ASTR 1230 (O'Connell) Lecture Notes
7. ASTRONOMICAL IMAGING
Orion and Mars over Monument Valley (Wally Pacholka)
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
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.
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
(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 at
most a few tenths of a second. Film can integrate the signal for
many hours (even up to a week), allowing detection of
sources thousands of times fainter than possible with the eye at
the same telescope.
(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.
Here is a brief history of the use of photography
[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.
The illustration below shows electron ejection from a metallic
surface, but the effect is important and has different manifestations
(e.g. "photo-conductivity") in a wide variety of materials. The
photoelectric effect provided definitive evidence for the existence of
EM photons --- i.e. packets of EM energy that behave like particles,
schematically shown in the illustration --- and Albert Einstein
received the Nobel Prize for his interpretation of the effect.
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
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.
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
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:
(i) They are highly
sensitive, with peak 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,
and they can be accurately calibrated.
(iii) They immediately convert a scene into a digitized computer
image, which can then be further analyzed by image
CCD's have now almost completely replaced film in professional
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
The requirement for low dark current while making long exposures
is the reason that inexpensive "snapshot" mass market digital cameras
do not work well for most astronomical applications. Better ones, e.g.
single-lens reflex designs with large pixels and small dark currents,
are more satisfactory. The high-quality CCD detectors used in professional
observatories typically cost over $100,000 and must be
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.
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,
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
- The colors in the combined image
do not necessarily correspond to what you would see with your eye; and
they are often deliberately exaggerated ("pseudo-color") to bring
out fine shading or physically significant details.
- One standard technique is to use narrow band filters to isolate
certain emission lines, characteristic of hot atoms of
certain elements (e.g. hydrogen, oxygen), to show the distribution of
different phases of hot gas.
An example of the compositing process is shown
- 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).
Click for a
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,
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
- Download, print, and read the webnotes for this lecture.
- Optional reading (not required):
7.1 Modern Observational Astronomy and 7.2
- Take the Review Quiz for week 8 on Collab.
- You should finish Lab 3 and move on to Lab 4 at the earliest opportunity.
- The Midterm Exam is Monday 10/31. See the
Exam Prep page for helpful hints
and a review of important topics.
For all interested students, there will be a REVIEW for the exam
on Sunday, 10/30 at 4 PM in Astronomy 265.
Views of the
Solar System---images by Damian Peach
Lunar and Planetary
Observation with CCD Imaging---images by Antonio Cidadao
El Cielo de
Canarias---Images & Time-Lapse Astro-Videos by Daniel Lopez
Observatory Images Page--best site for professional ground-based pictures
of astronomical objects
Telescope Image Archive---spectacular, widely-distributed images; site
includes a tutorial on astronomical filter imaging
Background information on imaging & CCD's:
Astrophotography (Jerry Lodriguss)
Colour (Digital Photography Tutorials) (Sean McHugh)
Digital Astrophotography Bibliography
(David Haworth, 2009)
Astrophotography Tutorial (Ray Shore)
& CCD Imaging For Amateur Astronomers (Sky & Telescope Magazine)
on Astronomical Image Processing using the "trillion-pixel" Sloan Digital
Sky Survey dataset
YouTube video showing how
Hubble Space Telescope color images are processed
Strobel's Astronomy Pages for background information on
More information on atomic physics & EM radiation (ASTR 1210, O'Connell)
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.