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 in the solar spectrum Relative 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 pressure-sensitive

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)