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|Publisher:||University of Alaska Press|
|Edition description:||New Edition|
|Product dimensions:||10.90(w) x 10.60(h) x 1.10(d)|
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Coloring the Universe
An Insider's Look at Making Spectacular Images of Space
By Travis A. Rector, Kimberly Arcand, Megan Watzke
UNIVERSITY OF ALASKA PRESSCopyright © 2015 Travis A. Rector, Kimberly Arcand, and Megan Watzke
All rights reserved.
HUMAN VERSUS TELESCOPE
Comparing Telescopic Vision with Human Vision
SEEING IS BELIEVING
Images of space can inspire awe and wonder. But in many urban and suburban spaces, the light from stars is mostly drowned out by the artificial glow (called light pollution) that humans have created. This is one reason that the pictures taken by telescopes on the highest mountaintops and remote deserts on Earth, as well as the armada of observatories in space, are so important. They are dramatic windows into our Universe.
The popularity of cosmic images is easy to spot. We see space pictures on computer screens and tablets, splashed on billboards, album covers, clothing, and everywhere in between. Despite our attraction and connection with space images, many people are not sure that what these images show is real. When one of our images was recently featured on a blog, several commenters were skeptical. "Really? Not Photoshopped? Amazing." Another person commented in Spanish, "Me estas jodiendo, no puedo creer que no esté trucada." (Rough translation: "Are you kidding me? I don't believe this hasn't been modified.") Others express doubt that we can even see that far away.
These are important questions for us to ask and, just as critically, ones that need to be answered. So where and why does the confusion, or even controversy, arise?
Let's look at an example. For many space aficionados, the picture on the previous page is iconic. It captures a famous object called the Horsehead Nebula, which gets its name from the distinctive dark shape at the center of the image. It is part of a large cloud of gas and dust called a nebula, found in the constellation of Orion, where hundreds of stars are being born.
The image was taken with an advanced digital camera from a telescope at the Kitt Peak National Observatory (KPNO) in southern Arizona. This is what the telescope and its camera can see. But let's pretend you had the ability to board a spaceship and fly to the Horsehead Nebula — what would you see? After a journey of more than a thousand light-years, you would finally arrive at your destination. You look out of the window of your spaceship at this same scene, but you're now at a distance one hundred times closer than before, when you were standing on the Earth. The image on the right shows what you would see.
You'd see some of the brighter stars but none of the dust and gas in the nebula, including the horsehead shape. Why?
THREE THINGS A TELESCOPE DOES
To better understand what's going on, it helps to know what a telescope does. Just as a pair of binoculars can make the upper-level seats in an arena almost as good as courtside, a telescope can make a distant object appear much closer. But a telescope does more than this. It doesn't just magnify an object; it also amplifies it. It makes something faint appear much brighter.
Some people might think that the reason why a telescope can see objects our eyes can't is because it magnifies something that is too small for us to see. And this is often true. But the Horsehead Nebula is actually not that small. The fields of view of these images of the Horsehead are about twice the size of the full Moon in the sky. You can't see it because it is too faint, not because it is too small.
Why couldn't you see the Horsehead Nebula even if you were much closer? For objects that appear to be larger than a point of light (for example, galaxies and nebulae, but not stars), how bright it appears has nothing to do with how far away it is. Moving closer to it will make it bigger, but not brighter. This may seem counterintuitive, but you can try it at home. Walk toward a wall. As you approach you'll notice that the wall is getting bigger but otherwise is the same brightness. The same is true of the Horsehead Nebula. If you can't see it with your eyes while standing on Earth, you still won't see it from your spaceship.
Why then can a telescope see it? A telescope offers several advantages over our eyes. As marvelous as the human eye is, it's not that well suited for nighttime observing. First, our eyes are tiny. The opening that allows light to enter, known as the pupil (the black area at the center of the eye), is only about one-quarter of an inch wide when fully open.
In comparison, the mirror that collects light for the Gillett Gemini North telescope, at one of the professional observatories atop Mauna Kea in Hawai'i, is about 27 feet across. What this means is that, at any given moment, this mirror is collecting more than a million times more light than your eye. The more light you can collect, the fainter an object you can see.
Human eyes also don't collect light for long. Our eyes function like a video camera, taking images about thirty times every second. So the exposure time for each image captured by the human eye is only one-thirtieth of a second. With digital cameras attached to the telescope we can collect light for as long as we like. The longer the exposure, the more light the telescope collects.
