First Light: Photography
& Astronomy
Introduction to Spectroscopy
Christopher G. De Pree
Agnes Scott College
RARE CATS, June 20, 2002
Overview


Advent of Photography
Applications of photography in
astronomy
 Imaging
 Spectroscopy

Early pioneers in astrophotography
 Harvard
College Observatory
 Annie Cannon

Impact on our understanding of the
universe
Advent of photography


“Camera” from camera obscura, Latin for
“dark room”
Drawing by Gemma Frisius


De Radio Astronomica et Geometrica
(1545)
Thomas Wedgwood (1771-1805) and Sir
Humphry Davis
Late 18th century first photographic
images
 Photosensitivity of silver nitrate and silver
chloride

Daguerreotypes


1831 French painter Louis Jacques
Mandé Daguerre
Photographs on silver plates
 Coated
with a light-sensitive layer
of silver iodide

Permanent photographs
 Developed
plate was coated with a
strong solution of table salt

Daguerre method produced a
single image on the silver plate
Evolution of the Form

British inventor William Henry Fox
Talbot
 photographic
method involving the
use of a paper negative


Calotype process (30 sec exp.)-multiple prints
Daguerre and Talbot announced
processes in 1839
 Within
three years the exposure
time in both processes was a few
seconds
Applications in
Astronomy

Imaging
 Long
exposure times--faint objects
 Tracking single objects
 Long time-scale monitoring

Spectroscopy
 Emission
mechanisms
 Identifying elements
 Helium
 Relative
in the solar spectrum
motions (Doppler Shift)
Impact on our
Understanding



What’s out there?
What is it made of?
How is it moving with respect to
us?
Imaging

Development of “fast” film
 Problem
of first photographic
techniques (no smiles)
 Rotation of the Earth
 Exposure times of a few minutes

Long exposure times
 Detect
faint sources
 Tracking required
 Telescope stability (mounts)
Tracking

Equatorial Mount
 Moves
to counteract Earth’s
Rotation
 Must be “polar aligned”

Altazimuth mount
 Not
a unique coordinate system
 More difficult to track
 Some
modern telescopes use (e.g.
VLA, VLBA)
Telescope Designs




Cassegrain
Coude
Herschelian (obsolete)
Newtonian
Beck Telescope





30” diameter primary mirror
Cassegrain or Newtonian
arrangement
German equatorial mount
Counterbalance
Tracking system (clock drive)
 Mechanical
(1930)
 Electrical (1966)
 Computer-controlled (1998)
Impact on astronomy
(Imaging)

Images create archives
 Classification
of galaxies
 Classification of stars
(spectroscopy)

Deep images show detail
Images taken over time

Look for change in successive
images
 Variable
stars
 New planets
 Asteroids and comets
Spectroscopy


Pass light through a prism
Elemental fingerprints
 Why
are spectra unique
 Electrons “hopping”
Where do spectral lines
come from?


Electrons “hopping around”
Molecules spinning and vibrating
Doppler Shift




Motion of the Source
Train whistle
Water waves and ducks
Light experiences the same effect
 Redshift--longer
wavelength
 Blueshift--shorter wavelength
Planetary Detection
Known Extrasolar
Planetary Systems
Use of Stellar Spectra

Harvard College Observatory
 Edward
Pickering
 “Computers”

Antonia Maury(1866-1952)
 Classification
scheme
 Position and width of absorption
lines

Annie Cannon (1863-1941)
 Established
the current stellar
classification scheme
Luminosity
Classification (Maury)

Absorption lines are pressuresensitive
 Lines
get broader as the pressure
increases.
 Giant stars are puffier, which
means lower pressure
 Giant stars have narrower
absorption lines
 Supergiant stars have absorption
lines that are even narrower

Used by Ejnar Hertsprung
Temperature
Classification (Cannon)

Harvard system (HD catalog)
 Classifying
stars by their spectra
 Spectral classification by surface
temperature

Systematic work on all stars down
to 9th magnitude
 Used
photographic plates (16x
fainter than human eye sees)
 Cannon could identify spectral
patterns at 3 stars/minute
Cannon’s Contributions


Most stars grouped into a small
number of classes
Personally classified spectra for
more than 225,300 stars
 Henry

Draper Catalogue
Discovered more than 300 variable
stars and five novae
Conclusions

Significant advances in astronomy
followed the advent of photography
 Deep
imaging
 Classification
of galaxies
 Archiving
 Time
variability of stars
 Discovery of planets, comets,
asteroids
Conclusions (cont.)
 Stellar
spectroscopy
 Classification
scheme
 Understanding of how stars evolve
 Motion (Doppler Shift)
• Extrasolar planets
 Spectroscopy
in general
• Rotation of galaxes
• Expansion of the universe

The next great leap
 Charge
Coupled Devices (CCDs)