Telescopes

What is a Telescope?

A telescope is an instrument that collects light and produces an image on the retina in our eyes. While how they do this will be discussed soon there are three properties of telescope that I will briefly describe on this page.

The aperture of a telescope is the size of the opening at the front of the instrument. It is this opening that dictates how much light is collected by the telescope.

A telescope can be likened to a bucket left out in the rain. The wider the top of the bucket is, the more rain it will collect. A telescope is like a light bucket. The larger the aperture is, the more light it will collect. For a beginner telescope an aperture of 70mm (2.8”) to 200mm (8”) would be appropriate. However,  you may go larger if your budget allows.

The focal length of a telescope is the distance between the lens at the front of the telescope to the point where the primary image is formed. The focal length affects how much of the sky can be observed. For longer focal lengths, the field of view is smaller that shorter focal lengths. A longer focal length produces a more magnified, but dimmer,  image.

The focal ratio of a telescope is the ratio of the instrument’s focal length to its aperture. It is determined by dividing the telescope’s focal length by its aperture. For example, a 1000mm focal length and an aperture of 200mm results in a focal ratio of F/5. The ‘F/’ indicates that it is a focal ratio. Camera lenses use this same ratio.

The Eye

The human eye can be considered a telescope. The simplest and most affordable way to get started in astronomy is to view the sky from a comfortable deck chair. With the addition of a planisphere, you can observe constellations and even a galaxy or two, if you know where to look.

Like telescopes, the eye has an aperture. This aperture is variable and controlled by the pupil. The pupil can dilate from 1mm to 8mm in a young person. Unfortunately, as we age, the pupil becomes more limited in its ability to open. As a result, a 60-year-old’s pupil may only dilate to 4mm. With a wider aperture, a younger person can see dimmer objects.

As our pupil dilates, we lose acuity. In daylight, our resolution is about one arc minute. However, at night it reduces to about two arc minutes. The reason for this is related to the eye structure that we will discuss next.

The important parts of the eye are the lens and the retina. The lens is at the front of the eye. Its purpose is to focus light on the back of the eye. Covering the back of the eye is the retina. The retina is the structure where the photoreceptors are located. There are two types of photoreceptors which are called cones and rods.

We focus images on the fovea centralis (or fovea for short) for the highest visual acuity. The central part of the fovea contains only cones. Moving away from the center of the fovea, the concentration of cones and the concentration of rods decreases. As a result, the fovea does not help observe the night sky.

Moving away from the fovea, the concentration of rods continues to increase until about twenty degrees from the center of the fovea. Hopefully, you have recognized the importance of this information for night sky observations. Due to the higher concentration of rods at twenty degrees to where we usually focus our attention, we must look about twenty degrees away from what we are observing. This will focus the light on the area providing the brightest image. Of course, you will not be about to perceive any color.

The technique of looking slightly away from the object you observing is called the Averted vision. It is best practiced at a dark site.

Types of Telescopes

Before looking at the different types of telescopes, we need to understand some essential principles.

Telescopes perform the same function as the human eye: they collect light and focus it on a point or plane. The primary benefit of using a telescope is that they collect more light and can be used to magnify a small area of the sky.

As some concepts are very important, we are going to repeat some points contained in the previous section. Telescopes can be compared to buckets left out in the rain. You could refer to telescopes as ‘light buckets’. A bucket with a large opening will collect more water than one with a smaller one. In the same way, a telescope with a larger opening will collect more light than one with a smaller opening. With telescopes, this ‘opening’ is called the instrument’s aperture. If you have dabbled in photography, it is precisely the same principle.

Telescopes focus the light they collect to a point. The focal length is the distance between the telescope opening and the point where the light is focused. So far, we understand the telescope’s aperture and focal length.

The ratio of the telescope’s focal length to the aperture is called the focal ratio. It is a measure of how ‘fast’ the telescope is. A smaller ‘f number’ results in a brighter image. For example, if we have a telescope with a 110mm aperture and the focal length is 770mm, its focal ratio is f/7 (i.e., 770/110). In this instance, the focal length is seven times as large as the aperature. Telescopes with a focal ratio of below five are fast, and above eight are slow. In between, they are considered, well, in between.

Refractors

Refractors are the type of telescope that most new telescope geeks visualize when telescopes are mentioned. They are, in principle, simple instruments that are perfectly adequate for various astronomical viewing targets.

