ASTR 1210, O'CONNELL. Study Guide 14 [Spring 2013]

ASTR 1210 (O'Connell) Study Guide

14. TELESCOPES

Summit of Mauna Kea, Hawaii

Spring 2013: 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.

lens-grinding technology

here

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

Collect more light: 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.
Resolve sources better: provide sharper images, permit seeing more detail. Rresolution depends on both the diameter of the telescope and its optical quality
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 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 combine to make a prism). The shorter the wavelength, the stronger the bending.
The image at the right 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. The largest refractor ever built was 40-inches in diameter (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 reflecting material, which reflects rays off its front surface to a common focus. See picture below. Invented by Gregory (1663); improved by Newton (1669).
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" (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.
Diffraction is worse the longer the wavelength of light and the smaller the telescope aperture. Click here for an illustration of how telescope size improves 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 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).

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)

highest resolution images yet obtained

very faint

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

full electromagnetic spectrum

National Radio Astronomy Observatory

Study Guide 10

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 secs, and it has low sensitivity. Astronomers have long sought more capable detectors to use with telescopes.

Film

Film was the main astronomical detector used between 1900 and 1980.
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
Provides permanent storage of info, though not digital
Extends the observable EM wavelength range to regions where the eye is not sensitive
Large formats (up to 20-in square for astronomy)
Nonlinear, nonuniform response makes determination of incident light energy difficult

Charge-Coupled Device Architecture

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

CCD detectors are a type of solid state electronics; widely used in still & video cameras.
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).
Works 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 easier
Only small formats available (2-in typical); but one can "mosaic" CCDs to create large area detector surface
Now are the standard detectors used in astronomy