PH 332 The Hubble Space Telescope Parmelee
I. Introduction
The Hubble Space Telescope is the best tool to date to study the outer reaches of our universe. The research completed using this telescope has made transformed the view of the universe forever. Even though it is a multi-million dollar project it shares the same basic principles and design of telescopes that are priced in the thousands rather than millions. However, due to its superior manufacturing and placement in space it became the most useful telescope man has ever made.
To understand what the Hubble Telescope is we will first present the history and principles of telescope construction. Next the history of the Hubble Telescope will be explained along with applying the principles of telescope construction to describe the construction of Hubble Telescope. These go hand in hand because the physical issues that the Hubble Telescope has had explain how the Hubble became what it is today. Finally, prominence of the research made possible by the Hubble telescope’s images in modern physics will be shown with examples of the change in the fundamental understanding of its structure and speed that were not and are not possible with ground based telescopes.
II. General Telescope Construction and History
The first telescope was created in 1608 by a Dutch eyeglass maker named Hans Lippershey. The design was a simple setup of a two lenses [1]. The telescope allowed people to study objects outside of Earth’s atmosphere. This is done by using lenses or mirrors to collect more light than the human eye alone can and focus it near the eye or other imaging device so that the object appears larger [2]. Early telescopes are similar to modern microscopes and very different from modern telescopes. Early telescopes are made of multiple optically clear convex lenses to focus the light (figure 1) and are called refracting telescopes. Although the common design is to use two convex lenses some of the early telescopes made the objective lens a concave lens [1].
Figure 1. Artistic representation of how lenses bend light to magnify objects. Source: [3].
As the amount of magnification in telescopes increased two main types of imaging issues with lenses became apparent [4]. The first is chromatic aberration, this because different wavelengths of light have different indices of refraction. This difference in the index of refraction causes each wavelength of light to have a different place where its part of the image will focus. The difference in the placement of the focused image results in a blurring of the image. The second is spherical aberration, which is due to a
2/7 PH 332 The Hubble Space Telescope Parmelee difference in the focus point of light from the inner to the outer edge of the lens. This distortion of the image causes blurring of the image and as the lens gets larger the distortion increases. [4]
Spherical mirrors can also be used to magnify an object; the principles of mirror magnification are the same as for lenses. The benefit of using mirrors is that chromatic aberration is no longer a problem because the light does not pass through them. However, they are still susceptible to spherical aberration if not manufactured correctly. A telescope using mirrors (figure 2) is called a reflecting telescope [4].
Figure 2. Artistic representation of how mirrors reflect light to magnify an object. Source: [3].
III. Hubble Telescope Construction and History
While the Hubble Telescope is fairly young, the concept was first proposed in 1946 by Lyman Spitzer. Spitzer was a researcher at Yale University and argued that by putting a telescope in space there would be many advantages over a ground based telescope. This is because of the Earth’s atmosphere which blurs, distorts, and filters light before it reaches the ground. Therefore by putting it into space the images will be improved as compared to even the most advanced earth based telescopes [5]. It was not until the 1977 that the possibility of a space telescope became a reality when Congress approved funding for the Large Space Telescope program that created the Hubble Telescope. The program was a joint project between the National Aeronautics and Space Administration (NASA) and the European Space Agency. Thirteen years later on April 24, 1990 the Hubble Space telescope finally launched to orbit aboard a NASA space shuttle after many delays had delayed the launch by seven years. This delay was due to delays in the manufacturing of components such as optics, and an explosion of a Space Shuttle putting a hold on all launches for several years [5].
The Hubble Telescope is an extremely high precision reflecting telescope whose optical design is a Ritchey-Chretien Cassegrain. This design is a simple two mirror setup with the openings for the instruments to record what the telescope is observing [6]. (figure 3). According to [6] the mirrors are ground so that no part of the mirror is more than 1/800,000 in off from a perfect curve. To visualize this, if the mirror was scaled to match the curvature of the earth the largest bump would be about six inches tall.
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Figure 3. Artistic representation of the layout of the mirrors in the Hubble Space Telescope. Source: [7].
While it was believed that the optics were perfect it was discovered that the primary lens was slightly mis- ground. The mirror was “about 1/50th the thickness of a sheet of paper” off from perfect causing spherical aberration [2]. To correct this NASA sent a crew of astronauts into space to install an additional set of optics called the Corrective Optics Space Telescope Axial Replacement or COSTAR for short to adjust for the aberration and remove one of the original sensors to make space for it.
The Hubble Telescope takes pictures of more than just the visible range of light with a total of different six sensors. This is done because many different ranges of light can be used to provide different information about the part of space being viewed. Each sensor has a different operating range and sensitivity level allowing each part of spaced being viewed at a time to be studied in multiple ways.
First, the Wide Field Camera 3 can see ultraviolet or near-infrared light, selectable be which mode out of two it is in. When paired with other instruments it allows the Hubble to have wide range of operation. Additionally it has a higher resolution and wider field of view than the older Wide Field Camera 2, which it replaced, allowing researchers to better study dark matter, star formation, and galaxies out of range of the other instruments [8].
Second, the Cosmic Origins Spectrograph than can only see in ultraviolet range and separate the light into component colors and measures their intensities. The intensities of the different colored light enables researchers to determine its density, motion, chemical composition and temperature. This sensor is designed to study stars, quasars and other points of light. The installation of this sensor improved the telescopes sensitivity to ultraviolet over ten times compared to before its installation [9].
Third, the Advanced Camera for Surveys views ultraviolet and visible light and consists of three different cameras, each of which serves a unique function. The wide field camera views a large portion of the universe at once from ultraviolet to visible light to study early activity of the universe. The high resolution camera was used in the hunt for and study of planets and possible planets; this camera was rendered inoperable by an electrical short. The solar blind camera views only the ultraviolet spectrum and blocks visible light to further enhance the ultraviolet sensitive to view stars and planets that radiate ultraviolet light [10].
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Fourth, the Space Telescope Imaging Spectrograph can see from ultraviolet to near infrared light and is often used in the hunt for black holes. Its operation is nearly the same as the Cosmic Origins Spectrograph except for a wider field of view and range of wavelengths of light [11].
Fifth, the Near Infrared Camera and Multi-Object Spectrograph views the infrared range of the light spectrum with use of three different camera each of which has a different field of view. Since this views the infrared range of light and warm material emits infrared light, the sensor must be kept below -321° Fahrenheit to minimize interference from its own heat. The infrared capabilities of this sensor allow researchers to view objects hidden by interstellar dust and into deep space [12].
Sixth, the Fine Guidance Sensors view the exact location of specific starts and keeps the Hubble pointed in the desired direction and can measure the distance between stars and their relative motion. There are three total sensors in this instrument: two are used for precise positioning of the telescope, the third is for measurements. This allows scientists to do anything requiring precise measurement on top of taking a picture for an extended period. Two example of this are: measure the size of stars, and find stars that wobble which indicates it has orbiting objects. [13]
In the time from the COSTAR installation to present, all of the onboard instruments have had their own corrective optics installed making COSTAR obsolete, so it was removed and brought back to earth. These modifications have been made over four separate repair operations. These four service trips to the Hubble served the purpose of routine maintenance, replacing broken solar panels, and to upgrade the sensors. [2]
IV. What have we learned because of it?
The information discovered from the Hubble Telescope has drastically changed peoples perspective on the universe in many ways including the age and size of the universe, our own solar system, black holes, galaxies and more. [14] The Hubble Telescope has more than “10000 scientific papers attributed to it” which makes it “by some measures the most productive scientific instrument ever built” [14]. This paper is not long enough nor is it meant to cover in detail but rather it is meant explain some key discoveries to show the importance of the telescope to modern physics.