Refracting telescopes are the earliest type of telescope. Refractor telescopes are constructed from a convex objective lens at the front of a long tube. A smaller eyepiece is attached at the back of the tube (eyepieces are discussed later in this article). The lens in the eyepiece focuses an image onto the observer’s retina. Depending on the lens’s geometry, the produced image is either the correct way up or inverted.

The objective lens refracts (or bends) incoming light from parallel to converging. The converging beams of light can then to focused to a point. Original designs of reflectors used a single element for the objective lens. However, this simple design produced aberrations due to the nature of light. These aberrations were later mitigated using two and three-element designs for the objective lens.

Before we discuss the two types of refracting telescopes, we need to know about the two different geometries of glass lenses. Glass lenses can be either convex or concave. The lenses’ geometry dictates how the light is refracted (or bent).

A convex lens is thicker in the centre and narrows on the edges. This type of lens bends light to converge to a point (as such it is also referred to as a converging lens). This is the type of lens found on the front of a refractor.

A concave lens is narrower in the centre and thicker on the edges. This type of lens causes light to diverge, hence referred to as a diverging lens. To differentiate concave from convex I think of concave as representing a cave. Now that this is understood, we can look at Galilean-style refracting telescopes.

Galilean Refractor

While Galileo Galilei did not invent the telescope that now bears his name, he was the first to use them to make scientific observations of the night sky. Galileo made his first telescope in 1609 before improving its design.

Galilean refractors are constructed using a convex objective lens ( i.e. lens to the front of a tube). A concave eyepiece lens is placed between the objective lens and the focal point to produce an image. This configuration results in an upright image. However, it produces a small field of view which is only useful at low magnification. Today this lens configuration is only used in Opera Glasses.

Keplerian Refractor

Keplerian refractors are also known as astronomical telescopes and are what are used today.

Like Galilean refractors, these telescopes use a convex objective lens. Where they differ is that a convex lens is used in the eyepiece. Also, the eyepiece is placed beyond the focal point. In this configuration, an intermediate image is produced at the focal point. The eyepiece forms an enlarged image of the intermediate image.

These telescopes have a major drawback. When light is refracted through a glass lens, all the light is not refracted by the same amount. Blue light is refracted more than red light. This results in irritating color fringing around bright objects when viewed through the telescope. In technical terms, this is called chromatic aberration.

Chromatic aberration can be mitigated using more than one lens in the objective lens. Initially, two lenses were used. In this design, the two lenses created different refractive indices (ie bend light in different ways) and arranged in such a way that the chromatic aberrations from both cancelled each other. This two-lens arrangement is called an achromatic doublet. Later objective lenses containing three lenses were constructed to improve the design further and almost totally remove the color aberrations. The three lens arrangements are called achromatic triplets. These designs use different glass types and coatings.

Using a long focal length is an alternative method to combat chromatic aberration. This not only creates long telescopes but also produces dim images.

Well-corrected refractors with large apertures are great instruments but are costly and physically large.

Reflectors

Instead of a glass lens as an objective lens reflectors use a concave mirror. A concave mirror produces converging rays of light. An advantage of reflectors is that they do not produce chromatic aberration. They typically cost less than corresponding refractors with the same aperture. However, reflector design reduces the effective aperture via obstructions. The obstructions also reduces contrast.

To further explain the effect of obstructions within these telescopes, consider a Newtonian telescope with a 200mm (8”) aperture and a 50mm secondary mirror. This configuration would result in an effective aperture of 193mm. That’s about a seven percent reduction in light reaching the primary mirror. Also, the contrast would be similar to an unobstructed telescope with a 150mm aperture.

Newtonian

A Newtonian telescope is a simple design. They employ a concave mirror at the end of an open tube. The light is reflected back towards their source. Near the open end of the telescope, a smaller flat mirror (secondary mirror) placed at 45 degrees to the light reflects it at 90 degrees to the incident rays, exiting the side of the telescope tube. The light is then passed through an eyepiece. Focussing is achieved by a focuser located between the eyepiece and the telescope tube.

The use of a spherical primary mirror produces elongation of stars (this is known as coma) and chromatic aberrations towards the outside edges. To mitigate this phenomenon, parabolic mirrors can be used. These lenses cost more to produce, so increase the telescope's price. Optical devices call field flatteners can be used to improve the defect further.

Catadioptric Telescopes

One technique to remove aberrations caused in the classic Newtonian telescopes is to place a lens at the front of the telescope. These compound telescopes are called catadioptric telescopes. The designs provide physically smaller and lighter instruments that are easier to transport. However, the lenses are expensive to produce.