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THE INTERNATIONAL MARSWATCH ELECTRONIC NEWSLETTER
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Volume 3; Issue 7
December 21, 1998
Circulation: 1430
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Dear Marswatch participant,
Mars exploration took another step forward recently with the launch
of the Mars Climate Orbiter on December 11. Check out the mission and
some spectacular launch shots at the official web site:
http://mpfwww.jpl.nasa.gov/msp98/orbiter/
Poised next for a January 3 launch is the Mars Polar Lander. If all
goes well, we'll all be getting snowy Christmas cards from near the
south pole of the red planet this time next year. Check out both the
lander mission and the story about the incredible "microprobes" that
the mission is carrying, at:
http://mpfwww.jpl.nasa.gov/msp98/lander/
http://nmp.jpl.nasa.gov/ds2/
Finally, continuing to ramp up towards the fast-approaching telescopic
observing season, enclosed is an article by Jeff Beish on the measurement
of the Martian polar caps from CCD images. Enjoy, and best wishes for
a safe and successful 1999!
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Measuring the Polar Caps of Mars From CCD Images
By: Jeff Beish, Former Mars A.L.P.O. Recorder
INTRODUCTION
Theoretical studies by classical planetary researchers and recent work by
modern space scientists reveal that the size of Mars' polar caps may be a
controlling factor in the dynamic atmosphere of the Red Planet. For years
Mars observers noted that the number of clouds in the Martian atmosphere
appeared to increase or decrease with the shrinking or growing of Mars'
polar caps.
To understand the Martian climate the A.L.P.O. Mars Section began to plot
the retreat and reformation of the planet's polar caps on a regular basis
and compare the results with the number of clouds observed during each
Martian year.
During each apparition dedicated Mars observers used astronomical
micrometers to make polar cap measurements at their telescopes. This is no
easy task for sure because Mars' small apparent size renders micrometer
measurements very difficult and is a laborious job that amateur astronomers
are not likely to enjoy.
Adding to this difficulty is the variable "astronomical seeing" conditions
that causes the apparent disk of Mars to blur or expand, and move about the
eyepiece image. Clouds and hazes in the Martian polar regions also hamper
measurements. It is still difficult to see more of the limbs or polar cap
edges clearly on Mars even using the recommended red filter techniques. To
reduce the difficulties in measuring the Martian polar caps we turned to
photographs to provide a better method to record Mars and produce more
accurate polar cap latitude measurements. However, the late Charles F.
("Chick") Capen demonstrated that a two degree (2() systematic error in
latitude was apparent using this method, even on photographs taken with a
variety of large professional telescopes [Capen, 1970].
Measurements made on film were compared with micrometer measurements using
the same telescope and image scale. We usually found the computed polar cap
latitudes differed significantly between the two methods. Also,
photographs taken in less than ideal "seeing" resulted in more errors than
those made with micrometers.
The equations used to reduce these measurements are sensitive to image
size and seeing conditions can vary the size of telescopic images [Beish,
1994]. We surmised that the effects of Earth's turbulent atmosphere,
unstable telescope conditions, and minute errors in the telescope drives
might cause the photographs to blur or shift in position and cause
erroneous measurements.
During the time of exposure film may record each change in size or
movement of the image, whereas the human eye often filters out or misses
these changes. So, we returned to using micrometers as our main polar cap
measuring device.
However, we did find some photographs that produced lower than expected
errors, indicating the photographic method was not completely useless. But
there is a catch here. During many years of cataloging and analyzing Mars
observations this author found a very few photographs that could be
effectively used in our program.
While systematic errors are reduced by using a large image scale,
increasing the photographic image scale requires increased exposure time.
Increased exposure time increases the integration time for the image to
record more image blur and/or movement on the film.
Also, the nonlinear response of film restricts exposure times to the
linear part of the film's "characteristic curve" or film density/brightness
curve in order to produce predictable results [Dobbins et al, 1988].
Furthermore, the plate scale on most of the photographs we received were
too small to be used anyway. It is clear that obtaining photographs with
a useable image scale too difficult for the average observer and
unproductive for the observing program. The answer to our dilemma seems to
be to increase image scale, keep the exposure time down, and produce images
with a linear response. As hard as we tried to improve it, conventional
photography had to be abandoned.
THE CCD CAMERA ARRIVES
While time and space prohibits a detailed discussion of CCD technology
here is a brief description of its leading characteristics.
First, the CCD chip is made from materials that produces a slight current
flow when it is exposed to light. Current flow within the chip's material
increases or decreases linearly with an increase or decrease of the light
hitting its surface. The material is separated into thousands of cells and
connected electrically to components that produce digital signals. These
signals are connected to a computer to be used later to determine the
amount of light recorded in each cell of the CCD chip.
Computer programs have been developed to analyze these images and to
refine the images by allowing electrical component noise and other
signal-to-noise to be reduced. Image brightness, contrast, and a host of
filtering techniques can also be employed.
A most important characteristic of the CCD camera is that exposure times
are reduced and the chip's material is sensitive to the red and infrared
light. This is good for filtering out the effects of Mars' atmosphere so
the polar cap can be imaged more clearly.
So, the CCD camera fits the requirement to produce a linear, fast
exposure, and is less susceptible to atmospheric turbulence (bad seeing
cells).
REDUCED SYSTEMATIC ERRORS WITH CCD CAMERA
Using CCD images of Mars and very short exposure times makes Martian polar
cap latitude measurements a pleasure. No more waiting for those moments of
steady seeing to fit bright Mars between two fine wires of the micrometer.
Each micrometer observation requires two measurements per set; 1) the disk
of Mars and 2) the width of the polar cap.
A typical observational period might require you to wait several minutes
to get one set of measurements. At least eight or ten measurement sets
should be made during any observational period. So, this could take up
quite a long time for an observer and may seem more like work than having
fun [Beish at al, 1986].
The typical CCD camera allows exposures of Mars in tenth's of a second as
opposed to 2 to 5 seconds for the film method and is ready to take the next
image right away. So, one doesn't even have to wind the film, wait for the
telescope to stop vibrating, then wait for that moment of good seeing to
take an image. With the CCD, you just shoot away and take as many images as
desired. Surely some of the images will be exposed in the moments with
steady seeing.
As stated above, the CCD camera chip records images into thousands of
cells, called "pixels," that can be stored on the hard disk of your
computer. This image array can be used to analyze every pixel of the
planet's image, including the background sky. If the image was taken in
steady sky with a very short exposure you can count the pixels at each limb
of Mars' image -- including the edges of the polar cap.
One additional advantage here is that while using a CCD camera one may
take many more images of Mars during an observational period than can be
taken using conventional film techniques. After all, one has to use longer
exposures times, then wind up the film to the next frame and align the
image again, then wait for the moment of good seeing. Good seeing may not
last as long as the exposure, so for a typical 2 to 5 second exposure you
may get some of the image blurred and some clear.
POLAR CAP MEASUREMENTS FROM A TYPICAL CCD IMAGE
Regardless of which method is used systematic errors are reduced
significantly when using CCD images to measure Mars' polar caps. A typical
set of CCD images of Mars taken in fair to good seeing conditions easily
replaces the time and effort used at the telescope using a micrometer.
Also, the reduction of the images can be done in the comfort of your home
instead of peering at Mars at the micrometer eyepiece.
CCD images of Mars can be taken with a deep red filter to cut
through the atmospheres of both Earth and Mars. Using a typical image
processing program the image is rotated on the screen so the disk is seen
pole to pole relative to the image frame. The cursor is placed at the
north and south edge or limb of the image and the pixel positions of each
are recorded. Next, read the cursor/pixel position of the east and west
edges of the polar cap.
Now, one has only to count the number of pixels between the extremes of
the image and apply this to the desired equations for determining the
latitude of polar cap boundary. So, one has only to record the two pixel
locations to determine the distance between the features on Mars.
Taking the difference of the positions, apply these values to the proper
conversion equations to determine the latitude. ALPO uses the following
equation:
Latitude = arccos (C/D), (1)
where C is the breadth of the cap and D is the apparent diameter of the disk.
The results in a straight forward latitude of the edge of each side of the
polar cap. Co-latitudes can be found by the equation: arcsin (1 -C/D). This
method and calculation has proven quite good and produces less than 0.5
degree systematic errors [Beish, et al, 1986].
Now, with the extent of the north-south of the disk in pixel positions
(178,10) and extent of the east-west positions (120,76) we can take the
difference: 178 - 10 = 168 and 120 - 76 = 44. Applying the differences or
distances between extents to equation (1):
Latitude = arccos (C/D)
= arccos (44/168)
= 74.8 degrees
SUMMARY
From the early 1960's ALPO observers measured Mars' polar caps to
understand the polar regions and Mars' atmospheric behavior. While the
author feels this tedious work is more suited for the professional
observer, never the less the job is important enough to fascinate amateur
planetary observers. Also, results could possibly help solve some of the
mysteries of the Red Planet Mars.
Several methods were employed in measuring the latitudes Martian polar
caps. The Bi-filar, reticule, or other types of wire and optical
micrometers were used. Photograph plates were used. ALPO researchers found
that micrometer measurements were more accurate and produced fewer
systematic errors than by using photographs. However, even this method is
limited by the effects of human errors, equipment handling, and effects of
weather on the observer.
Difficulties arose from using photographs because the Earth's atmosphere
tends to blur and move the image on the film plane. Also, the time
required to take the images, wind the film and wait until the typical
amateur telescope settles down wastes valuable time.
This author missed numerous potentially great Mars images while waiting
for the telescope to settle down, refocusing, and other problems Murphy's
law dealt me causing something to go wrong at the right moment. Good seeing
also has a way of coming and going at the worst possible moments.
Compare the above with the CCD camera. One only has to find the image and
obtain the desired focus on the computer monitor then settle down in a warm
room. Wait until periods of good seeing -- then shoot. If one is
experienced with the CCD system then ten or more images can be taken while
the film camera method is still waiting for the shutter to close.
Of course, this is obviously not the whole story. The CCD camera is
subject to electronic component noise, pixel defects, and other problems.
Image processing programs can be used to take out noise or defects and
enhance the images way beyond the dreams of the most planetary
photographers. You still have the same telescope and seeing problems. Some
of these problems can be taken care of with the proper techniques before
and after the observing session begins, flat fields, etc., but that is a
subject for a more technical discussion on CCD technology.
During the past three apparitions more and more observers are employing
the CCD camera to record Mars. The result has been more useable and higher
quality images of Mars being received by the ALPO Mars Section.
Our experience indicates that it takes a considerable time to do all this,
and sometimes even more than the other two methods discussed above.
However, when one considers we can produce larger images in a shorter
amount of time, and measurements can be done in the comfort of the
laboratory instead of the observatory, is becomes obvious that CCD
technology offers advantages over other methods discussed herein. Now,
measuring the Planet Mars for science can be fun!.
References
Capen, C.F, and Capen, V.W., "Martian North Polar Cap, 1962 - 68," Icarus,
13, No. 1, July 1970, 100-108.
Beish, J.D., D.C. Parker, and C.F. Capen, "Calculating Martian Polar Cap
Latitudes," J.A.L.P.O., Vol. 31, No. 7-8, April 1986.
Dobbins, T. A., D.C. Parker, and C.F. Capen, Introduction to Observing and
Photographing the Solar System, Willmann-Bell, 1988, 145 - 147.
Beish, J.D., "Systematic Errors in Polar Cap Measurements," The Martian
Chronicle, May 1994.
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A calendar of events for the coming Mars apparition can be found in
the previous IMW newsletter, at:
http://astrosun.tn.cornell.edu/marsnet/imw/imw3.6.html
Next newsletter: Another article by Jeff on the role and importance of
amateur observations of Mars...
For more information on starting your own Mars observing program,
or contributing to the scientific study of Mars, check out the A.L.P.O.
WWW home page at:
http://www.lpl.arizona.edu/~rhill/alpo/mars.html
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Other Useful WWW sites:
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Latest MGS images:
http://www.msss.com/mars/global_surveyor/camera/images/index.html
Main MGS Home Page:
http://mars.jpl.nasa.gov/mgs/index.html
Mars-98 Orbiter:
http://mpfwww.jpl.nasa.gov/msp98/orbiter/
Mars-98 Lander:
http://mpfwww.jpl.nasa.gov/msp98/lander/
Mars-98 MVACS Science Payload Home Page:
http://mvacs.ess.ucla.edu/index.html
New Millenium Mars Microprobe Mission (DS2):
http://nmp.jpl.nasa.gov/ds2/
Pathfinder Home Page:
http://mars.jpl.nasa.gov/default.html
JPL Mars Missions Page
http://www.jpl.nasa.gov/mars
Mars-01 and Mars-03 APEX/Athena Science Payloads
http://astrosun.tn.cornell.edu/athena/index.html
A.L.P.O. Mars observations:
http://www.lpl.arizona.edu/~rhill/alpo/mars.html
1996-97 Marswatch highlights:
http://mpfwww.jpl.nasa.gov/mpf/marswatch.html
1996-97 Marswatch ftp site:
http://marswatch.tn.cornell.edu/marsidea
MarsNet:
http://astrosun.tn.cornell.edu/marsnet/mnhome.html
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I will continue to maintain the email distribution list as well
as the various Cornell and JPL Marswatch-related WWW archives. I
am working with the Astronomical League and the JPL Mars-98 Project
to set up a WWW site for uploading and downloading of 1999 Marswatch
images. More on this soon...
If you are receiving duplicate copies of this mailing, or you want
your name removed from the distribution list, please send me email.
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Jim Bell
Cornell University
Department of Astronomy
Center for Radiophysics and Space Research
402 Space Sciences Building
Ithaca, NY 14853-6801
phone: 607-255-5911; fax: 607-255-9002
email: jimbo@marswatch.tn.cornell.edu
WWW: http://marswatch.tn.cornell.edu
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