A refracting or refractor telescope is a type of telescope that uses a lens as its objective to form an image. Refracting telescopes are also referred to as dioptric (i.e. dealing with refraction of light, especially by lenses).
These were the earliest type of telescope used. In fact, all the startling discoveries that Galileo did, involved this type of telescope, and his own design known as Galilean telescope.
All refracting telescopes use the same principles. The combination of an objective lens and some type of eyepiece is used to gather more light than the human eye is able to collect on its own, focus it, and present the viewer with a brighter, clearer, and magnified virtual image.
The objective in a refracting telescope refracts or bends light. This refraction causes parallel light rays to converge at a focal point. Because the image was formed by the bending of light, or refraction, these telescopes are called refracting telescopes or refractors.
Problems with the refractor design
Refractors suffer from residual chromatic and spherical aberration. This affects shorter focal lengths more than longer ones. A 100 mm (4 in) f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 100 mm (4 in) f/16 has little color fringing.
In very large apertures, there is also a problem of lens sagging, a result of physical weight of the glass causing a distortion in the shape of the lens because the lens can only be supported by the edges. Well, gravity is not always helpful! Since a lens can only be held in place by its edge, the center of a large lens sags due to gravity, distorting the images it produces. The largest practical lens size in a refracting telescope is around 1 meter (39 in).
There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself.
Even though the foundation remains the same, refractors have different design variations, newer ones mostly an improvement over the older ones.
This type of refractor, designed by Galileo, uses a plano-convex (convergent, positive) objective lens and a plano-concave (divergent, negative) eyepiece lens. A Galilean telescope, because the design has no intermediary focus, results in a non-inverted and upright image.
Galileo’s best telescope magnified objects about 30 times. Because of flaws in its design, such as the shape of the lens and the narrow field of view, the images were blurry and distorted. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. The Galilean telescope could view the phases of Venus, and was able to see craters on the Moon and four moons orbiting Jupiter.
The Keplerian telescope, invented by Johannes Kepler, is an improvement on Galileo’s design. It uses a convex lens as the eyepiece instead of Galileo’s concave one. The advantage of this arrangement is that the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief, but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design, but to overcome aberrations the simple objective lens needs to have a very high focal ratio (f-number).
The achromatic telescope uses an achromatic lens (a lens that brings two wavelengths, typically red and blue, into focus in the same plane) to correct for chromatic aberration. As discussed in the post, Optical Elements, an achromatic lens (usually an achromatic doublet) is a composite lens made with two types of glass with different dispersion, usually a Crown (low dispersion) and Flint (high dispersion), placed one after the other (usually in that order) to cancel out the chromatic aberration.
Based on the type of achromatic doublet lens used as the objective, the achromatic refractors can be classified as one of five types. We will keep the descriptions short as these are not the refractors most commonly used today. The descriptions are defined in terms of the radii of the lens surfaces as Rn where n ranges from 1 to 4, corresponding to the four surfaces of the doublet lenses (e.g. a bi-convex lens has two curved surfaces) in the order they are placed.
A Littrow doublet is composed of an equiconvex Crown with R1=R2, and a flint with R3=-R2 and a flat back. It can produce a ghost image (read below) between R2 and R3 because they have the same radii. It may also produce a ghost image between the flat R4 and rear of the telescope tube.
Ghost image is an image that is generally formed by the reflected component of a ray incident on a transparent surface. Remember the law of refraction from the post Fundamentals of Optics? When the reflected component strikes another surface like a mirror, an image is formed, called a ghost image because it is not of something that was part of the object under observation.
A Fraunhofer doublet uses a Crown with R1 greater than R2 and a Flint with R4 usually greater than R3. And, R2 is close, but not equal, to R3.
A Clark doublet uses an equiconvex Crown with R1 equal to R2, and a Flint with R3 close to R2 and R4 much greater than R3. R3 is slightly shorter than R2 to create a focus mismatch between R2 and R3, thereby reducing ghosting between the crown and flint.
An Oil-spaced doublet uses oil between the Crown and Flint to eliminate the effect of ghosting, particularly where R2 is equal R3. It can also increase light transmission slightly and reduce the impact of errors in R2 and R3.
A Steinheil doublet has a Flint as the first of the doublet. In contrast to the Fraunhofer doublet, it has a negative lens first followed by a positive lens. It needs stronger curvature than the Fraunhofer doublet.
An apochromatic refractors has apochromatic objective, built with special, extra-low dispersion materials. Apochromatic lenses are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane, as opposed to just two (red and blue) in achromatic lenses. The residual color error (tertiary spectrum) can be up to an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration. Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture and used most widely today.
Most of the problems with refracting telescopes are avoided or diminished in reflecting telescopes, which can be made in far larger apertures and which have all but replaced refractors for astronomical research. So, let’s study these “shiny” telescopes in the next post, “Reflecting Telescopes“.