|http://arstechnica.com/science/news/2010/04/hubble-turns-20-a-retrospective-in-pictures.ars (click to enlarge)|
The Hubble Space Telescope is responsible for many discoveries that revolutionized our view of the universe. A few observations here and there of phenomena in a minute portion of the sky is all that is needed to infer how our entire universe works. To illustrate, a few of Hubble's images lead to the discovery of dark matter. Yes, physics is that elegant.
Since its launch in 1990, the Hubble Space Telescope has provided the whole world with a detailed view of the universe that was never before possible. Given that the Hubble Space Telescope is outside of the Earth’s atmosphere, the images are clearer and sharper than any telescope located on the Earth’s surface. This is because the light that the ground telescope receives goes through air first. The molecules in the air distort and diffract light, and they absorb certain wavelengths of light, such as ultraviolet and gamma radiation. Space telescopes don’t have this problem outside of the atmosphere. Since it is in orbit above Earth’s atmosphere, there is no background light from Earth, and light is unaltered and undistorted by the molecular gases in our atmosphere.
The Hubble Space Telescope was launched into low Earth orbit by the space shuttle Discovery on April 25th 1990, It travels about 5 miles per second, takes 97 minutes to complete an orbit around Earth.  Even though it was meant to have been launched in 1983, it was delayed seven years because of financial constrains, and a cautious Congress and populace after the Challenger disaster a few years prior in 1989.
It is expected to re-enter into Earth’s orbit within the next few decades, due to orbital decay and drag, at which time a new space telescope is planned to replace it. Retrieval is considered not practical as it would risk the lives of people and would be too costly to do, as the shuttle program is already retired.
The Hubble Space Telescope works to collect light from a wide spectrum and focus it, allowing us to see farther and clearer than the human eye possibly could. It is a Ritchey-Cretien Cassegrain reflector, meaning incoming light that travels from distant stars will come and bounce off a primary concave mirror and hit a secondary convex mirror, which focuses light through a small hole in the center of the primary mirror. The primary mirror’s diameter and size is one of the main determinants for how much light the telescope can collect, and compared to the large ground telescopes in observatory towers, the one on the Hubble would be considered small, at just 2.4 meters.  After incoming light passes through the hole in the primary mirror, it can be collected by the instrumentation on the Hubble Space Telescope. The mirrors must be very smooth, uniform don to 1/800,000th of an inch, and are treated with various coats to increase transmissibility of light, and to protect the mirror itself from warping due to solarization and aging due to ultraviolet light. The complex structure of the two mirrors, apertures, and trusses that support it is called the Optical Telescope Assembly, or OTA. 
Even small aberrations of the mirrors can cripple the Hubble Space Telescope. For example, after its launch, they discovered that the main mirror’s curvature had problems, but that was fixed after the first service mission in 1993. Since its launch, the Hubble Space Telescope’s fine instrumentation have required five different service missions: Service Mission 1 in 1993, Service Mission 2 in 1997, Service Mission 3A in 1999 to put in a new computer and gyroscopes, 3B in 2002 to replace the FOC with the Advanced Camera for Surveys, repair the NICMOS Cryocooler, and the most recent one, 4, in May 2009 to repair the Advanced Camera for Surveys, the Space Telescope Imaging Spectrograph, replace the Fine Guidance Sensoring System and two gyroscopes.  To maintain accuracy and for the data retrieved by the telescope be useful for scientific observation, constant upkeep and calibrations of the instruments must be made. The instruments themselves will now be described.
NICMOS and Cryocooler
NICMOS, or Near Infrared Camera and Multi-Object Spectrometer was installed on the Hubble Space Telescope in February of 1997, during the Servicing Mission 2. In short, it seeks out heat in the form of infrared radiation.
