ASTR 1230 (O'Connell) Lecture Notes


7. ASTRONOMICAL IMAGING


Orion/Mars/MonumentValley

Orion and Mars over Monument Valley (Wally Pacholka)


A. INTRODUCTION

Astronomers have developed a wide array of ingenious instruments for attaching to telescopes in order to make measurements of the sky. For 270 years, the human eye was employed exclusively as the detector (more details on the performance of the eye can be found here). 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 the most widely-used kind of optical band equipment and detectors: imagers and CCDs.


B. IMAGING

ASTRONOMICAL IMAGERS (CAMERAS)

In order to convert a telescope into a camera, all you have to do is place a photosensitive detector (film or other) in the focal plane of the primary optics and add a shutter and filter wheel. No eyepiece or other optical device is needed, although many professional cameras do employ additional optics to produce better images over wide fields of view or to partially correct seeing blur, for instance.

PHOTOGRAPHY

HubbPhot
  • 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:

  • As a consequence, the impact of photography on astronomy was profound. Here is a brief history of the use of photography in astronomy. [The picture at the right, ca. 1930, shows Edwin Hubble guiding the camera at the Newtonian focus of the Mt. Wilson 100-in telescope, which he used to prove the existence of external galaxies.]

  • All optical band detectors, including film, rely on the photoelectric effect. This is the energy boost given to an electron in a photosensitive surface when struck by a light photon. In the classical photoelectric effect, enough energy is imparted to eject the electron from the surface altogether. The rate of electron ejection is proportional to the incident EM flux at the detector. Hence, if you can trap and measure the ejected electrons somehow, you can estimate the incident photon flux.

  • Film consists of a thin photosensitive emulsion coated on a sheet of glass or plastic. Ejected electrons are stored by crystals of silver bromide in the 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 for astronomical imaging from around 1900 to 1980. 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.

  • Visit this link for more information on the photographic process.


    ELECTRONIC IMAGING AND CCDs

  • Astronomers began using various types of electronic detectors to supplement film in the 1920's. World War II greatly accelerated these technologies, especially in the form of the photomultiplier tube which could amplify a single photon into an easily-detectable burst of millions of electrons. These and similar 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 (CCDs) were introduced in the 1970's.

    CCD
  • A CCD is one example of a larger class of detectors called solid-state, semiconductor arrays. It is a light-sensitive silicon wafer with built-in microcircuitry (see diagram above and picture at right; click for an enlargement).

  • During an exposure, photoelectric interactions free electrons from their tight bonding to individual atoms, allowing them to move through the material. A CCD is designed to trap the released electrons in small voltage wells or pixels. After the exposure, the collected electrons are shifted rigidly across the CCD (so the image isn't smeared), amplified, and stored in a computer memory. The smallest element in the stored image corresponds to the size of a pixel on the detector surface. The cartoon below illustrates the electron shifting technique.

  • CCD's and related array devices are very powerful and are the basis of modern digital and video cameras. (In regular cameras, single images are intended to be shifted out relatively slowly; in video cameras, images must be shifted out 24-60 times per second.) For astronomers they have the following key advantages:

  • One complication for the low light levels important to astronomers is the presence of dark current in CCDs. This is a residual electrical signal in the absence of incident light. To suppress this, astronomers operate CCD cameras at low temperatures of about -100oC.

  • The largest CCD (now routinely manufactured in sizes up to 4000x4000 pixels) is still much smaller than typical photographic "plates." To cover large fields, astronomers have built mosaic CCD cameras containing many individual CCD chips.

    COLOR IMAGING

  • CCD's have no inherent color resolution---i.e. their response to different EM wavelengths changes slowly over the optical band. Your eye can sense color because there are three different types of cone cells in your retina, each sensitive to a different wavelength range of the spectrum between 4500 and 6000 Å, corresponding to the sensations of blue, green, and red, respectively.

  • To obtain color information, professional astronomers place different colored filters in front of a single CCD detector, take multiple images of a scene, and then combine the images with software. The process is described in this tutorial by the Hubble Space Telescope staff. Each black and white image is assigned a color by the software, and these are combined together to make a full color version. [Such "color separation" images are much easier to construct using CCD's than was the case with film because of their linearity and immediate conversion to digital format.]

  • Studio-quality commercial CCD/video cameras use a similar color-separation technology but employ beam-splitting optics and three separate black and white CCDs fed by red, green, and blue filters to take simultaneous 3-color images that preserve the spatial resolution delivered by the CCD pixels.

  • Less expensive CCD/video color cameras instead make color images with a single CCD chip and single exposure. The CCD surface is covered by a "Bayer array" of color filters that makes each electronic pixel sensitive only to one color band (see below). After readout, software creates full-color image pixels by combining a number of the electronic pixels together. The spatial resolution of the resulting image is lower than the CCD can deliver, but the results can be beautiful.

  • There are many excellent websites offering tutorials on astrophotography or examples of images made with small telescopes and inexpensive CCD cameras. Some good ones are listed at the bottom of this page. Here's an example image of the "Horsehead" Nebula from Robert Gendler:


    C. SKY SURVEYS

    48in One of the most important tasks for astronomical imagers is simply to map the sky---i.e. to find out what's there. Systematic, telescopic large-area surveys began over 200 years ago with, for example, the New General Catalog (NGC) of 7000 extended objects (star clusters, nebulae and galaxies) by Herschel and his sons (pre-photographic). The photographic Henry Draper Catalog of objective prism spectra for 300,000 stars (ca. 1900) was immensely valuable in clarifying stellar evolution.

    With the development of 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 wide-field photographic telescope, the 48-in Schmidt. This obtained matched photographs with blue and red filters on large 14-in plates with fields 6 degrees on a side. It recorded stars to about 20th magnitude. At right is a picture of Edwin Hubble guiding the 48-in Schmidt.

    Several follow-up surveys, also with large format photographic plates, were made. 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 has now shifted to all-electronic surveys, which instantly produce digital output. All-sky maps have also been made to various depths in a number of other EM bands, from radio to gamma ray. Useful Web sites:



    Assignment: Web links:


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    Last modified January 2013 by rwo

    Rosette nebula image taken by T.A.Rector, B.Wolpa, and M.Hanna, with the KPNO 0.9-m Mosaic Camera (copyright © AURA/NOAO/NSF). CCD transfer animation by C. Tremonti. Horsehead Nebula image by R. Gendler. Text copyright © 2001-2013 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1230 at the University of Virginia.