ASTR 130 (O'Connell) Lecture Notes
8. ASTRONOMICAL INSTRUMENTS & SKY SURVEYS
Bubble Nebula; image from the Hubble Space Telescope
A. INTRODUCTION
Astronomers have developed a wide array of ingenious instruments for
attaching to telescopes in order to make measurements of the sky.
Historically, the human eye was employed exclusively as the
detector. Modern instruments, however, almost exclusively
use other kinds of detectors. The kind of instruments used depend on
the particular band of the EM spectrum for which they are designed.
Entirely different technologies are used in the radio region, for
example, than in the optical band. Here, we discuss only the two most
powerful types of optical band equipment: imagers (cameras) and
spectrographs.
B. IMAGING
PHOTOGRAPHY
- Although the human eye is a marvelously sensitive and adaptable
instrument, even poor quality film easily outperforms
it for astronomy. The first astronomical photographs were made late
in the 19th century. Photography offered two revolutionary
capabilities: first, it provided permanent records of
observations. Second, it permitted very long exposure times
and hence the detection of faint objects far beyond the capability of
the human eye. Under dark conditions, the eye integrates a light
signal for a few tenths of a second. Film can integrate the signal
for hours (even up to a week), providing a detectability thousands of
times fainter than the eye. A third important, if less basic,
property is that film can be made sensitive to a much wider range
of EM wavelengths than is the eye. The impact of photography on
astronomy was profound.
- All optical band detectors, including film, rely on the
photoelectric effect. This is the ejection of an electron
from a photosensitive surface when struck by a photon of EM energy.
In the case of film, the electrons are stored by crystals in an
emulsion until the chemical reactions during development
cause them to precipitate grains of silver, which form the permanent
image.
- Film was the detector of choice in astronomy from around 1900 to
1985. [The picture at the right shows Edwin Hubble guiding the camera
on the Mt. Wilson 100" telescope ca. 1930.] However, it had
limitations which astronomers had long struggled with. First, it was
relatively insensitive in that it responded to only about 1% of the
incident light. (We would say that film has a quantum
efficiency of only 1%.) Second, it was very difficult to
calibrate photographic signals quantitatively in terms of the amount
of incident EM flux.
ELECTRONIC IMAGING
- Starting in the 1920's, astronomers began using various types of
electronic detectors to supplement film. World War II greatly
accelerated these technologies, especially in the form of the
photomultiplier tube which could amplify a single photon into
a large burst of electrons.
- But none of these were easily converted into large format,
two-dimensional detectors until charge-coupled devices (CCD's)
were introduced in the 1970's. A CCD is a light-sensitive silicon
wafer with built-in microcircuitry (see picture below right). During
an exposure, it traps the released electrons in small voltage wells.
After the exposure, the collected electrons are rigidly shifted across
the CCD (so the image isn't smeared), amplified, and stored in a
computer memory.
- CCD's are very powerful devices and are the basis of most modern
video cameras. For astronomers they have the following key
advantages: (i) they are highly sensitive, with quantum
efficiencies over 80%; (ii) they are highly linear, meaning
that their response is directly related to the EM flux deposited;
(iii) they immediately convert a scene into a digitized computer
image, which can then be further analyzed by image processing.
CCD's have now virtually replaced film in professional astronomy. One
complication for the low light levels important to astronomers is the
presence of ``dark current'' in CCD's (a residual signal in the
absence of incident light), which requires that they be operated at
temperatures of about -100oC.
- The largest CCD is still much smaller than typical photographic
"plates." To cover large fields, astronomers have built mosaic
CCD cameras containing many individual CCD chips. Below is an extract
from a 3-color image of the Rosette Nebula taken at the Kitt Peak
National Observatory with a mosaic CCD camera containing 8450x8450
pixels. [Color separation images of astronomical objects are much
easier to construct using CCD's because of their linearity and
immediate conversion to digital format than was the case with film.]
The image below was derived from 3 individual images with filters
centered on hydrogen (coded red), sulfur (blue), and oxygen (green)
emission lines. Click for a full-screen version.
C. SKY SURVEYS
One of the most important tasks for astronomical imagers is simply to
map the sky---i.e. to find out what's there. Systematic, all-sky
surveys began over 200 years ago, e.g. the New General Catalog
(NGC) of 7000 diffuse objects (nebulae and galaxies) by Herschel
and his sons (pre-photographic). The photographic Henry Draper
Catalog of 300,000 stars (ca. 1900) was immensely valuable in
clarifying stellar evolution.
