ASTR 1210 (O'Connell) Study Guide


14. TELESCOPES


Summit of Mauna Kea, Hawaii



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 other types of "telescopes" to exploit a large part of the whole electromagnetic spectrum. Cosmic sources produce radiation across the entire range of this spectrum. Some telescopes for other spectral bands (e.g. the ultraviolet and near-infrared) are quite similar to optical telescopes. Others (e.g. for the radio and gamma-ray bands) use very different technologies.


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: in order to detect fainter objects. This is the most important function of telescopes.

  2. Resolve sources better: provide sharper images, permit seeing more detail. Resolution 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). The collecting area of each consists of 36 independent 36-in hexagonal mirrors. See image at right and this diagram.

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

The Large Binocular Telescope: two 330-in (8.4-m) diameter monolithic mirrors on a common mount, providing the largest existing collecting area. One of the mirrors is shown above. UVa is a partner in this project.

Large mirror technology

Other EM spectral bands


E. Next Generation Telescopes

Two very large telescopes based on the segmented-mirror concept of Keck have been designed: the Thirty Meter Telescope and the European Extremely Large Telescope (39-meters or 1550-in). The ELT is now under construction in Chile, but the TMT is embroiled in a dispute over environmental and cultural impacts at its preferred Mauna Kea site. The Giant Magellan Telescope (at right), also under construction in Chile, is a multiple-mirror design with 7 8.4-meter spin-cast mirrors and an equivalent collecting area of a single 22-meter mirror. The six off-axis segments are challenging to figure to the correct surface shape.

Another large telescope with a very different design and operations mode is the Large Synoptic Survey Telescope. To achieve a wide field of view (3.5o) it employs a unique 3-mirror design in which the primary and tertiary mirrors have been figured on a single piece of spin-cast glass 8.4-meters in diameter. The telescope is intended to repeatedly image the entire usable sky every three nights, searching for transient or moving targets (including near-Earth asteroids) while building up an ultra-deep combined image of the sky. Continuous output from its 189 imaging CCDs (see below) will generate an unprecedented data volume (15 TB/night). Under construction in Chile, LSST should be in operation by 2021.

The James Webb Space Telescope, expected to be launched in 2021, is the follow-on to HST. It features a 6.5-m diameter primary mirror (a 25 square meter collecting area, 5.5 times that of HST) composed of 18 hexagonal segments that must be deployed on-orbit. JWST carries four imaging and spectroscopic instruments and is optimized for the near-infrared spectral region. A large sunshield permits a low overall structural operating temperature through cooling to space. JWST will orbit around the Earth-Sun L2 point, about 900,000 km from Earth.


F. 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. Descriptions of the two most important kind of imaging detectors are given next:

Photographic Film

Charge-Coupled Device Architecture

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

CMOS solid state detectors, with characteristics similar to CCDs but less suitable for astronomical use, are employed in commercial still and video cameras and cell phones. These are mass-produced in quantities of billions.

Many other types of electronic detectors are now used in the UV, IR, and X-Ray bands of the astronomical EM spectrum.


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


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Last modified July 2018 by rwo

Text copyright © 1998-2018 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.