It is composed of three cameras, each with their own field of view, with 256x256 HgCdTe Rockwell sensor, sensitive to wavelengths of .8 micrometers to 2.5 micrometers. They are housed in a cryogenic dewar, maintaining a constant working temperature between 58 and 60 degrees Kelvin. The dewar is composed of three shields to keep NICMOS cool: the VCS, or Vapor Cooled Shield, the Thermo-Electric Cooled Inner Shield, and the Thermo-Electric Cooled Outer Shield. The hybrid nitrogen and aluminum dewar was supposed to keep it at just 58 degrees Kelvin, but a gap in the Vapor Cooled Shield resulted in an unanticipated heat load. Fluctuations in temperature where the cameras are housed means the cameras will need to calibrate more often.
This is where the Cryogenic Cooler comes into play, maintaining temperature stability within .5 degrees Kelvin. It circulates neon gas through a cooling loop, using high-speed centrifugal machines that do 7000 revolutions a second to compress gas, removing heat from the gas. 
NICMOS is useful because it captures information about infrared light, which reaches Earth from very far away, unaffected by interstellar dust, unlike visible light.
Advanced Camera for Surveying
The Hubble Telescope’s FOC was replaced by the ACS in 2002, during the Service Mission 3B. It has three cameras that are sensitive to a wide spectrum of light, from ultraviolet to near infrared (wavelengths of 1,200 to 10,000 angstroms), and since it has high contrast even near bright stars, it can be used to study galaxies and black holes very far away, where light from the ancient universe has just arrived at Earth. ACS has many components that make it versatile and useful for scientific observations. For instance, ACS is sensitive to ultraviolet light because it has a solar blind camera, or SBC, to block out visible light.
The Wide Field Channel of the ACS has two cameras with a resolution of 2048x4096 pixels each (for a total of 4k by 8k pixels), and its wide view frame is used to survey galaxies and the positions of stars within those galaxies. It is responsible for the Ultra Deep Field images of the universe.
The High Resolution Channel of the ACS has one camera with a resolution of 1024x1024, and is used primarily to detect ultraviolet light. The HRC has a coronagraph component that increases the contrast near stars tenfold. The High Resolution Channel allows us to “zoom” in, with a smaller field of view, but the images have a greater angular resolution. While the WFC and HRC are mainly on the red and blue regions of light, there’s a Multi-Anode Microchannel Array has no electronic noise and is sensitive to ultraviolet light, but not visible light. The images are 1k by 1k pixels, which is less than the WFC and HRC. Only the Solar Blind Camera is working right now. 
COS, STIS, and Ultraviolet Light
Spectrographs allows us to see precise information about incoming light that would otherwise be very faint and unusable. Incoming light is broken down to different wavelengths of light, and the amounts of each light at specific wavelengths are plotted on a graph. The light would have a unique fingerprint of different wavelengths, which can be studied.
COS, or the Cosmic Origins Spectrograph, looks at specific stars.
STIS, or Space Telescope Imaging Spectrograph, primarily has a larger field of view, focusing mainly on galaxies or a system of stars.
The Hubble Space Telescopes main mirrors are coated with magnesium fluoride 1 x 10-6 inches thick to improve the reflection of light at ultraviolet wavelengths.  Ultraviolet light, with a wavelength of 300 nanometers to 10 nanometers, is very high energy, so a special material that had a high energy gap and was resistant to absorbing the high energy from the photons was required. Magnesium fluoride not only helps maintain the integrity of ultraviolet light, the resistance to absorbing photon energy prevents warping of the mirrors due to solarization. 
Pointing and Guidance Systems
A system of fine guidance sensors, magnometers, solar sensors, and gyroscopes help keep the telescope position and point itself to collect light and data from specific areas in our universe. Since the Hubble Space Telescope is constantly moving in low orbit at a rate about 5 miles per second, a wealth of stabilizing components is needed to ensure the telescope is pointed at one spot for sufficient periods of time. The position and orientation of the telescope is also crucial because solar energy needs to hit the panels to power the telescope, and to keep the Sun’s heat from hitting only one side of the telescope. 
