ASTR 1210 (O'Connell) Study Guide


14. TELESCOPES


Summit of Mauna Kea, Hawaii


Spring 2014: this guide is optional reading only


The telescope is the single most important invention for astronomy. It is a beautiful example of the interplay between technology (fabrication of quality glass, optics design, polishing techniques, large mechanical structures, computers) and basic science.

This lecture describes the main features of optical-band telescopes---i.e. those which operate in or near the part of the EM spectrum to which our eyes are sensitive. This is the only kind of telescope which was in widespread use before 1950.

Since that time, astronomers have developed "telescopes" to exploit a large part of the whole electromagnetic spectrum. Some of those (e.g. for the ultraviolet and near-infrared) are quite similar to optical-band telescopes. Others (e.g. for radio and gamma-ray) are very different.


A. Introduction and History

The telescope was invented in 1608 by Lipperhey in Holland.

The first astronomical use of a telescope was by (Galileo, in Italy in 1609. The telescope instantly and utterly transformed astronomy (see Study Guide 7).

Purposes

  1. Collect more light: detect fainter objects. This is the most important function of telescopes.

  2. Resolve sources better: provide sharper images, permit seeing more detail. Rresolution depends on both the diameter of the telescope and its optical quality

  3. Magnify sources: make the images of distance objects larger for easier study


B. Designs

Basic principle

Types of telescopes

There are therefore two basic types of telescopes:


Focal plane


C. Image Quality

The crispness of images made by a telescope depends on several factors: fabrication of the optics, the size of the telescope compared to the wavelength of light, and the Earth's atmosphere.

The "resolution" of a telescope image is quantitatively defined to be the smallest measurable detail in an image (in seconds of arc).

Optical Figuring

Diffraction of light waves

Diffraction

"Seeing" Produced by Earth's Atmosphere

"Seeing"


LBT 8.4-m Mirror Blank

Mirror blank for one of the two mirrors of the Large Binocular Telescope.
Click for enlargement.

D. Current Telescope Milestones

The Hubble Space Telescope: 94-in reflector in space (launched 1990)

Keck Observatory: Two 400-in "segmented mirror" telescopes (1993, Hawaii). Mirrors consist of 36-in independent hexagonal mirrors. See image at right and diagram.

The Very Large Telescope (VLT): Four 320-in monolithic mirror telescopes (2001, Chile)

The Large Binocular Telescope: two 8.4-m diameter monolithic mirrors on a common mount, now nearing completion. One of the mirrors is shown above. UVa is a partner in this project.

Other EM spectral bands


E. Detectors

The human eye is a sophisticated, auto-focus, auto-exposure, electrical camera system. However, for all its versatility and importance to us in everyday life, it is a seriously limited astronomical detector: it is small, its maximum integration time is only about 0.1 sec, and it has low sensitivity. Astronomers have long sought more capable detectors to use with telescopes.

Film

Charge-Coupled Device Architecture

"Charge-Coupled-Devices" (CCD's)

Many other types of electronic detectors based on similar concepts are used in the UV, IR, and X-Ray bands of the EM spectrum



Sunset over the William Herschel Telescope (La Palma, Spain; N. Szymanek)


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Last modified March 2014 by rwo

Text copyright © 1998-2014 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.