Optics 101

Chandra X-ray Observatory - Space Based Telescope Exploring The Invisible Universe

NASA’s Great Observatories
Space Based Observation
Orbit Details
Observatory Elements
The Spacecraft System
The Telescope System
Science Instruments
High Resolution Camera
Advanced Charge-Coupled Device Imaging Spectrometer
Chandra X-ray Observatory Technical Details

To the human eye, space appears serene and void. It is neither. To the “eye” of an X-ray telescope, the universe is totally different - a violent, vibrant, and ever-changing place. Temperatures can reach millions of degrees. Objects are accelerated by gravity to nearly the speed of light and magnetic fields more than a trillion times stronger than the Earth’s, causing some stars to crack and tremble.

NASA’s Great Observatories

The Chandra X-ray Observatory is part of NASA’s fleet of “Great Observatories” along with the Hubble Space Telescope, the Spitizer Space Telescope and the now deorbited Compton Gamma Ray Observatory. Chandra allows scientists from around the world to obtain unprecedented X-ray images of exotic environments to help understand the structure and evolution of the universe. The observatory not only helps to probe these mysteries, but also serves as a unique tool to study detailed physics in a laboratory that cannot be replicated on Earth. Already surpassing its five year life, NASA's Chandra X-ray Observatory is rewriting textbooks and helping advance technology.

Figure 1. Chandra X-ray Observatory

Named in honor of the late Indian-American Nobel laureate, Subrahmanyan Chandrasekhar, the Chandra X-Ray Observatory provides information on the nature of objects ranging from comets in our Solar System to quasars at the edge of the observable universe.

Space Based Observation

Since X-rays are absorbed by the Earth’s atmosphere, space-based observatories are necessary to study these phenomena. To meet this scientific challenge, the Chandra X-ray Observatory was carried into low-Earth orbit by the Space Shuttle Columbia on July 23, 1999. Chandra was then deployed from the Shuttle’s cargo bay 155 miles above the Earth. Two firings of an attached Inertial Upper Stage rocket and several firings of its own on-board Integral Propulsion System after separating from the Inertial Upper Stage placed Chandra into its working orbit.

Orbit Details

Unlike the Hubble Space Telescope’s circular orbit that is relatively close to the Earth, Chandra was placed in a highly elliptical - or oval-shaped - orbit. At its closest approach to Earth, the observatory reaches an altitude of about 6,000 miles. At its farthest point of approximately 86,400 miles, Chandra travels almost one-third of the way to the Moon.

It takes Chandra 64 hours to complete one full orbit, during which the observatory takes uninterrupted observations for about 55 hours. Chandra cannot take science observations while it travels through Earth’s radiation belts that surround the planet, since the radiation can disturb Chandra’s sensitive instruments.

Observatory Elements

The Chandra X-ray Observatory has three major elements:

  • the spacecraft system
  • the telescope system
  • the science instruments.

The Spacecraft System

The spacecraft module contains computers, communication antennas and data recorders to transmit and receive information between the observatory and ground stations.

The onboard computers and sensors, with ground-based control center assistance, command and control the vehicle and monitor its health.

The spacecraft module also provides reaction wheels to aim the entire observatory, a set of small momentum unloading system thrusters to control momentum buildup, an aspect camera that tells the observatory its position relative to the stars, and a Sun sensor that protects it from excessive light. Electrical power is provided by solar arrays that also charge three nickel-hydrogen batteries that provide backup power.

The Telescope System

At the heart of the telescope system is the High Resolution Mirror Assembly. Since high-energy X-rays would penetrate a normal mirror, special cylindrical mirrors were created.

The two sets of four nested mirrors resemble tubes within tubes. Incoming X-rays graze off the highly polished mirror surfaces and are funnelled to the instrument section for detection and study.

The mirrors of the Chandra X-ray Observatory are the largest of their kind and the smoothest ever created. If the surface of the state of Colorado were as relatively smooth, Pike’s Peak would be less than 1 inch tall. The largest of the eight mirrors is almost 4 feet in diameter and 3 feet long. Assembled, the mirror group weighs more than 1 ton.