With the development of very large telescopes, astronomers realized
they needed very sensitive, all-sky imaging surveys, made with
specialized telescopes. The modern prototype was the Palomar
Observatory Sky Survey (POSS), completed in the 1950's with a
specialized photographic telescope, the 48-in Schmidt. This took
matched photographs with blue and red filters on large plates with
fields 6 degrees on a side and reached about 20th magnitude. At
right is a picture of Edwin Hubble guiding the 48in Schmidt.
Several follow-up surveys, also with large format photographic plates,
have been made or are in progress. The whole sky has now been mapped
to about 20th magnitude. All of this material is being converted to
digital format for computerized retrieval. Emphasis is now shifting
to all-electronic surveys, using CCD's to instantly produce digital
output. All-sky maps have also been made to varioius depths in a
number of other EM bands, from radio to gamma ray. Useful Web
sites:
All-sky map in infrared. Click image
for info.
D. SPECTROSCOPY
Spectroscopy is the study of the distribution with wavelength
or spectrum of the EM energy released by a cosmic source. We
already saw in Lecture 5 that the shape
and peak wavelength of the EM spectrum of a star are related to its
temperature.
- This is true of any self-luminous, dense object: its EM
spectrum is smooth or continuous and changes only slowly with
wavelength. You can think of the atoms and electrons in such an
object as interacting so strongly that the individual signatures of
each type of atom are blended away.
- On the other hand, in a dilute or thin gas---the
atmosphere of the Sun, for example---the atoms do not interact
strongly, and if there are enough atoms of a given type, they will
impress their individual signatures on the emergent spectrum. The
internal electronic structure of each type of atom consists of
discrete energy states, and these states are unique
to that type.
- In the spectrum of a dilute gas, the states appear as
sharp features called "spectral lines." They are seen either in
emission (if the gas is observed in isolation) or
absorption (if it is observed against a continuous source).
The 3 general kinds of spectra are summarized in the illustration
below:
- Because each type of atom produces a unique spectrum, the
chemical composition of cosmic objects can be deduced from
their spectra even though they may be millions of light years away.
The figure below shows part of the spectrum of the Sun at yellow
wavelengths. The dark lines are produced by atoms of magnesium, iron,
titanium, and other elements. Quantitative analysis of such spectra
tells us the detailed chemical makeup of the Sun and other stars. For
example, the element helium (the second lightest after
hydrogen) was discovered in the spectrum of the Sun before it
was studied in an Earth-bound laboratory.
- In addition, spectra also contain information on the velocity,
pressure, density, temperature, ionization state, magnetic field
strength, and other physical characteristics of cosmic objects.
Spectroscopy is therefore an exceedingly powerful tool. It is the
source of much of the astrophysical information we have about the
universe.
Homework:
- Background information on these topics can be found in ASTR 121/124
textbooks. Texts are available for consultation in the Astronomy
Library and in the Day Lab (267 Astronomy).
- Download, print, and read the webnotes for this lecture.
- In preparation for the Midterm Exam (Monday, 3/26), study the Review
and Sample Questions handed out in class (or download from the
ASTR 130 home page). Review the reading assignments in the ASTR 130
Manual.
- You should be working on Labs 3 and 4 and the first optional lab.
Web links:
Last modified
8 March 2001 by rwo
Images of EM spectra copyright © 2000 Harcourt, Inc., from
the ASTR 121-4 textbook by Fraknoi et al.. Rosette nebula image taken
by T.A.Rector, B.Wolpa, and M.Hanna, with the KPNO 0.9-m Mosaic
(copyright © AURA/NOAO/NSF). Text copyright © 2001 Robert W.
O'Connell. All rights reserved. These notes are intended for the
private, noncommercial use of students enrolled in Astronomy 130 at
the University of Virginia.
With the development of very large telescopes, astronomers realized
they needed very sensitive, all-sky imaging surveys, made with
specialized telescopes. The modern prototype was the Palomar
Observatory Sky Survey (POSS), completed in the 1950's with a
specialized photographic telescope, the 48-in Schmidt. This took
matched photographs with blue and red filters on large plates with
fields 6 degrees on a side and reached about 20th magnitude. At
right is a picture of Edwin Hubble guiding the 48in Schmidt.
Several follow-up surveys, also with large format photographic plates,
have been made or are in progress. The whole sky has now been mapped
to about 20th magnitude. All of this material is being converted to
digital format for computerized retrieval. Emphasis is now shifting
to all-electronic surveys, using CCD's to instantly produce digital
output. All-sky maps have also been made to varioius depths in a
number of other EM bands, from radio to gamma ray. Useful Web
sites:
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