Typically a single exposure is not more than ten to twenty minutes, but multiple exposures can be added together to make a single image with an exposure time that is, in effect, much longer. To create the most sensitive image ever made, astronomers collected over fifty days worth of observation time with the Hubble Space Telescope of a single portion of the sky. Known as the Hubble eXtreme Deep Field (XDF), this image represents a cumulative exposure time of about 2 million seconds.
As we'll talk about more in chapter 7, the human eye is complex. It isn't as sensitive to faint light, and it only detects amounts that are above a certain threshold. To prevent confusion, our brain filters out the "noise" below that level. In comparison, modern electronics detect nearly all of the light that enters a telescope's camera, even if it takes hours to collect the light. All of these factors enable telescopes to go far beyond the limits of human vision. The faintest objects in the XDF are about 10 billion times fainter than what the human eye can see.
Finally, the Universe and the amazing objects in it glow in other types of light — from radio waves to gamma rays — that are impossible for our eyes to see. We'll talk about these kinds of light, and the telescopes that observe them, in chapter 10. It's taken the ingenuity of scientists and engineers over the course of many decades to develop our abilities to capture the views of the Universe that we enjoy today. Without these technical tools, many phenomena and objects would simply be invisible to us entirely.
It's no exaggeration to say that telescopes give us superhuman vision. Nearly every astronomical image contains objects too small and/or too faint for us to see. And these images often show us kinds of light our eyes can't detect. How do astronomers take what the telescope sees and convert it into something we can see? Are these images showing us what the Universe really looks like? Turn the page and let's find out.CHAPTER 2
THIS IS NOT A SELFIE
How Telescopes and their Cameras Work
When a news story reports something like, "Astronomers have taken a picture of a new galaxy," it's easy to imagine scientists pointing a camera toward outer space, pressing a button ("click!"), and the image appears on the back of the camera. Perhaps even a flash goes off (it is nighttime after all). In reality, taking an image of a cosmic source is not the same as taking a picture with your phone.
HOW A "VISIBLE-LIGHT" TELESCOPE WORKS
As we mentioned in the last chapter, there are many different kinds of telescopes that detect different types of light — everything from radio waves to gamma rays and a bunch in between. In chapter 10 we'll talk about these other kinds of light. But we're going to start first with how a telescope looks at "visible" light, which is the small slice of light we can see with our eyes.
Starting with visible-light telescopes is useful because they are the most familiar. If you have ever seen a telescope on top of a mountain, it probably detects visible light. The telescope you can buy at a toy store, science center, or a planetarium? Also a visible-light telescope. And, the most famous telescope in the world — the Hubble Space Telescope — can observe in visible light (and then some, as we'll talk about later).
Telescopes are sophisticated pieces of technology and engineering, but they can all be broken down into some simple pieces. Visible-light telescopes have either lenses or mirrors, or some combination, to collect and redirect light. To where does the telescope send this collected light? On a simple backyard telescope, it is sent to your eye. On more complex telescopes, the light is delivered to a camera or detector that can capture and store it.
Another way to think about it is that the telescope itself acts as a giant telephoto lens. In short, a telescope is a device that collects light from a distant object. The camera then uses that light to make an image. For this reason, telescopes are often referred to as "light buckets." (Remember, the more light you can collect, the more you can see. This is why astronomers strive to build bigger telescopes — to have, in essence, bigger buckets for collecting light.)
There are many ways to build telescopes. And over the years scientists and engineers have come up with some ingenious designs. For ground-based telescopes, the challenge is to build them with high-quality optics that function well in a wide range of conditions. Telescopes must be built onto sturdy mounts that are capable of holding the telescope stable in winds of up to 40 miles per hour. If the telescope shakes, the image will be blurry. Because the Earth is rotating, telescopes are also designed to carefully track the night sky as it slowly moves. Without this tracking, stars would appear as streaks across the image. To prevent these streaks, telescopes monitor a guide star, a bright star in the telescope's field of view. Modern telescopes have computers that carefully control and adjust the telescope's movement to keep the guide star in the same spot in the image.
For telescopes on the ground, another issue is that the Earth's atmosphere can make an image blurry. This happens because of the motion of warm and cold air in our atmosphere. It's the same effect that causes objects to look distorted when seen through the hot air rising off of a hot road or from behind a jet engine. It's also what makes stars "twinkle." Astronauts in space don't see twinkling stars.
