Refracting Telescopes
Dutch spectacle makers, Hans Lippershey and Sacharias Jansen, built the first telescope in 1608. In 1609, Galileo learnt of their invention and adjusted it in order to study the sky.
The Galilean telescope used a concave eyepiece that adjusted the magnifying power and a convex objective lens that adjusted the size of the image. The concave lens flipped the image to upright, which made it useful to observe objects on Earth, as the Dutch opticians did. However, the shape and the narrow field of view of the lens meant that Galileo's telescope produced blurry and distorted images.
The Galilean telescope, similar to a basic refracting telescope, has two major problems in its design. To be able to see a celestial object a long way away, enough light needs to be focused on the image. The larger the diameter of the objective lens, the brighter the image is. To allow enough light to focus on an image, the lens of a refracting telescope needs to be lengthened. As a result, the thickness of the lens must also be increased to keep its curvature. This creates distortion as the light is refracted when entering and exiting the lens. Another flaw in a refracting telescope is that different colours focus at different points. This is called chromatic aberration and it can be fixed with a glass lens but not completely erased.
In 1611, Johannes Kepler invented the Keplerian telescope. It used two convex lenses and was a lot longer than the Galilean telescope. The image was inverted but when exploring space this does not matter. It also meant that the telescope could include more in one frame. It had a fixed focal length of about one metre and could be fitted with eyepieces of various magnifications to view objects of different brightnesses.
The Galilean telescope used a concave eyepiece that adjusted the magnifying power and a convex objective lens that adjusted the size of the image. The concave lens flipped the image to upright, which made it useful to observe objects on Earth, as the Dutch opticians did. However, the shape and the narrow field of view of the lens meant that Galileo's telescope produced blurry and distorted images.
The Galilean telescope, similar to a basic refracting telescope, has two major problems in its design. To be able to see a celestial object a long way away, enough light needs to be focused on the image. The larger the diameter of the objective lens, the brighter the image is. To allow enough light to focus on an image, the lens of a refracting telescope needs to be lengthened. As a result, the thickness of the lens must also be increased to keep its curvature. This creates distortion as the light is refracted when entering and exiting the lens. Another flaw in a refracting telescope is that different colours focus at different points. This is called chromatic aberration and it can be fixed with a glass lens but not completely erased.
In 1611, Johannes Kepler invented the Keplerian telescope. It used two convex lenses and was a lot longer than the Galilean telescope. The image was inverted but when exploring space this does not matter. It also meant that the telescope could include more in one frame. It had a fixed focal length of about one metre and could be fitted with eyepieces of various magnifications to view objects of different brightnesses.
Reflecting TeleScopes
In the reflecting telescope, the objective lens is replaced with a concave mirror. It is referred to as the objective or primary mirror and made of a silver or aluminium coated surface. Light is reflected by the mirror and cannot be influenced by a faulty lens or chromatic aberration. The mirror can be placed outside of the telescope, allowing for a large mirror with a better light-gathering power. The secondary mirror receives light from the primary mirror and is designed to reflect it through the eyepiece.
The Gregorian telescope was the first to use the reflecting technique. It was designed by Scottish astronomer, James Gregory in 1663, and built by Robert Hook in 1673. This telescope contains two concave mirrors. The primary mirror collects light and focuses it onto the secondary mirror, which reflects it back through an eyepiece in the middle of the primary mirror. This halves the length of the telescope, making it portable and easy to control when on a tripod. The Cassegrain mirror, designed in 1672, was very similar to the Gregorian telescope, however, its secondary mirror was convex.
In 1668, Sir Isaac Newton changed the angle of the secondary mirror to 45 degrees. In other reflecting telescopes, it was difficult to view an image, without blocking out the light. However, Newton's secondary mirror angled the light's path up, through an eyepiece at the top of the telescope.
The Gregorian telescope was the first to use the reflecting technique. It was designed by Scottish astronomer, James Gregory in 1663, and built by Robert Hook in 1673. This telescope contains two concave mirrors. The primary mirror collects light and focuses it onto the secondary mirror, which reflects it back through an eyepiece in the middle of the primary mirror. This halves the length of the telescope, making it portable and easy to control when on a tripod. The Cassegrain mirror, designed in 1672, was very similar to the Gregorian telescope, however, its secondary mirror was convex.
In 1668, Sir Isaac Newton changed the angle of the secondary mirror to 45 degrees. In other reflecting telescopes, it was difficult to view an image, without blocking out the light. However, Newton's secondary mirror angled the light's path up, through an eyepiece at the top of the telescope.
The Hubble Space Telescope
The Hubble Space Telescope (HST) was activated in 1990 by NASA and ESA. It was created to provide a view of space, unobstructed by the Earth's atmosphere. It uses visible, infrared and ultraviolet light in its findings and has a 2.4 metre mirror. The original images it recorded were unexpectedly disappointing, however, it was discovered that there was a problem with the main mirror. In 1993, the problem was fixed with a set of extra mirrors, which corrected the error in the primary mirror.
To reduce the problems caused by the Earth's atmosphere, modern telescopes are built with adaptive optics. This means that the shape of the mirrors can be adjusted to suit the atmospheric conditions, for example the temperature.
To reduce the problems caused by the Earth's atmosphere, modern telescopes are built with adaptive optics. This means that the shape of the mirrors can be adjusted to suit the atmospheric conditions, for example the temperature.