Reflecting telescopes.pptx

reflecting telescopes

1. Reflecting telescopes

2. A reflecting telescope is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image . The reflecting telescope was invented in the 17th century by Isaac Newton as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration . DEFINITION

3. Almost all of the major telescopes used in astronomy research are reflectors . Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. VARIATIONS

4. HISTORY In late 1668 Isaac Newton built his first reflecting telescope . He chose an alloy of tin and copper as the most suitable material for his objective mirror. He chose a spherical shape for his mirror instead of a parabola to simplify construction. He found that the telescope worked without colour distortion and that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus with it.

5. DESIGNS The Gregorian telescope , described by Scottish astronomer and mathematician James Gregory in his 1663 book Optica Promota. This one produces an upright image, useful for terrestrial observations.

6. DESIGNS The Newtonian telescope is one of the simplest and least expensive designs for a given size of primary, and is popular with amateur telescope makers as a home-build project.

7. DESIGNS The cassegrain telescope was first published in a 1672 design attributed to Laurent Cassegrain . It has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. The folding and diverging effect of the secondary mirror creates a telescope with a long focal length while having a short tube length.

8. Technical considerations A curved primary mirror is the reflector telescope's basic optical element that creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length . Film or a digital sensor may be located here to record the image, or a secondary mirror may be added to modify the optical characteristics and/or redirect the light to film, digital sensors, or an eyepiece for visual observation.

9. The primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to a spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective first surface mirror . Technical considerations

10. Some telescopes use primary mirrors which are made differently. Molten glass is rotated to make its surface paraboloidal, and is kept rotating while it cools and solidifies. The resulting mirror shape approximates a desired paraboloid shape that requires minimal grinding and polishing to reach the exact figure needed. Technical considerations

11. Use in astronomical research Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements. Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this: Light of different wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration . Reducing this to acceptable levels usually involves a combination of two or three aperture sized lenses The cost of such systems therefore scales significantly with aperture size. An image obtained from a mirror does not suffer from chromatic aberration to begin with, and the cost of the mirror scales much more modestly with its size.

12. Reflector vs. Refractor telescopes Pros and cons of each Reflector:

13. REFractor: