CCD's Versus Professional Plate Film - by Ricky Leon Murphy:
Introduction
Early Photography
Advantages of using
plate film
The remarkable CCD
Digital Processing
CCD versus Plates
Conclusion
References
Internet Resources
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Astrophotography
Introduction:
When I think of comparing
photographic plates with CCD imagery, I think of three words: Hubble
Space Telescope. On closer inspection of this subject, I see the line
between CCD and photographic plate use in Astronomy is not so sharply
delineated. Photographic plates use in Astronomy has enjoyed a very rich
history. Using photographic emulsion to capture starlight was the day
astrophysics was born. Supernova was discovered, as was variable stars.
Comparing photo plates also led to the discovery of Pluto, and even a
score of asteroids. However, even with the rich history of plates,
Astronomy must bow to technology. CCD imagery is progressing at a rapid
pace. It allows immediate results that can be shared, and makes the use
of orbiting observatories possible. Ultimately it may not be the CCD
that will end the reign of plate film; economics of plate film
manufacturing may ultimately determine the demise of this once widely
used format.
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Early photography:
In the 1850’s, a very
early version of the photographic process was used for the first time in
Astronomy. Leon Foucault and Armand Fizeau photographed the Sun and its
Sunspots on a type of film called a daguerreotype (Belkora, 2003), which
used silver halide crystals on a copper plate. Later work by William
Huggins would result in the photography of stellar spectra which gave
birth to astrophysics (Belkora, 2003). Since then, film evolved rapidly
to conform to ease of use and sensitivity and eventually resulted in a
gelatin film that holds silver halide crystals in a form we know and
love: the roll of film. Silver halide emulsion on a glass plate was
still used professionally mostly because of rigidity – the film would
not bend resulting in an evenly exposed image without artifacts. The
plate process also allows much finer grains of silver emulsion possible,
even as small as 9 microns in diameter.
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Advantages and
uses of plate film today:
One of the most important
advantages of plate film use is the fact that plate film surveys of the
entire sky already exist. In the 1950’s, the first set of an all-sky
survey was initiated resulting in nearly 2000 exposures of the entire
sky called the POSS (Palomar Oschin Sky Survey). With the availability
of finer grained emulsion after 1970, it was decided to perform a second
all sky survey in the 1980’s called the POSS-II (http://astro.caltech.edu/~wws/poss2.html).
One of the advantages that plate film still has over the CCD camera is
the field of view. Each plate of the POSS and POSS-II is a generous 14
inch square that covers an area of 6 degrees (http://lyra.colorado.edu/sbo/sboinfo/readingroom/poss.html).
The results are images of extremely high resolutions, even by today’s
standards. Another advantage of plate film is the availability of grains
that are sensitive to specific wavelengths. For example, there are three
widely used plate styles: IIIa-J emulsion for blue sensitivity, IIIa-F
for red sensitivity, and IV-N for near infrared sensitivity (http://www.eso.org/org/dmd/pos/).
While the plates are not very useful for computer software to analyze,
it is possible to provide excellent digital images from the existing
plate film. Because the plates are clear, they can be illuminated from
the rear and photographed either by regular film or by CCD. A group of
European astronomers successfully tested wide field photography on
standard film using the plates as a master (Zodet, Quebatte, and Heyer,
1994). One of the issues of a duplicate image, especially using CCD’s,
is the introduction of noise. To help eliminate this, it is possible to
stack the images in the digital realm resulting in a much cleaner image
(Bland-Hawthorn and Shopbell, 1993). The high resolution plates have
been archived digitally and are available through the DSS (digitized sky
survey) thanks to the process of drum scanning.
Astronomers from around the world can access previously scanned sky
surveys through
the MAST (Multimission Archive at Space Telescope) at
this website:
http://archive.stsci.edu/index.html. The benefit is recent
photographs can be compared to pervious photographs of an area of
interest to look for changes such as stellar position, supernova,
variable star discovery, or even the discovery of new asteroids. While
using plate to plate comparison, Clyde Tombaugh discovered Pluto by
discovering the shift of a very faint object. This same process is now
possible in the digital realm thanks to MAST.
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The remarkable CCD sensor:
CCD stands for charged
coupled device. The CCD chip is made of silicone, and operates by
absorbing photons of light that cause electrons to be released by the
silicone (Howell, 2000). The silicone chip is divided into very small
segments called pixels, and the electrons associated with each pixel are
measure by the internal electronics of the CCD camera resulting in an
assigned value. This value is interpreted by software and is converted
to an image. There are several advantages to using CCD sensors versus
standard film:
-
The sensor is very
sensitive to light and is comparable to standard film with 1600 ISO
-
Greater range of
brightness over film usually referred to as grayscale
-
Linear response –
twice the exposure equals twice the light gathering capacity
-
Responds to the
entire visible spectrum, and even into the near infra-red
-
Capable of storing
additional information such as bad pixel data for easy correction
-
Immediate results
-
Defects of the CCD
chip can be subtracted using software
(Covington, 1999). While
the list of features of the CCD seems impressive, there are several
disadvantages. For example, the CCD operates with a continuous power
supply that results in increased heat. The CCD sensor will pick up this
heat and read it as noise. While the noise can be subtracted with
software, the best solution is to cool the chip using heat sinks, water
cooling, or even liquid nitrogen. Also, while CCD sensors are becoming
larger and cheaper, they are still limited by its field of view. To
equal the field of view provided by a 14 inch by 14 inch plate film, a
mosaic of CCD sensors must be constructed. The price of such a sensor is
(pardon the pun) astronomical. The cooling requirements of such a chip
are also greater. None the less, such mosaics have been constructed and
are currently in use.
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Digital Processing:
Digital processing is not
limited to CCD images only. Photographic plates are scanned and saved in
a digital image format just as CCD images are interpreted by software,
and saved in a digital image format. Image processing software can take
these two images and perform a variety of adjustments and measurements.
Astrometry (measuring the precise coordinates of stars in an image) and
photometry (measuring the brightness of a star) can be performed on any
digital image. Improved image quality is possible by a process called
“image stacking” multiple images of the same subject (Wodaski, 2002).
While the processes of image enhancements are beyond the scope of this
essay, it is important to understand that once in the digital realm,
measurements and image processing can be performed regardless if the
source is from a photographic plate or CCD sensor.
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CCD use versus Plate use:
Currently there are no
active projects exposing photographic plates. The sensitivity and
resolution capabilities of the CCD sensor are far greater than any
plate. Most observatories are joint ventures of several countries, so
results are available electronically almost immediately. Only the CCD
camera is capable of this. In addition, we have orbiting observatories
like the Hubble Space Telescope that make full use of the CCD.
Photographic plates used in orbit would not be possible. An example of
the advances of CCD technology is the camera used on the CFHT
(Canada-France-Hawaii Telescope); using a mosaic of forty 2,048 by 4,612
pixel CCD chips that result in a total of 350 million pixels (Cuillandre,
2004). While the field of view of this remarkable sensor is still not as
great as plate film, the image quality, sensitivity and range of colors
are far greater than any photographic plate ever produced.
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Conclusion:
The advances of the CCD
sensor are not the only reason that plate film is no longer used. The
International Astronomy Union learned that Kodak has discontinued
production of several photographic emulsions (IAU Symposium, 1993). In
addition, consumer use of inexpensive, high quality CCD cameras have
resulted in a shift in policy at several film manufacturing companies as
well as film based camera manufacturers. In today’s computer driven
market, it may not be financially possible to continue to support film.
Regardless of the future trends, the photographic plates that exist are
an invaluable tool for Astronomy today. The Digitized Sky Survey uses
high resolution scans of the exposed plates from the 1950’s and the
1980’s and is available to anyone online. In the meantime, CCD
technology is certain to progress resulting in greater CCD resolution at
a cheaper price. Perhaps one day, a CCD sensor will finally achieve the
6 degree field offered by the photographic plates of the past.
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References:
Belkora, Leila.
Minding the Heavens. Institute of Physics Publishing. Bristol, 2003.
Bland-Hawthorn. Patrick
L. Shopbell. “Deep Sky Surveys: A Motivation For Stacking Digitized
Photographic Plate.” The Astronomical Journal, Volume 106, Number 5.
November, 1993.
Covington, Michael.
Astrophotography for the Amateur. Second Edition. Cambridge
University Press, 1999.
Cuillander, Jean-Charles.
“Mauna Kea’s Colorful Universe.” Astronomy Magazine, pages 34 to 41.
August, 2004.
Fierro, I.H. Bustos. J.H.
Calderon. “CCD-Based Astrometric Measurements of Photographic Plates.”
Revista Mexicana de Astronomia y Astrofisica, 39, 303-310. 2003.
Heyer, H.H. J. Quebatte
and H. Zodet. “Photographic Wide-Field Imaging from Schmidt Plates.”
IAUS, 161. 1994.
Howell, Steve B.
Handbook of CCD Astronomy. Cambridge University Press, 2000.
IAU Symposium 161.
“Astronomy from Wide-field Imaging.” Brandenburg, Germany. 23-27 August
1993.
Wodaski, Ron. The New
CCD Astronomy. New Astronomy Press, 2002.
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Internet Recourses:
The Digitized Palomar
Observatory:
http://taltos.pha.jhu.edu/~rrg/science/dposs/dposs.html
The SBO Palomar Sky
Survey Prints:
http://lyra.colorado.edu/sbo/sboinfo/readingroom/poss.html
Data Management &
Operations Division POSS II Project:
http://www.eso.org/org/dmd/pos/
Sloan Digital Sky Survey:
http://cas.sdss.org/dr2/en/proj/advanced/skysurveys/poss.asp
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