Linking Amateur and Professional Mars Observing Communities.
Volume 3; Issue 7
December 21, 1998
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:
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,
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!
By: Jeffrey D. Beish
Former Mars A.L.P.O. Recorder
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 usable 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 a 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; first, the disk of Mars and then 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
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 usable 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!.
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.
Jim Bell will continue to maintain the email distribution list as well as the various Cornell and JPL Marswatch-related WWW archives. If you are receiving duplicate copies of the International MarsWatch Electronic Newsletter, or you want your name added to or removed from the distribution list, please send him an email at firstname.lastname@example.org.
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