Stunning amateur photographs of the solar system’s planets have been common in magazines and Internet sites but Pluto photographs are comparatively rare. Many amateurs like myself probably don't realize Pluto may be within reach of their telescopes. My challenge was to see what it would take to image Pluto with my amateur astronomy tools.
First a little background on my Pluto challenge. Long ago as a young boy I read the story of Pluto’s discovery in a book titled "The Search for Planet X". I don’t remember the author but I’ve always remembered the tale of Clyde Tombaugh’s tireless search for the suspected "Planet X". It wasn’t until Tombaugh’s discovery on January 23, 1930 that our view of the solar system grew from eight to nine planets. With a diameter of 1454 miles Pluto is a bit smaller than the Earth’s moon. All planets with the exception of Pluto orbit the Sun in a common plane known as the “plane of the ecliptic”. The plane of Pluto’s orbit is inclined 17 degrees to the plane of the ecliptic. Pluto is approximately 3.6 billion miles from the Sun (that’s 5.4 hours travel time at the speed of light!). At that distance Pluto is pretty darn cold at –390 degrees Fahrenheit (F). For comparison dry ice is -109 degrees F and liquid nitrogen is -321 degrees F. Pluto’s dim light cannot be seen with the human eye. It has one moon called Charon. Somehow astronomers determined the length of a Plutonian day to be 6.4 Earth days. Pluto’s orbit around the sun takes 248.5 Earth years. These days Pluto traverses an area of the night sky that it hasn’t visited since 20 years prior to signing of the United States Declaration of Independence! NASA has plans to send a science mission to Pluto. If the launch occurs on schedule in 2006, we may know more about Pluto in 2015.
To imagine Pluto take the Earth’s moon, chill it down to an unimaginably cold temperature, push it out towards the constellation of Ophiuchus to a point in the solar system that will take its moonshine 5 hours to get back to us and will only be viewable through a telescope. Although not as visually exciting as the rare Venus Transits or the passing of Halley’s Comet, photographing Pluto any evening offers a celestial snapshot that can’t be repeated in many lifetimes!
My search was quite different from Clyde Tombaugh’s. I had the advantage of state of the art "amateur" astronomy equipment: Charged Couple Device (CCD) astronomical camera, computer controlled telescope and a research grade computerized star catalog. Where Tombaugh had calculated guesses where Pluto might be, my telescope’s computer calculated precisely where Pluto is at the touch of a button. If Tombaugh spotted suspicious objects in his photographic plates he had to manually compare them to catalogued archives. I can take photographs and have the computer scan them for suspicious objects. In fact I could be indoors sleeping while the computer and telescope worked for me! Amateur astronomy technology is remarkably advanced since Tombaugh’s discovery but even so Pluto’s dim light and distance makes it a challenging solar system object to observe.
My Pluto challenge began on the evening of July 7th. I planned to use my telescope’s computer to point to the star field containing Pluto, take a series of CCD images (digital photographs), wait a number of hours and take a second image series of the same star field. Guided telescopes counteract the rotation of the Earth and effectively freeze stars in the telescope’s field of view (FOV). Stars are very, very distance objects and as a result do not move within the FOV over time. However, objects within the solar system are relatively close and always in motion around the Sun. If photographs taken at different times include solar system objects such as asteroids or comets there’s a very good chance that their position will change relative to background stars. In fact, some objects pass so close to the Earth that their motion can be observed in a single photograph. Recall that Pluto is at the outer fringes of the solar system. It’s orbital motion against the background stars is minute but observable. The trick is to wait several hours between photographs an compare them for changes.
Clyde Tombaugh used a method called "blink comparison" which overlaid glass photographic plates of the same star field taken at different times. The forward and back switching produced a blinking effect for any objects whose position had changed. Blinking is also used to sense changes in brightness. As a result blinking is a common technique used to search for other celestial events such as supernovae.
My first attempts involved six 60 second CCD images. My computer program combined the images to produce the equivalent of a six minute exposure. The combined exposure below yielded a rich star field that I expected to contain Pluto.
The next step was to compare the star field to a computerized catalog. I could do this step manually but there’s a faster way called “plate solving”. Plate solving is a process of comparing two or more stars in the image to a catalog of stars with known positions. All I had to do was tell the star catalog roughly where I thought the image’s star field was located and let the computer do the hard work. Even better, I could instruct the star catalog to locate Pluto and the surrounding star field. The star catalog would then display a field of stars, their coordinates and where Pluto is located within the field. I then overlayed my image and asked the computer to plate solve the image. Once plate solved, the star catalog would directly aid the search for Pluto (lower left) within the CCD image.
Even so, computers and people can make mistakes. I wanted to be sure I had Pluto so I planned to wait several hours and repeat the process. There would be no thrill without definitive motion against the background stars. A couple hours later, clouds moved in rapidly and I scrambled to gather a quick image set but it was not to be. The second set resulted in poor quality hazy stars and terrible contrast. I would have to try again another night. As it turned out the July weather combined with my schedule produced a long delay.
My next attempt was July 30th under the blaze of a full moon. I had a short window of dark sky to capture an image set before the moon rose above the horizon. The set went very well and produced a good plate solve. The star catalog indicated that Pluto was in the star field.
I had done a little research on Pluto’s rate of movement against the background stars and expected to sense some motion in 3 or more hours. At 1:00am I was competing with a bright full moon. My second image set was successful but the background contrast was poor due to the moonshine. I blink compared the 1:00am with the 10:30pm images. A slight flicker occurred. I had it…I thought. Was it a flicker or a noisy digital hiccup on the image? There were other changes throughout the image. I couldn’t be sure I had Pluto yet. I had to work the next day. I needed sleep. There’d be other nights.
The next day, I watched the sky carefully. It was thick with clouds most of the day. I was tired and a little glum over the previous night’s missed opportunity. Towards sundown the clouds began to part. By 10:00pm the sky was crisp, clear and I was at it again! By 10:30 I had a new set of images. I didn’t need to wait 3 more hours as I could compare images from the previous night. This provided an ample 23 hours for Pluto to move against the background stars. After a bit of image adjustment the blink comparison fired up. Wow! There was a definite hopping pinpoint of light that could only be Pluto!
Considering the many technological advancements aiding my search I can only applaud Tombaugh’s persistence. Although my challenges are usually work and weather related I will revisit Pluto from time to time for those one of a kind photographs.
Epilogue: After I finished this article, my wife discovered that the story of Clyde Tombaugh’s search is still available through Amazon and possibly other used bookstores. The book title is “Clyde Tombaugh and the Search for Planet X” by Margaret K. Wetterer & Laurie A. Caple. It’s written for Grades 4-8. Maybe I’ll read it again for old times sake.
Some interesting web addresses: