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) are very different.

A. Introduction and History

The telescope was invented in 1608 by Lipperhey in Holland.

Although lenses had been used in eyeglasses for several centuries, realization of higher precision optical devices suitable for viewing distant objects depended on improvements in lens-grinding technology. Details of the early work of Lipperhey, Galileo, and others are given here.

Note: the microscope was invented in 1654, also in Holland, and was responsible for opening up a second kind of "invisible world." Modern medicine would not exist without the microscope.

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.

• 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

• An objective or "primary" optical element forms an image (i.e. an accurate representation of the original scene) at a usable focus, where it can be studied by eye, recorded by film or other detectors (as in a camera), or fed into yet other instruments

The objective element can be either a lens or a mirror.

Types of telescopes

There are therefore two basic types of telescopes:

• Refracting telescopes: the objective is a lens (shaped, transparent glass) which refracts (or bends) incoming light rays to a common focus. The image at the left below shows how a flat glass surface bends light rays. In this case, two flat surfaces at a angle have been combined to make a prism. The shorter the wavelength of the light, the stronger the bending.

The image at the right below shows how a glass surface can be continuously curved to bring all the light rays passing through it from a given point on a distant object to a common focal point.

Galileo's telescopes were refracting but only about one inch in diameter. The largest refractor ever built was 40 inches in diameter (Yerkes Observatory, 1896).

 Refraction of Light By a Prism (click for a descriptive animation) Shaped Convex Lens

• Reflecting telescopes: the objective is a shaped mirror coated with a metallic film, which reflects light rays off its front surface to a common focus. See the picture below. Invented by Gregory (1663); improved by Newton (1669).

Many early reflecting telescopes employed metal mirrors, but almost all modern designs use glass mirrors.

At first sight, a reflecting design is counterintuitive because the focus is in a position where placing equipment would interfere with the incoming light beam. This is true, but the effects, both in terms of blocking the light and diminishing the image quality, are small if the mirror is large enough. Most reflecting telescopes use a "secondary" mirror to redirect the light beam outside the main body of the telescope. There are many different designs for reflecting systems, each with its own advantages and disadvantages.

The mirror is easy to support from behind, unlike a lens which must be supported from its edges and tends to sag. Reflectors have many other advantages. (Click here for more information on these.) All large telescopes are therefore reflectors. The largest reflector is 400-in (10-m) in diameter (built 1993); a telescope 1550-in (39.3-m) in diameter is now under construction at the European Southern Observatory.

Reflection of Light by a Figured Mirror
• Applet. Here is a Java applet illustrating the differences between refraction, reflection, and diffraction.

Focal plane

• For distant objects (including all astronomical objects), the incoming rays are parallel to one another. Such rays are focussed in a plane which is one focal length from the objective. This is called the "focal plane." Click on the button below for a Java applet illustrating image formation for objects at different distances.

• In the focal plane, the light rays from a distant object form a one-to-one representation of the distant scene which is called an image.

• Ordinarily, a camera or other instrument in placed at the focal plane. For "visual" use of a telescope, an eyepiece can be used to magnify the focal plane image so it can be viewed by the eye. See the illustration above.

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

• Light is a wave. In order to produce a good image, telescope optics must be figured to a minimum tolerance of about 1/4 of the wavelength (distance between crests) of the light they are intended to focus. For optical telescopes, this is about 10-5 cm.

• The intended shape of an optical surface, e.g. the curve in the convex lenses shown above, must be reproduced to high precision in order to obtain good image quality.

• Scale comparison: if a 320-in (8-m) diameter telescope mirror were scaled up to the size of the continental United States, i.e. about 3000 miles diameter, then the maximum ripple allowed in its polishing would be only about 2 inches!

Diffraction of light waves

Diffraction

• A fundamental limit on resolution is set by the physics of light. Since it is a wave phenomenon, light spreads out or diffracts when it passes through an aperture (like ocean waves around a breakwater). This smears out images.

Applet. Here is a nice interactive Java applet illustrating diffraction.

• Diffraction is worse the longer the wavelength of light and the smaller the telescope aperture. Click here for an illustration of how telescope size affects resolution.

• A 10-in diameter telescope with perfect optics can resolve 1 arc-sec. A 100-in diameter telescope could resolve 0.1 arc-sec.

• Note that almost all stars are so distant that they are smaller in angular size than 0.1 arc-sec and therefore appear as point sources in such a telescope. Only a handful of stars can be resolved by even the largest telescopes.

"Seeing" Produced by Earth's Atmosphere

"Seeing"

• The Earth's atmosphere also refracts light; and because it is constantly moving, there is always a blurring and jittering of images in a telescope. Astronomers call this "seeing." Seeing actually dominates diffraction in most cases and usually limits telescope resolution in practice to 0.5-2 arc-seconds.

• Above is an enlarged image of the bright star Betelgeuse seen though a large telescope. It is a large blob, broken up into smaller near point-like units. Click on the image for a video of the seeing effects:

• To partly overcome seeing effects, special equipment such as adaptive optics can be used. Or, telescopes can be placed in space (where there's no atmosphere), though this is much more costly.

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)

HST is not a large telescope by modern standards. But it has produced the highest resolution images yet obtained at visible wavelengths, with blur sizes of only about 0.05 arc-seconds. This is because of its high quality optics and the fact that it is outside the Earth's atmosphere, so it does not have to contend with seeing. Its high resolution and the absence of the natural and artificial atmospheric background also allows it to detect very faint sources.

Here is a composite version of some of HST's best images.

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

Here (from the Magellan Observatory) is a brief description of how the latest generation of huge telescope mirrors are manufactured:

The Magellan main mirrors are f/1.25 paraboloids and a radical departure from the nearly solid-glass mirrors of the past. Each is 21,000 pounds of borosilicate glass with a lightweight honeycomb structure inside. It took 6 months to build the mold for each mirror, 2 days to fill it with chunks of glass, 1 week to melt the glass and spin it into shape (in a specially designed rotating oven), and 3 months for the glass to cool. Each was then polished for 8 months while its surface was constantly tested for accuracy. Relative to their size, the main mirrors are about as thin as a dime. The aluminum surface of each mirror is a mere four-millionths of an inch (0.1 micron) thick. Each also sits in a "cell" that peforms two important functions. First, the cell's thermal control systems prevent warping from thermal expansion and contraction. Second, the support systems in the cells maintain the mirrors in their proper shape, so there is no distortion or cracking. The actual shape of the mirror surface is controlled to within two-millionths of an inch (0.05 microns).

Other EM spectral bands

The telescopes we've discussed so far operate only in the optical (or "visible") and adjacent spectral bands, but astronomers now exploit most of the full electromagnetic spectrum. The first instruments outside the visible range were radio telescopes (1950's). Now astronomers operate not only radio telescopes (e.g.the National Radio Astronomy Observatory, with headquarters in Charlottesville) but also microwave, infrared, ultraviolet, X-ray, and gamma-ray telescopes.

Because the Earth's atmosphere screens out many parts of the the EM spectrum (see Study Guide 10), telescopes for the gamma-ray, X-ray, ultraviolet, and parts of the infrared and microwave spectrum must be placed on spacecraft outside the atmosphere.

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

Photographic Film

• Film was the main astronomical detector used between 1900 and 1980.
• It detects only 1-2% of incident photons (not much better than the eye) but allows long integrations (hours)
• Requires chemical development of image after exposure, a serious complication
• Provides permanent storage of image information---a tremendous benefit. But information on film is not in digital form.
• Extends the observable EM wavelength range to regions (the near-UV and near-IR) where the eye is not sensitive
• Large formats are possible (up to 20-in square for astronomy)
• Nonlinear, nonuniform response makes quantitative measures of incident light energy difficult

Charge-Coupled Device Architecture

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

• CCD detectors are a type of solid state electronics, using a light-sensitive silicon wafer fabricated with embedded microelectronic integrated circuits by photolithography. .
• See image above. The CCD surface is composed of millions of independent, light-sensitive pixels.
• After exposure, pixel contents are shifted in 2 dimensions across the surface to an output amplifier and storage device.
• Astronomical applications were pioneered during development of the Hubble Space Telescope (1974-85).
• Work well at both very short (TV) and very long (astronomy) exposure times.
• 50-100x more sensitive than film
• Provides digital image storage for immediate computer processing
• Determination of incident light energy is easier
• Only small formats are available (2-in typical); but one can "mosaic" CCDs to create a large area detector surface
• CCDs are now the standard detectors used in optical-band astronomy.

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)

(Optional) Reading for this lecture:

Study Guide 14

Bennett textbook Chapter 6

Reading for next lecture:

Bennett textbook: pp. 203-204; Secs. 9.3, 9.5.

Study Guide 15