How Telescopes Work – Refracting Telescopes
We find out about the Universe by detecting and analyzing the electromagnetic radiation emitted from celestial bodies. Translation – we look up and see what is there. Our eyes alone and unassisted are not sufficient to accomplish this task. As a result, we use a wide range of astronomical instruments to provide the information we cannot otherwise obtain.
Telescopes provide an observer with two advantages over unaided vision – magnification and extra light. Simply put, objects look larger and brighter when observed using a telescope.
The average unaided human eye can see a light bulb which emits one watt of visible light at a distance of about 40 kilometers. At that same distance, the same person can see a pair of such lights as separated from each other if the two lights are placed about 10 meters apart. If this person now uses a 6 inch diameter telescope (not an unusual amateur telescope) to aid their sight, the light bulbs could be seen and distinguished at a distance of 1000 kilometers.
Although there are various types of ancient astronomical instruments, the story of telescopes begins with the invention of the refracting telescope.
Galilean Refractor
Contrary to popular belief, Galileo did not invent the first refracting telescope. Instead, he actually improved on an earlier design by Hans Lippershey, who was a Dutch optician. He invented the ‘kijker’ (‘looker’ in Dutch) in 1608, and applied for a patent from the Dutch Government. It is even possible that one Giovanpattista della Porta of Naples invented a telescopic device in 1589, but no direct record of his device has survived.
These earliest telescopes were of very low quality and magnifying power, as well as narrow field of view. These limitations stemmed in part from the poor quality of the available glasses and lenses, and in part from the design of what is now known as the Galilean refractor.
The Galilean refractor consists of two lenses, a positive objective lens which focuses light to a point, thereby forming an image, and a negative eyepiece lens, which serves to magnify the image.
Parallel rays of light from a distant object are brought to a focus in the focal plane of the positive objective lens. However, the negative eyepiece lens intercepts these rays and renders them parallel once more, so that they appear to be coming from a distant object, but the rays are traveling at a larger angle to the optical axis. This increased angle leads to an increase in the apparent size of the object, so that the image is magnified.
The magnification produced by a Galilean refractor is not very large – most telescopes of this type have a magnification of 2-6 times, and Galileo’s most powerful telescope had a magnification of about 30. Still, the Galilean refractor proved sufficient to discover the craters on the Moon, the moons of Jupiter, the rings of Saturn, the disk of Mars, and the phases of Venus. No matter who actually invented the telescope, it is not disputed that Galileo turned it into an instrument for the discovery of wonders in space.
Keplerian Refractor
In rather short order, a vastly improved version of the refracting telescope was developed. In 1611, only three years after the Galilean refractor was invented, the general optical arrangement used in modern refractors was introduced by Johannes Kepler.
As in the Galilean refractor, a positive objective lens forms an image of a distant object. In the Keplerian telescope, however, this image is enlarged by an eyepiece, which is essentially a magnifying glass.
More precisely, parallel rays of light from a distant object are brought to a focus by the positive objective lens and then diverge as they approach the eyepiece lens. The positive eyepiece renders the rays parallel, but traveling at a larger angle to the optical axis. As in the Galilean refractor, the virtual image is thereby magnified with respect to the object, and is located at infinity, but is inverted.
This combination of optics produces higher quality images, larger magnification, and larger fields of view than does the Galilean refractor, which accounts for the near abandonment of that optical arrangement for observation of the heavens.
Optical Aberrations
The Keplerian refractor far outperforms the Galilean telescope. However, the two classes of refractors share a number of intrinsic optical defects, called aberrations. The primary optical aberrations affecting the performance of a refracting telescope are:
1. Spherical aberration
2. Chromatic aberration
3. Coma
4. Astigmatism
Let’s take a quick look at each of these, and how they affect the image produced by a refracting telescope.
1. Spherical aberration:
It is much easier to produce spherical surfaces, so real lenses almost always have such surfaces. A lens with a spherical surface focuses the incident light along its axis, so that at any given position, the image is a broad disk rather than a sharp point. This effect is called spherical aberration.
2. Chromatic aberration
A property of all optical glasses is that they have different refracting powers for different colors of light. As a result, different colors of light will focus at different distances from the lens, resulting in a blurring effect similar to that of spherical aberration, but in this case the blurred image of a point of light (star) will show the colors of the rainbow. This is called chromatic aberration.
3. Coma
Thus far we have described optical aberrations which occur for on-axis images, that is, when the telescope is pointed directly at the object being observed. However, the field of view also includes off-axis images, and additional aberrations appear for off-axis images.
Coma is one such off-axis aberration. Light passing through the center of the lens does not focus at the same distance from the lens as does light passing through the edges of the lens. As a result, the image is smeared out toward the edge of the field of view. This aberration is called coma because the smeared images appear like little comets.
4. Astigmatism
Astigmatism is an aberration which is a bit harder to draw on a piece of paper. Light from an off-axis object that passes through the vertical axis of the lens comes to a focus at a different distance from the lens than does light from the same object that passes through the horizontal axis of the lens. Accordingly, a star does not come to a sharp focus, but rather forms an image that changes from a vertical line to a horizontal line as the position of an eyepiece is changed. In between these limits a fuzzy image is formed which is called the circle of least confusion. This is essentially the flawed image that looks the most like the actual object.
