflects the light to a focal plane at the side of the top of the telescope tube. It is one of the simplest and least expensive designs for a given size of primary, and also the most popular one among reflectors.
The Cassegrain telescope (sometimes called the “Classic Cassegrain“) was first published in an 1672 design attributed to Laurent Cassegrain. It has a parabolic primary mirror, and a hyperbolic (imagine two cones meeting at each other’s tip) secondary mirror, placed inside the focus of the primary, that reflects the light back down through a hole in the primary.
Although the Classic Cassegrain design might seem similar to the Gregorian one, the difference is that in the Gregorian design, the secondary mirror is placed beyond the focus of the primary whereas in the Cassegrain design, the secondary mirror is placed within the focus of the primary. The resulting differences are that the Gregorian produces an upright image but needs a longer tube whereas the Cassegrain creates an inverted image using a smaller tube. The shortening of the tube, or folding of optics has made telescopes with the Cassegrain design very compact and hence quite popular.
Other variations of the Cassegrain telescope are: Ritchey–Chrétien, Three-mirror anastigmat and Dall–Kirkham.
The Ritchey–Chrétien telescope is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of coma and spherical aberration at a nearly flat focal plane if the primary and secondary curvature are properly figured, making it well suited for wide field and photographic observations. Almost every professional reflector telescope in the world is of the Ritchey–Chrétien design.
A three-mirror anastigmat is a telescope built with three curved mirrors, enabling it to minimize all three main optical aberrations – spherical aberration, coma, and astigmatism. This is primarily used to enable wide fields of view, much larger than possible with telescopes with just one or two curved surfaces.
A telescope with only one curved mirror, such as a Newtonian telescope, will always have aberrations. If the mirror is spherical, it will suffer from spherical aberration. If the mirror is made parabolic, to correct the spherical aberration, then it must necessarily suffer from coma and astigmatism. With two curved mirrors, such as the Ritchey–Chrétien telescope, coma can be eliminated as well. This allows a larger useful field of view. However, such designs still suffer from astigmatism. This too can be cancelled by including a third curved optical element. When this element is a mirror, the result is a three-mirror anastigmat. In practice, the design may also include any number of flat fold mirrors, used to bend the optical path into more convenient configurations.
The Dall–Kirkham Cassegrain telescope’s design uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey–Chrétien system, it does not correct for off-axis coma. Field curvature is actually less than a classical Cassegrain. Because this is less noticeable at longer focal ratios, Dall–Kirkhams are seldom faster than f/15 (remember lens speed from the post “Rudiments of Telescopes“?). Takahashi Mewlon telescopes are Dall-Kirkham instruments with f/12 and are highly regarded. They require a corrector for wide field applications.
Looking at the designs above, you might have noticed that a secondary mirror seems like an obstruction, especially because of the distortions like diffraction that it induces. Well, the scientists were of the same opinion. So, some came up with designs that try to avoid obstructing the incoming light by eliminating the secondary or moving any secondary element off the primary mirror’s optical axis, commonly called off-axis designs.
The Herschelian reflector has the primary mirror tilted. Although this introduces geometrical aberrations, this design avoids the use of a Newtonian secondary mirror. Since the speculum metal (alloy) mirrors of earlier times tarnished quickly and could only achieve 60% reflectivity, Herschel, inventor of this design, felt that need of eliminating a secondary mirror altogether. However, that is no longer a concern, owing to the advancement in material science.
The Schiefspiegler telescope (“skewed” or “oblique reflector”) uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. However, while eliminating diffraction patterns this leads to an increase in coma and astigmatism. These defects become manageable at large focal ratios — most Schiefspieglers use f/15 or longer, which tends to restrict useful observation to the moon and planets.
The following two off-axis designs are not much in use and can be excluded from much details. Just some brief descriptions below so that you aren’t taken back when you hear about them somewhere else. Don’t worry if you don’t understand a thing, just know they exist!
The Stevick-Paul telescopes are off-axis versions of the three-mirror systems with an added flat diagonal mirror. A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally (with no focus) so as to send parallel light on to the tertiary. The concave tertiary mirror is positioned exactly twice as far to the side of the entering beam as was the convex secondary, and its own radius of curvature distant from the secondary. Because the tertiary mirror receives parallel light from the secondary, it forms an image at its focus. The focal plane lies within the system of mirrors, but is accessible to the eye with the inclusion of a flat diagonal. The Stevick-Paul configuration results in all optical aberrations totaling zero to the third-order, except for the field curvature which is minor.
The Yolo, like the Schiefspiegler, is an unobstructed, tilted reflector telescope. The original Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis. The Yolo design eliminates coma, but leaves significant astigmatism, which is reduced by deformation of the secondary mirror by some form of warping harness, or alternatively, polishing a toroidal (formed by revolving a circle in 3D) figure into the secondary.
If you are thinking of mercury, you are right! Liquid mirror telescopes are telescopes with mirrors made with a reflective liquid. The most common liquid used is mercury, but other liquids will work as well (for example, low melting alloys of gallium). The liquid and its container are rotated at a constant speed around a vertical axis, which causes the surface of the liquid to assume a paraboloidal shape (try stirring a glass filled with water and you will see the water assuming a paraboloidal shape), suitable for use as the primary mirror of a reflecting telescope. The rotating liquid assumes the paraboloidal shape regardless of the container’s shape. To reduce the amount of liquid metal needed, and thus weight, a rotating mercury mirror uses a container that is as close to the necessary parabolic shape as possible. Liquid mirrors can be a low cost alternative to conventional large telescopes. Compared to a solid glass mirror that must be cast, ground, and polished, a rotating liquid metal mirror is much less expensive to manufacture.
Reflectors with large apertures can be giant and bulky, though slightly less than comparable refractors. Portability of a telescope can play a deciding role in your choice of telescope. Mind you, for frequent stargazing (going out, looking for clear and dark skies), a bulky telescope could potentially hold you back and eventually kill your interest. But that’s just perspective. You could still be fine with a gigantic telescope that is “powerful” enough to give you views of the sky that you have only dreamed of till now.
Anyways, the attempt to make the telescopes compacter, and to correct some aberrations marring the refractors and reflectors, gave birth to the Catadioptric telescopes, which we will study next in the post “Catadioptric Telescopes“.