The High Resolution Mirror Assembly is contained in the cylindrical “telescope” portion of the observatory. The entire length of the telescope is covered with reflective multi-layer insulation that assists heating elements inside the unit in keeping a constant internal temperature. By maintaining a precise temperature, the mirrors within the telescope are not subjected to expansion and contraction, thus ensuring greater accuracy in observations.

Figure 2. High Resolution Mirror Assembly

The assembled mirrors were tested at NASA’s Marshall Space Flight Center in Huntsville, Ala. Marshall’s X-ray Calibration Facility verified the mirrors’ exceptional accuracy, which allows Chandra to detect objects separated by one-half arc second. This is comparable to reading the letters of a stop sign 12 miles away. The Chandra X-ray Observatory represents a scientific leap in ability over previous X-ray observatories like NASA’s Einstein, which orbited the Earth from 1978 to 1981. With its combination of large mirror area, accurate alignment and efficient X-ray detectors, Chandra has eight-times greater resolution and is 20 to 50 times more sensitive than any previous X-ray telescope.

Science Instruments

Two instruments, each of which can serve as an imager or spectrometer, are located at the narrow end of the telescope cylinder to collect X-rays and study them in various ways. By studying the X-ray rainbows, or spectra, and recognizing signatures of known elements, scientists can determine the composition of the X-ray producing objects, and learn how the X-rays are produced.

High Resolution Camera

The High Resolution Camera records X-ray images, giving scientists an unequaled look at violent, high-temperature occurrences like the death of stars or colliding galaxies. The camera is composed of two clusters of 69 million, tiny lead-oxide glass tubes. The tubes are only one-twentieth of an inch long and just one-eighth the thickness of a human hair. When X-rays strike the tubes, particles called electrons are released. As the electrons are accelerated down the tubes by high voltage, they cause an avalanche of about 30 million more electrons. A grid of electrically charged wires at the end of the tube detects this flood of particles and allows the position of the original X-ray to be precisely determined.

Advanced Charge-Coupled Device Imaging Spectrometer

Complementing the High Resolution Camera is the Advanced Charge-Coupled Device Imaging Spectrometer, which is also located at the narrow end of Chandra. This detector is capable of recording not only the position, but also the color (energy) of the X-rays. The imaging spectrometer is made up of 10 charge-coupled device arrays.

These detectors are similar to those used in home video recorders and digital cameras, but are designed to detect X-rays. Commands from the ground control center allow astronomers to select which of the various detectors to use. The imaging spectrometer can distinguish up to 50 different energies within the range Chandra operates.

In order to gain even more energy information, two screenlike instruments, called diffraction gratings, can be inserted into the path of the X-rays between the telescope and the detectors. The gratings change the path of the X-ray depending on its color (energy) and the X-ray cameras record the color and position. The High Energy Transmission Grating concentrates on the higher and medium energies and the Low Energy Transmission Grating disperses low energy X-rays. Both gratings can be used in conjunction with either the imaging spectrometer or the High Resolution Camera.

Chandra X-ray Observatory Technical Details

  • Size: 45.3 feet long by 64.0 feet wide (solar arrays deployed)
  • Weight: 10,560 pounds
  • Life: Originally planned for five years, but now planning for at least 10 years
  • Orbit: 6,000 by 86,400 miles, 64-hour period per orbit
  • Power: Two three-panel, silicon solar arrays (2,350 watts). Three 40-amp-hour nickel-hydrogen batteries for power in eclipse
  • Data recording: Solid-state recorder; 1.8 gigabits (16.8 hours) of recording capability
  • High Resolution Mirror: Assembly of four sets of nested, grazing incidence paraboloid/hyperboloid mirror pairs, constructed of Zerodur material
  • Weight of assembly: 2,104 pounds
  • Focal length: 10 meters (about 33 feet)
  • Outer diameter: 1.2 meters (about 4 feet)
  • Advanced Charge-coupled Imaging Spectrometer: Ten charge-coupled device arrays provide simultaneous imaging and spectroscopy
  • High Resolution Camera: Micro-channel plates detect X-ray photons
  • Transmission Gratings: One high/medium and one low energy, gold grating

Source: AZoOptics

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