1.0 The First Telescopes

Though there were many different variations of magnifying devices throughout history, two groundbreaking optic telescopes stood out; The Keplerian telescope, and Galilean Telescope. These telescopes, though based on previous devices, were more successful and gained more respect in the scientific world compared to their counterparts. These telescopes use a set of two lenses and a tube in which the light travels toward the eye.

A modern telescope

1.1 Galileo's telescope

Ray Diagram of the Galilean telescope showing a virtual upright image

Galileo's first telescope was created in 1609 based on other of magnifying devices in appearing in Europe. His telescope used both converging and diverging lenses (converging for objective, diverging as eyepeice). The eyepeice is placed before the focal point of the objective but it is situatied so that it's own focal length is in the same place as the focal length of the objective. Because of the lenses and their distances from one another (relating to the focal lengths) the image which they result in is erect, virtual, and magnified. Athough Galileo's telescope was a great step in telescopic devices, the available magnification is limited and can only reach about a power of 30 which makes for a very narrow feild of view, making this telescope more resonable for terrestrial use rather than astronomical.


1.2 Kepler's Telescope

Although it creates an inverted image, the Keplerian Telescope is more effective than the Galilean for astronomical purposes. This telescope was a rendition of the original galilean telescope made by Johannes Kepler in 1611. Instead of using diverging and converging lenses, the Keplerian uses converging lenses for both the objective and eyepiece. The original image goes through the objective and creates a smaller image within the tube which is then magnified into the eye through the eyepiece to create an inverted image for the viewer. Because the distance of the lenses from one another does not directly relate to the focal lengths, the Keplerian telescope allows for more magnification than the Galilean and with more room for adjustment, this telescope could be used to view a wide range of distances with relative clarity, while the galilean was more suitable for closer objects. Click Here for an interactive diagram.

1.3 Today's Telescopes

Today, telescopes are everywhere, and not only are they used by scientists, but they are also commonly used for recreational activities such as star gazing. Although both galileo's and kepler's telescopes were both utilized refracting, there has been many different adjustments and forms of telescopes that do not involve this technique. Although there are many, the most common types of telescopes used today are both refracting and reflective. The difference is; while refracting telescopes use lenses to refract the image, reflecting telescopes use various mirrors to reflect the image into a suitable size for viewing.

1.4 Today's Refractors

Refracting telescopes are commonly used as refile scopes as seen above

Today refracting telescopes are used anywhere from toys to rifle scopes,
they are rugged, easy to focus and result in clear, sharp images. Refractors have two lenses at each end of a sealed tube and depending on their usage, obtain different lens types or tube lengths. Although refractors are very useful in everyday life, there are some downfalls in their design. Chromatic Aberration is an effect that is always present in refracting telescopes and it Chromatic Aberration results in a rainbow of colours around the image but can be reduced by having a longer focal length in the telescope, this in fact is the reason that historical telescopes (like the kepler and Galilean were so long). Today, many telescopes use another technique to reduce this effect, and that is by utilizing multiple lenses within the telescope to adjust the refracting index of each colour. Another disadvantage is that the glass lenses are in face liquid meaning that since they are held only by their edges, over time they will sag and distort the image.

1.5 Reflective Telescopes

One form of a refractive telescope

Although reflector telescopes may give similar images as refractors, the method in which they
magnify is very different. When the light enters the chamber, it goes straight to the rear and is reflected off of a parabolic mirror which centralized the image into one point and then is reflected up by a flat mirror, back through an opening in the primary mirror by a convex mirror, or just travels back to the original point in a smaller more condensed form. SInce the light does notrefract at all, the problem of Chromatic abbreviation is eliminated in reflective telescopes because the light does not change speed. Also since the primary mirror is at the rear of the chamber, it can be supported more than a lens would, allowing people to build them bigger with less fear of sagging. However with every advantage comes a disadvantage. First of all, these telescope are very difficult to focus and often come out of alignment. Also, since the telescope s not fully sealed, the mirror(s) may have to be frequently cleaned and looked after. And finally, the secondary mirror that may be used to reflect back to the viewer, may get in the way of the image and cause frustration. Although there may be some minor issues with reflective telescopes, overall, they are more technologically advanced and effective for creating images from afar than refracting telescopes.

The rim of indigo around this bird is an example of what Chromatic Aberration can do to images

1.6 Chromatic Aberration

Chromatic Aberration occurs when light is refracted through a lens and as the light refracts, some of the colours travel at different speeds. This causes the lens to act as if it was a prism andseparate some of the colours so they create what seems to be a 'rainbow halo' around the image. Chromatic Aberration is seen in all Refracting telescopes but can be diminished by adding more lenses to the telescope, or by increasing the focal length, and therefore the entire telescope as well.

1.7 References