In June of 1889, about one year before his untimely death, the brilliant Dutch post-impressionist, Vincent Van Gogh, furiously completed The Starry Night while staying at the Monastery Saint-Paul de Mausole, a mental asylum located in Southern France. The painting depicts a humble village nestled between the blue tranquility of undulating hills and a magical sky filled with comet shaped clouds and cartwheeling stars the size of Ferris wheels. Even though Van Gogh only sold one painting during his lifetime, this priceless work of art has become an icon. In it he captured a childlike wonder that adults can recognize for who has not stood outside and been swayed by twinkling stars celebrating overhead. Beautiful deep space images can elicit similar excitement from astronomical enthusiasts. However, the photographers who produce them are more interested in the stars when they are peaceful.
The Starry Night (1889) was not the only painting Van Gogh created depicting the night firmament. In fact, this canvas was not his favorite because it was’t as realistic as he had originally envisioned. For example, one year earlier he produced The Starry Night over the Rhone (1888) and Cafe Terrace at Night (1888). Both of these have common elements but each are also unique- the earlier versions include people and the stars take on a diminished role, for example. Nevertheless, all three of these works have captivated millions and each day hundreds of art lovers crowd around them, at their respective museums, making personal interpretations to themselves and others who will listen.
Interestingly, what makes memorable art can also lead to forgettable astronomical images. More specifically, the dazzling fireworks in each of Van Gogh’s paintings represent stars that are shimmering and twinkling.
We live at the bottom of an ocean of gasses primarily composed of Nitrogen (78%), Oxygen (21%), and Argon (1%) plus a host of other components including water (0 – 7%), “greenhouse” gases or Ozone (0 – 0.01%) and Carbon Dioxide (0.01-0.1%). It extends upward from the surface of the Earth to a height of about 560 miles. Seen from Earth orbit, our atmosphere appears as a soft blue glow just above our planet’s horizon. Every thing we observe that exists beyond our planet- the Sun, Moon, nearby planets, stars and all else, is viewed through this intervening medium we call the atmosphere.
It’s constantly in motion, changing density and composition. The density of the atmosphere increases as it approaches the Earth’s surface, although this is not at all uniform. It also acts like a prism when light transverses. For example, light rays are curved when they pass through regions of different temperature, bending toward the colder air because it’s denser. Since warm air rises and cooler air descends, the air remains turbulent and thus light rays from space change direction constantly. We see these changes as star twinkling.
Nearer the ground, cooler or warmer winds blowing horizontally can also create rapid air density changes that randomly alter the path that light takes. Thus, winds blowing from the four corners contribute to star jiggling, too. But, the air can also cause the stars to quickly shift focus thereby causing them to suddenly dim, brighten or change color. This effect is called scintillation.
Interestingly, the air can be in motion although we cannot feel its breezes- wind forces high above our heads can also cause the stars to shake. For example, the jet stream, a band of relatively narrow globe straddling currents located about six to nine miles up, is constantly changing its location. It generally blows from west to east, but its relative north-south position remains in a state of constant revision. This can result in highly unstable atmospheric conditions that cannot be sensed on the ground yet the jet stream will produce a sky filled with twinklers if it flows over your location!
Because planets are closer than stars, their size can be seen as a disk that is larger than the refractive shift caused by wind turbulence. Therefore, they seldom twinkle or do so only under the extreme conditions. For example, both stars and planets are viewed through much thicker layers of atmosphere when they are near the horizon than when they are overhead. Therefore, both will shimmer and dance as they are rising or setting because their light passes through much denser quantities of air. A similar effect occurs when viewing distant city lights.
The twinkling we see on star-studded nights is magnified hundreds of times by a telescope. In fact, twinkling can severely reduce the effectiveness of these instruments since all that can be observed are out of focus, randomly moving blobs of light. Consider that most astronomical photographs are created by holding the camera shutter open for minutes or hours. Just as you need to remind your subject to stand still while taking their picture, astronomers want the stars to remain motionless else their photographs are smeared, too. One reason observatories are located on mountaintops is to reduce the amount of air their telescopes must peer through.
Astronomers refer to the effect of atmospheric turbulence as seeing. They can measure its effect on their view of space by the calculating the diameter of photographic stars. For example, if the picture of a star could be taken with an instantaneous exposure, the star would, theoretically, appear as a single point of light since no telescope, to date, can resolve the actual disc of a star. But, taking stellar images require a long exposure and while the camera’s shutter is open, twinkling and scintillation will cause the star to dance around plus move in and out of focus. Since its gyrations are random, the star will tend to create a round pattern that is symmetrical on all sides of its true location in the middle.
You can demonstrate this yourself if you have a moment and are curious. For example, if you take a pencil or magic marker tied by a short string to a pin that is stuck into a piece of cardboard or very heavy paper, then swiggle the writing instrument about without removing the pin, over time you would create something that looks roughly like a circle. Your circular doodle will result because the string limits your maximum distance from the central pin. The longer the string, the larger the circle. Stars behave like this as their light is recorded on a long exposure photograph. Good seeing creates a short optical string (bad seeing makes the string longer), the star’s true location becomes a central pin and the star behaves like a writing instrument whose light leaves a mark on the camera’s imaging chip. Thus, the poorer the seeing and the more dancing that occurs during the exposure, the larger the disc that appears on the final image.
