Question

11.) In a reflecting telescope, the light from the astronomical object is collected and focused by the most important component which is

   a.) mirror

   b.) lens

   c.) collimator

   d.) camera holder

Answer__________

12.) Telescopes in low earth orbit have the advantage of

   a.) no atmospheric disturbances

   b.) visibility at all wavelengths

   c.) no scattering of city lights in the images

   d.) all of the above

Answer 1

. It usually has a paraboloid primary mirror but at focal ratios of f/8 or longer a spherical primary mirror can be sufficient for high visual resolution. A flat secondary mirror reflects the light to a focal plane at the side of the top of the telescope tube. It is one of the simplest and least expensive designs for a given size of primary, and is popular with amateur telescope makersas a home-build project.

• Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or in a catadioptric telescope.
• In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
• 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 (see achromat and apochromat for more details). 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.
• There are structural problems involved in manufacturing and manipulating large-aperture lenses. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter.In contrast, a mirror can be supported by the whole side opposite its reflecting face, allowing for reflecting telescope designs that can overcome gravitational sag. The largest reflector designs currently exceed 10 meters in diameter.

The advantage of using a telescope in space is that you don't have to look through the Earth's atmosphere. For very detailed observations the atmosphere is pretty murky and horrible so it's a real advantage to get above that. You've probably seen HST pictures, and they really are much more detailed than you can get from the ground.

For other wavelenghts there is no choice as our atmosphere can block them completely (eg. Far infra-red and X-rays and Gamma-rays). Telescopes for these have to be in space. For most radio wavelengths the atmosphere is very little problem, so instruments like Arecibo and the VLA are not limited by the atmosphere at all.