Currently, 3 gas-bearing gyroscopes rotate up to 19,000 rpm help stabilize the telescope, and with the help of up to three fine guidance sensors, the Hubble Space Telescope can help determine star locations that are 10 times more precise than a ground telescope. The guidance system also helps determine the position of stars, the distances between them, and the distance scale of the universe. Magnometers and CSS also help determine the telescope’s position relative to Earth’s magnetic field and the sun, respectively. Magnetic torquers and reaction wheel actuators help “lock” the telescope on to a planet or point in space. 
Design for Power
The Hubble Space Telescope is powered by 57 kilograms of nickel-hydrogen batteries when solar power is not available for 36 minutes in the Earth’s shadow.  Each orbit is 97 minutes, so it spends roughly 37 percent of its time without solar energy, so batteries are important. They must be able to power and sustain the telescope when solar power isn’t available. There are two modules consisting of a total of six batteries. Each module weighs 460 pounds and measures 36 inches long, 32 inches wide, and 11 inches high. There are three batteries per module, with each battery having 22 individual nickel-hydrogen cells placed in series.
Even though each battery has a total of 88 amp-hrs of capacity, on the Hubble Space Telescope, the practical maximum is 77 amp-hrs due to a limitation with heat dissipation. With a total of six batteries, the total energy supply becomes 450 amp-hrs. The batteries have lasted almost 13 years more than originally planned, 18 total. The ones that will replace them will be even better, built with the “wet slurry” process over the dry method. The “wet slurry” process allows for better porosity of the solid metallic powder over the dry method, in which the metallic powder is simply impacted into the battery cells. 
With the new batteries, the Telescope be used for normal scientific purposes on battery power for nearly 5 orbits, which is nearly 7.5 hours of operation, assuming the batteries are fully charged.  This means power for the Hubble Space Telescope is supplied redundantly in the sunlight, so if one source of power fails, the telescope will still be in working condition.
When the Hubble Space Telescope is within the day time portion of its orbit, about 61 minutes of the 97 minute orbit, it uses an array of four solar panels to power the telescope and to charge the batteries. They were replaced in the Servicing Mission 3B in 2002 with smaller, less flexible panels that produced less than a third more power than the pre-existing ones. Solar arrays supply 5k watts of energy to power the Hubble Space Telescope. This extra power supply makes running more instruments simultaneously on the telescope possible, so more institutions and scientists can use the telescope at the same time. 
1. Hubble Space Telescope Servicing. 5 Oct 2010. Available:
2. Servicing Misson 4. 5 Oct 2010. Available:
3. Servicing Mission 4 <Batteries>.
4. Servicing Misson 4 <Gyropscopes>. 5 Oct 2010. Available:
5. Servicing Mission 3B. 5 Oct 2010. Available:
6. Servicing Mission 3A. 5 Oct 2010. Available:
7. Servicing Mission 2. 5 Oct 2010. Available:
8. Servicing Mission 1. 5 Oct 2010. Available:
9. NASA Hubble. 5 Oct 2010. Available:
10. The Hubble Program – Technology. 5 Oct 2010. Available:
11. Hubble Finds Ring of Dark Matter. 5 Oct 2010. Available:
12. WMP, the Expansion of the Universe. 12 Nov 2010. Available: http://map.gsfc.nasa.gov/universe/uni_expansion.html
13. Hubble Servicing Missions. 25 Oct 2010. Available:
14. Hubble the Telescope. 25 Oct 2010. Available:
15. Hubble Telescope Essentials. 26 Oct 2010. Available:
16. Optical Assembly. 12 Nov 2010. Available:
17. Hubble Provides New Evidence for Dark Matter in Small Galaxies. 14 Nov 2010. Available:
18. NICMOS. 12 Nov 2010. Available:
19. Hubble – Fine Guidance Sensors. 5 Nov 2010. Available:http://hubblesite.org/the_telescope/nuts_.and._bolts/instruments/fgs/index.php