ASTR 1210 (O'Connell) Study Guide


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

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).

Galileo's lenses were only about one inch in diameter and of relatively poor quality. But telescope technology had advanced over the subsequent 300 years to the point where the largest operating telescope in the year 1900 was the Yerkes Observatory 40-inch telescope shown at the right.


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

    • Light gathering power depends on the telescope diameter2

    • Thus, a 10-in diameter telescope collects (10/5)2 = 22 = 4 times as much light as a 5-in telescope.

    • An 8-in telescope (widely used by amateur astronomers) collects 1600x more light than the human eye. Because there are many more faint stars than bright ones, an 8-in scope can detect over 2000x as many stars (10 million compared to 5000) as the unaided eye.

  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


"Seeing" Produced by Earth's Atmosphere


Sky Background

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 diameter reflector in orbit around the Earth (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.

The James Webb Space Telescope, was launched on Christmas Day in 2021.

See this graphical comparison of the collecting areas of the largest existing and planned telescopes.

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 innovative telescope with a very different design and operations mode is the Large Synoptic Survey Telescope of the Vera C. Rubin Observatory. 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 2024.

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, are employed in commercial still and video cameras and cell phones. These are mass-produced in quantities of billions. They are significantly less well suited to astronomical use (being harder to calibrate, for example), but astronomical applications are now being explored.

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 June 2023 by rwo

Text copyright © 1998-2023 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.