Fortunately there's a technique, known as adaptive optics, that can correct for most of the atmosphere's blurring effects. An adaptive-optics system makes extremely rapid but slight adjustments to the shape of the telescope's mirrors to counteract the blurriness our atmosphere causes. A computer calculates adjustments to the telescope's mirrors as it monitors a bright reference star. Some adaptive optics systems create their own reference star (or stars) by shining a laser into the sky in the same direction that the telescope is looking. The laser energizes sodium atoms in the atmosphere about 60 miles above the Earth, creating an artificial reference star. By using the reference star as a benchmark, the adaptive optics system knows how much to correct for the effects of the atmosphere.
Adaptive optics can improve the resolution, or "sharpness," of an image by a factor of five times. This enables telescopes on the ground to achieve images as sharp — or sharper — than Hubble. Currently, adaptive optics systems only work well for observations of near-infrared light, which falls just beyond what the human eye can see. So Hubble is still the champion for high-resolution images with visible light.
STARLIGHT, CAMERA, ACTION!
Whether using adaptive optics or not, the goal of the telescope is to bring the light to the camera. The camera is attached to the telescope at a location where the collected light comes into focus. Some telescopes allow for more than one camera to be attached, although usually only one can be used at a time.
The purpose of the camera is to capture that light and assemble an image from it. The digital cameras on a telescope are, at the most fundamental level, essentially the same as the digital cameras you own. In fact, the technology in everyday digital cameras was first developed and used on telescopes. At the heart of any digital camera — on a telescope or not — is its detector, the piece of hardware that actually captures and stores light.
Digital cameras use detectors that are made of semiconducting materials that are sensitive to light. They work in a way similar to a solar panel, which generates electricity when light hits it. A detector is divided up into millions of little squares, each one called a pixel. Each pixel is its own solar panel, and you can measure how much light has hit the pixel by measuring how much electricity it has generated. And by measuring the light collected by each pixel you can assemble an image of all the light that hit the detector.
When an exposure is taken, the camera's shutter is opened and light from the telescope is allowed to go into the camera. As the light hits the detector it generates electricity in each pixel. After the exposure is done, the shutter is closed and then the electronics in the camera measure how much electricity is in the individual pixels. This can take from several seconds to several minutes depending on the camera.
It's difficult to build detectors that are large, so many astronomical cameras have more than one detector in them. It's like having more than one camera look at the sky at once, allowing you to either see a larger portion of the sky or see objects in greater detail (or both).
CALIBRATING THE CAMERA
Imagine you turned on your television to watch a football game and everything was distorted. The sky was yellow, the grass was blue, and the people were green. You'd know that something was wrong with your TV because normally the sky isn't yellow (unless the game is in Los Angeles), the grass isn't blue (unless you're watching Boise State play football), and people aren't green (unless you're in Roswell, New Mexico).
But what if you were looking into space at something you've never seen before? You might not know anything was wrong. For this reason, astronomers use a variety of tools and techniques to carefully calibrate their cameras. They want to be certain what we're seeing is real and accurate. These calibrations are mostly done during the daytime, when the telescope and camera are put through a range of tests to characterize how they're going to work on that particular night. After the night's observations are done, the calibrations are used to correct for known imperfections in the telescope and camera.
Now we've seen how telescopes, along with their cameras and detectors, let us capture far more light than we ever could with just our eyes. Telescopes are incredible tools that, when carefully tuned and maintained, can give us superhuman sensitivity to objects and phenomena in space. However, just as a canvas and paints cannot make art without an artist, neither can a telescope produce images of the cosmos without the astronomers who use them. Now let's explore how telescopes are used to make color images.CHAPTER 3
COLORING THE UNIVERSE
Broadband Images and How We Use Color
Digital cameras are remarkable tools for detecting faint light from stars and galaxies across the Universe. But there's one problem. Measuring the electricity in a pixel tells you the intensity of the light but not the color. In other words, digital cameras can only see in black and white (or, more precisely, gray scale). How then do we get a color image? This is an important difference between the cameras you own and the ones used on telescopes.
SHOW YOUR TRUE COLORS
First off, what is color anyway? What does it mean for something to be, say, green? Even though most of us experience it every day, it's worth talking about the nature of the light we see with our eyes, which is known as "visible" light. When you take visible light and spread it out into a spectrum (using a prism, for example, or looking at a rainbow), the colors correspond to the energy of the light. Red light is the lowest energy kind of light we can see. Next up in energy is orange, then yellow, green, blue, indigo, and finally violet. Violet is the highest energy form of light we can see. You may have learned "ROY G. BIV" in school to remember this.
Excerpted from Coloring the Universe by Travis A. Rector, Kimberly Arcand, Megan Watzke. Copyright © 2015 Travis A. Rector, Kimberly Arcand, and Megan Watzke. Excerpted by permission of UNIVERSITY OF ALASKA PRESS.
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