So, poor seeing will cause star sizes to appear larger in photographs than those taken during good seeing. Seeing measurements are called Full Width Half Maximum or FWHM. It is a reference to the best possible angular resolution that can be achieved by an optical instrument in a long exposure image and corresponds to the diameter of the star’s size. The best seeing will provide a FWHM diameter of about point-four (.4) arcseconds. But you would need to be located at a high altitude observatory or on a small island, like Hawaii or La Palma, to get this. Even these locations only rarely have this type of very high quality seeing.
Amateur astronomers are also concerned about seeing. Typically, amateur’s must tolerate seeing conditions that are hundreds of times worse that the best observed from remote astronomical installations. It’s like comparing a pea to a baseball in the most extreme instances. This is why amateur photographs of the heavens have stars that are much larger in diameter than those from professional observatories particularly when backyard astronomers use telescopes with long focal lengths. It can also be recognized in wide field, short focal length, non-professional images when they are enlarged or studied with a magnifying glass.
Amateurs can take steps to improve their seeing by eliminating the temperature difference between local sources of heat and the air above their telescopes. For example, amateurs often ready their instruments outside just after sunset and let the glass, plastic and metal in them become the same temperature as the surrounding air. Recent studies have also shown that many seeing problems start just above the telescope’s primary mirror. A constant, gentle current of air passing over the primary mirror has been demonstrated to significantly improve telescopic seeing. Preventing body heat from rising in front of the telescope also helps and locating the instrument in a thermally friendly location, such as an open field of grass, can produce surprising results. Open-sided telescopes are also superior to those with primary mirrors at the bottom of a tube.
Professional astronomers have seeing improvement strategies, too. But their solutions tend to be extremely expensive and push the envelope of modern technology. For example, since the atmosphere inevitably produces poor seeing, it’s no longer far-fetched to consider placing a telescope above it in Earth orbit. That’s why the Hubble Space Telescope was constructed and launched from Cape Canaveral aboard the Space Shuttle Challenger in April 1990. Although it’s primary mirror is only about one hundred inches in diameter, it produces sharper images that any telescope located on Earth, regardless of their size. In fact, the Hubble Space Telescope images are the benchmark against which all other telescopic pictures are measured. Why are they so sharp? Hubble pictures are not affected by seeing.
Technology has improved significantly since the Hubble Space Telescope was placed into service. During the intervening years since its launch, the US government has de-classified their method for sharpening the sight of spy satellites that keep tabs on Earth. It’s called adaptive optics and it has created a revolution in astronomical imagery.
Essentially, the effects of seeing can be negated if you nudge the telescope or change its focus in the exact opposite direction to the nasties caused by the atmosphere. This requires high speed computers, subtle servo motors and optics that are flexible. All of these became possible during the 1990’s. There are two basic professional strategies for reducing the effects of poor seeing. One alters the curve of the primary mirror and the other moves the light path that reaches the camera. Both rely on monitoring a reference star near the position that the astronomer is observing and by noting how the reference is being affected by seeing, fast computers and servomotors can introduce optical changes on the main telescope. A new generation of large telescopes is under design or construction that will enable ground based instruments to take space pictures that rival the Hubble telescope.
One method features hundreds of small mechanical pistons positioned beneath and spread across the rear of a relatively thin primary mirror. Each piston rod pushes the back of the mirror ever so slightly so that its shape changes enough to bring the observed star back to dead center and in perfect focus. The other approach used with professional telescopes is a bit less complicated. It introduces a small flexible mirror or lens located close to the camera where the light cone is relatively small and concentrated. By tipping or tilting the small mirror or lens in opposite unison with the reference star’s twinkling, seeing problems can be eliminated. The optical adjustments that either solution initiates are made constantly throughout the observing session and each alteration occurs in a fraction of a second. Because of the success of these technologies, enormous land based telescopes are now considered possible. Astronomers and engineers are envisioning telescopes with light collecting surfaces as large as football fields!
Interestingly, amateur astronomers also have access to simple adaptive optics. One company, headquartered in Santa Barbara, California, pioneered the development of a unit that can reduce the effects of poor seeing or mis-aligned telescope mounts. The firm’s adaptive optics devices work in conjunction with its astronomical cameras and use a small mirror or lens to shift the light reaching the imaging chip.
Astronomer Frank Barnes III was also concerned about seeing when he produced this striking image of a star cluster and nebula located in the constellation of Cassiopeia. It is a small portion of the Soul Nebula, which was designated as IC 1848 in J.L.E. Dreyer’s landmark second Index Catalog (IC) (published in 1908 as a supplement to his original New General and first Index compilations).
Frank reported that his seeing was favorable and produced star sizes with a FWHM of between 1.7 to 2.3″ over each of his thirty-one, thirty minute exposures. Note the size of the stars in this image- they are very small and tight. This is a confirmation of reasonably good seeing!
By the way, the colors in this picture are artificial. Like many astronomers plagued by local night-time light pollution, Frank exposed his pictures through special filters that only permit the light emitted by certain elements to reach his camera’s detector. In this example, red represents Sodium, green identifies Hydrogen, and blue reveals the presence of Oxygen. In short, this picture not only shows what this region in space looks like, but what it is made of.
It is also noteworthy that Frank produced this remarkable picture using a 6.3 mega-pixel astronomical camera and a 16-inch Ritchey-Chretien telescope between October 2 through 4, 2006.
Written by R. Jay GaBany