Optical Telescope

Writer：55 Cherry

When it comes to astronomical instruments, what would you think of immediately? Telescopes? You might have heard of them for many times. But do you know that indeed they can be classified by their various characteristics? Telescopes aid the observations of distant objects by collecting electromagnetic radiation¹ such as infrared, visible light and ultraviolet. To categorize them, therefore, we can make use of the wavelength of the electromagnetic radiation they are sensitive to. For example, optical telescopes are telescopes that gather and focus mainly visible light to form brighter and finer images of the objects by the principle of optics². Optics employs one or more curved optical elements, which are usually made of glass lenses and/or mirrors.

Main Types

There are three main types of optical telescopes: refractors or refracting telescopes which use lenses as objectives (dioptrics), reflectors or reflecting telescopes which use mirrors as objectives (catoptrics) and Schmidt which use mirrors as objectives but also use correcting lenses to correct for some problems that observers might encounter during their observation (catadioptrics). All three types of telescopes adopt more or less the same principle of optics. Light goes in telescopes as parallel rays. Then, it converges, crosses and diverges. It is finally refracted by an eyepiece back into a narrower bundle of parallel rays before entering the eye. This is because if the light rays are too strongly divergent, the eye cannot focus. Also, if the light rays entering the eye are parallel, the eye is most relaxed and the amount of light collected by the eye is maximized. So, to collimate the light into parallel rays, the eyepiece should be placed at where its focal point³ coincides with that of the objective.

The above picture shows a Schmidt telescope.

Main Functions

It is time to talk about the three main functions of telescopes. First, telescopes are light collectors. The telescopes when compared to the eye can collect light hundreds or thousands of times more. Second, they provide us with an angular magnification of the images. However, they do not sharpen the features of the images. Third, they give higher resolution of the images. But it should be aware that higher magnification is totally different from higher resolution. Images look larger but not finer after being magnified. Conversely, more details from the object can be received by higher resolution even though the telescope is under the same magnification.

The above photos of Jupiter illustrate the difference between low (left) and high (right) resolution under the same magnification.

The functions of collecting light and higher resolution are directly related to the (aperture) diameter of the primary objectives of the telescopes. The larger the objectives, the more light the telescopes can collect and the finer details they can resolve. One extra piece of information to note is that the sizes of telescopes refer to the diameter of their primary objectives.

The upper left and upper right pictures are the world's largest refractor and reflector respectively.

Problems

Though telescopes sound so powerful, there are still a number of problems in the images formed by them. They are called optical aberrations. There are altogether six different kinds of optical aberrations, namely field curvature, spherical aberration, coma, astigmatism, image distortion and chromatic aberration.

Field Curvature

Light rays striking the lens at increasingly larger angles to the principal axis⁴ focus in a curved surface instead of a plane. A flat object normal to the optical axis cannot be brought into focus on a flat image. Currently, many photographic lenses are designed to have a focal length that increases with the ray angle so as to minimize field curvature.

Spherical Aberration

Light rays striking near the edge of a spherical lens suffer greater refraction than those striking near its principal axis⁴. These rays then focus at a point closer to the lens along the principal axis⁴.

Spherical aberration can be reduced or avoided by making the surface of the lens flatter such that light rays which strike closer to the edge of the lens and those which strike closer to its principal axis⁴ suffer similar refraction. The shape of lens can be parabola, which completely eliminates spherical aberration.

Coma

It is also called comatic aberration. Light rays striking at an angle to and at different distances from the principal axis⁴ focus at different points along and at different heights above the principal axis⁴. The resulting image is elongated which looks similar to the coma of a comet. It occurs even for parabolic surfaces. Comatic aberration becomes even worse for light rays striking at increasingly larger angles to the principal axis⁴ and near the edge of a spherical lens.

To reduce coma, the lens can be machined to an aspheric lens, which is not a shape resembling any conic section. Besides, this shape can also eliminate spherical aberration.

Changing the shape of the mirror to a hyperbola, which does not suffer from spherical aberration, can reduce coma. Nowadays, all research-class reflecting telescopes employ hyperbolic mirrors, both primary and secondary, known as the Ritchey–Chrétien design.

Astigmatism

For incident light rays not along a single plane but at a given angle to the principal axis⁴, the rays are foreshortened differently and do not present symmetrically. Light in different planes focus at different positions. If there is no common focus, we would choose the circle of least confusion, where the image projection is circular. Lenses that have different radii of curvature in different planes can often correct astigmatism.

Image Distortion

Distortion is a deviation from ordinary projection, which straight lines remain straight in an image. It arises from a difference in image scale across a field of view. There are two different types of distortion: barrel distortion and pincushion distortion. In barrel distortion, image scale decreases at the edge of the field. The visible effect is that the center of the image appears to bulge outward. In pincushion distortion, the image scale increases at the edge of the field. The apparent effect is that the corners of an image appear to bend outward.

Chromatic Aberration

Chromatic aberration is also called achromatism or chromatic distortion. Light rays striking from the same direction but of different wavelengths focus at different positions. This is because a lens has different refractive index for different wavelengths of light.

To reduce or avoid chromatic aberration, an achromatic lens (objective and eyepiece) comprising two individual lenses with different dependences of refractive indices with wavelengths can be used. It brings two wavelengths (typically red and blue) into focus in the same plane. Besides, an apochromatic design, which usually makes use of different dispersive properties to obtain crossings of different colours, can also bring different wavelengths to a common focus on the same plane.

Are you interested in optical telescopes after reading all these? Do grasp the chance of using an optical telescope if you are given so in the future!!!

1: Electromagnetic radiation is the flow of energy at the universal speed of light through free space or through a material medium in the form of the electric and magnetic fields that make up electromagnetic waves such as radio waves, visible light, and gamma rays.

2: Optics is the branch of physics that involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it.

3: A focal point is a point where parallel light rays originating from a point on the object converge and focus.

4: A principal axis is an axis that passes through the center of curvature of each surface of the lenses and/or mirrors of an optical system.