A. INTRODUCTION

B. IMAGING

• 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 revolutionary capabilities to astronomers:
  
  (i) First, it provided permanent records of observations.
  (ii) 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 many hours (even up to a week), providing a detectability thousands of times fainter than the eye.
  (iii) 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.

  As a consequence, 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 above right shows Edwin Hubble guiding the camera on the Mt. Wilson 100" telescope ca. 1930.] However, it had limitations with which astronomers had long struggled. 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.

• 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.

• These devices were highly sensitive and very useful in many applcations. But none of them 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 at 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. The cartoon above illustrates the electron shifting technique.

• CCD's are very powerful devices and are the basis of most modern digital and video cameras. For astronomers they have the following key advantages:
  (i) They are highly sensitive, with quantum efficiencies over 80%. This means they can detect much fainter sources in a given exposure time than can film.
  (ii) They are highly linear, meaning that their response is directly proportional 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 software. 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 low temperatures of about -100^oC.
  The requirement for low temperatures while making long exposures is the reason that mass market digital cameras do not work well for most astronomical applications.

• 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. The image below was derived from 3 individual images with filters centered on emission lines of hydrogen (coded red), sulfur (blue), and oxygen (green). [Color separation images of astronomical objects are much easier to construct using CCD's than was the case with film because of their linearity and immediate conversion to digital format.] Click for a full-screen version.
  

C. SKY SURVEYS

• The Skyview Virtual Observatory (retrieve images of sky in all EM bands from radio to X-ray)

• The Sloan Digital Sky Survey. The prototype optical-band electronic survey, with a camera employing a total of 54 CCD arrays, the most sophisticated CCD mosaic assembled to date. The camera layout is shown at right (click for enlargement). The SDSS will catalog about 100 million cosmic objects to magnitude 23 and obtain spectroscopy of about 1 million individual galaxies.

• The 2MASS All-Sky Infrared Survey. A recently completed survey of the entire sky (with 2 telescopes) in the infrared bands between 1.0 and 2.5 microns wavelength. Mike Skrutskie (UVa) was Principal Investigator for the survey. See Lecture 6 for an all-sky image created from the survey.

D. SPECTROSCOPY

• 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 the (wrapped) optical-band spectrum of the Sun extending from near-ultraviolet to near-infrared wavelengths. The dark lines are produced by atoms of hydrogen, calcium, 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.
  
  The solar spectrum. Click on the figure for an enlargement of the spectrum of the red giant star Arcturus.

  
  

• 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.

Anglo-Australian Observatory Images Page--best site for ground-based pictures of astronomical objects
Background information on imaging & CCD's:
Astrophotography (J. Lodriguss)
Primer on Astrophotography (J. Pennington)
Introduction to CCD's UVa ASTR 511 (R. O'Connell)
Introduction to CCD's (G. Bothun, Univ. of Oregon)
Astrophotography & CCD Imaging For Amateur Astronomers (Sky & Telescope Magazine)

Nick Strobel's Astronomy Pages for background information on electromagnetic radiation and spectroscopy
More information on atomic physics & spectrosopy (ASTR 121)
Example of a state of the art spectrograph (SINFONI for the VLT)