State two drawbacks of astronomical type of telescope.
Answers
It is easy to get the optics out of alignment.A reflector telescope's tube is open to the outside and the optics need frequent cleaning.Often a secondary mirror is used to redirect the light into a more convenient viewing spot. The secondary mirror and its supports can produce diffraction effects: bright objects have spikes (the ``christmas star effect'').Two famous reflector telescopes are shown below. The first picture is of the 5-meter (200-inch) Hale Telescope at Palomar Observatory. The number refers to the diameter of the objective (almost 17 feet across!). This was the largest telescope for several decades (from 1948 to 1976), though a larger telescope built in 1976 never lived up to its promise. Advances in ground-based telescopes did not really take off until the opening of the 10-meter Keck Telescope in 1993, so the Hale Telescope reigned supreme in astronomical research from 1948 to 1993, though the telescope is still very much in demand for world-class research. Clicking on the image will send you to a gallery of images of the telescope in another window. The telescope is the vertical piece in the middle with the mirror close to the floor. The huge diagonal piece is used to balance the telescope.
The second picture shows the path light travels in the 10-meter Keck Telescope at the W.M. Keck Observatory. The objective is composed of 36 hexagonal mirrors put together to act as one large mirror 10 meters across. Clicking on the image will give you more information about this telescope in another window. The small image next to it shows the 10-meter objective. The person in the red clothing at the center gives you a sense of scale. The Keck Observatory consists of two 10-meter telescopes placed next to each other on the top of Mauna Kea: Keck I opened in 1993 and Keck II opened in 1996. Lessons learned in building the Keck telescopes have been used to build several optical/infrared telescopes of similar or larger size since then. See the links at the bottom of the page for information about some truly huge ground-based telescopes in the works.
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