One of the primary goals of the Hubble Telescope was to determine the age and size of the Universe. While the exact size is unknown, its age is estimated at about 13.7 billion years old. In addition the rate of expansion of the universe has been studied in great detail leading to a ground breaking discovery that the rate of expansion is accelerating and not slowing down as previously suspected and this acceleration. This discovery earned the 2011 Nobel Prize in Physics for three astronomers who were studying supernovas and realized the data they measured did not fit what they expected from the universe. Resulting from the breakthrough it has been hypothesized that the universe will never stop expanding. [15]
Without the Hubble Telescope it would be impossible to detect black holes even though they have been part of scientific theory for centuries [16] The Hubble gave scientists the ability to produce clearer images of the area surrounding suspected black holes than earth based ones. This was critical because black holes cannot be seen, only their effects on their surroundings can be seen. An example of these effects is a cloud of dust and gas centered at a single point in space (figure 4). The Hubble gave strong evidence that black holes are at the center of all large galaxies and the size of the black hole increases as the size of the galaxy increases.
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Figure 4. Image taken from Hubble Telescope showing a disc of dust and gas circling and coalescing around a black hole. Source: [16].
These are just two of the thousands of discoveries of the universe that are a direct result of the Hubble Space Telescope. Just these two discoveries changed much of what has been supported and taught about the universe over the course of human history.
V. Summary
Using the basic principles of optics that have been around for centuries combined with modern manufacturing and computers scientists were able to create the most useful telescope known to man. While it is not the biggest, the Hubble’s placement in space made it superior to even the most advanced ground-based telescopes. Additionally, even though primary lens had an improperly ground section the creativity of scientists who added corrective optics transformed the information captured in images to be beyond what anyone could have expected. To date the Hubble has allowed some of the greatest breakthroughs in Astronomy and Physics of the Universe giving us the ability to better understand the structure of the universe that we are only a small part of.
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References
[1] Lauren Cox. (July 13, 2013). “Who Invented Space?”. Available: http://www.space.com/21950-who- invented-the-telescope.html
[2] Hubble Site. "Hubble Essentials”. Available: http://hubblesite.org/the_telescope/hubble_essentials/
[3] Michael Sitko. “Mirrors & Lenses”. Available: http://www.physics.uc.edu/~sitko/CollegePhysicsIII/23-Mirrors&Lenses/Mirrors&Lenses.htm
[4] Randall D. Knight, “Optical Instruments,” in Physics for Scientists and Engineers, 2nd ed. San Francsico, CA: Pearson Education Inc., 2008, ch. 24, pp. 739-756.
[5] National Aeronautics and Space Administration. “A Brief History of the Hubble Space Telescope”. Available: http://history.nasa.gov/hubble/
[6] Hubble Site. “Hubble’s Amazing Optics”. Available: http://hubblesite.org/the_telescope/nuts_.and._bolts/optics/
[7] The Space Telescope Science Institute. Available: http://amazing- space.stsci.edu/resources/explorations/groundup/lesson/basics/g28a/graphics/g28a_hst.gif
[8] Hubble Site. “Seeing More – Seeing Deeper”. Available: http://hubblesite.org/the_telescope/nuts_.and._bolts/instruments/wfc3/
[9] Hubble Site. “Pinpoint Precision”. Available: http://hubblesite.org/the_telescope/nuts_.and._bolts/instruments/cos/
[10] Hubble Site. “A Bigger Clearer Picture”. Available: http://hubblesite.org/the_telescope/nuts_.and._bolts/instruments/acs/index.php
[11] Hubble Site. “Fingerprinting the Cosmos”. Available: http://hubblesite.org/the_telescope/nuts_.and._bolts/instruments/stis/index.php
[12] Hubble Site. “Keeping Cool While Seeing Red”. Available: http://hubblesite.org/the_telescope/nuts_.and._bolts/instruments/nicmos/index.php
[13] Hubble Site. “Getting a Grip on Stars”. Available: http://hubblesite.org/the_telescope/nuts_.and._bolts/instruments/fgs/index.php
[14] European Space Agency. “The Science from the Hubble Space Telescope”. Available: http://spacetelescope.org/science/
[15] European Space Agency. “Measuring the age and size of the Universe”. Available: http://spacetelescope.org/science/age_size/
[16] European Space Agency. “Black holes, quasars and active galaxies”. Available: http://spacetelescope.org/science/black_holes/
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