For over 20 years, the Hubble Space Telescope has brought scientists a wealth of data and beautiful imagery of celestial objects near and far. It has been called “the single most important instrument ever made in astronomy” [source]. With the 23rd anniversary of its launch date (April 24, 1990) fast approaching, we take a look at the design of its optics, scientific instruments, and steering system.
Why put a telescope in space?
Space telescopes were conceived as early as 1923, and by 1946 were presented by Lyman Spitzer as a way to observe space without the optical distortions caused by changes in the atmosphere [source]. (These changes are why stars seem to twinkle.) Modern optics can correct for these distortions, but there is a more significant problem for scientific research: the atmosphere partially blocks ultraviolet, gamma, and X-rays, which means that we can never get a complete view of the radiating objects of the universe using telescopes on the surface of the Earth. Placing a telescope in orbit outside of the atmosphere avoids both problems.
Since one of the primary objectives of the Hubble is to produce extremely detailed wide-field images of space, Hubble’s engineers carefully considered size and potential aberrations when designing its optical assembly. Hobbyist telescopes are often refracting telescopes, which means they use lenses to focus an image. However, lenses are impractical for large research telescopes for a number of reasons. First, the length of the telescope increases with the size of the lens, and they become unwieldy very quickly. Additionally, the lenses themselves are impractical for very large telescopes because they deform under their own weight. Finally, lenses produce chromatic aberrations and absorb certain wavelengths of light, which is something we were trying to avoid by putting our telescope in space in the first place! Reflecting telescopes, which use mirrors to focus an image instead of lenses, suffer from none of these problems, and therefore a reflector was the obvious design choice for the optical assembly of the Hubble.Unfortunately not all reflector designs are created equally: different shapes and arrangements of mirrors introduce different optical aberrations. Since astronomers need to make accurate measurements of the positions of objects in space, they prefer telescopes which produce symmetric images of stars, as there is no obvious center of an asymmetric image to use as the endpoint of a measurement. Although many designs of reflecting telescopes in use today use parabolic mirrors, the parabolic shape produces comatic aberrations when focusing light from sources that are at an angle to the optical axis of the mirror. The visible effect of this aberration is that stars that aren’t centered in the field of view appear to have tails. Hyperbolic mirrors, on the other hand, produce no such aberrations. In a general reflecting telescope, a primary mirror is placed at the back of the telescope facing the aperture, and the secondary mirror set in front with the reflecting surfaces facing one another, possibly at an angle. The specific arrangement in the picture above makes for a particularly compact telescope called a Cassegrain reflector. Unlike the picture, which has two hyperbolic mirrors, a classical Cassegrain uses a parabolic primary mirror. However, since parabolic mirrors produce the undesirable comatic aberrations, the modified design above using only hyperbolic mirrors has become the standard choice for large research telescopes, including the Hubble. This modified design is called a reflector of Ritchey-Chrétien type. The downside to hyperbolic mirrors is that they are difficult and expensive to manufacture, so the classical Cassegrains (and other designs using parabolic mirrors) are more practical for amateur astronomers.
Behind the primary mirror live the instruments that act as our eyes in the sky. The scientific application of the term “telescope,” in fact, includes these instruments, not just the optical assembly. Currently on board the Hubble are five scientific instruments, and each of them has been designed to process a different range of wavelengths that together span a spectrum from ultraviolet to infrared. The instruments can be separated into two categories: cameras and spectroscopy devices. The essential difference between the two is that spectroscopy devices separate the incoming light into different wavelengths before it reaches the sensor of the device. Since describing each of Hubble’s instruments in detail would take five more blog posts, I will instead explain what kinds of sensors have been chosen for different wavelengths. Interested readers can learn about the technical specifications and applications of the instruments themselves, as well as earlier generations of instruments, at the dedicated Hubble websites of NASA, ESA (European Space Agency), and STSI (Space Telescope Science Institute), as well as at hubblesite.org.Light is divisible into three natural ranges: ultraviolet, visible, and infrared, and each of these ranges requires a different type of sensor. Each of the five instruments on Hubble, the Wide Field Camera 3 (WFC3), the Advanced Camera for Surveys (ACS), the Cosmic Origins Spectrograph (COS), the Space Telescope Imaging Spectrograph (STIS), and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), has at least two sensors, sensitive to different wavelengths, which are operated as different “channels” of the device. The cameras and the STIS use CCDs to record wavelengths roughly between 200 and 1100 nm. This includes the full visible spectrum, so they basically operate like a hand-held digital camera. In order to record the shorter wavelengths of ultraviolet light with high resolution, the COS’s sensors utilize microchannel plates, which are slabs of highly resistive material with a densely distributed array of tiny parallel channels. The channels are mere micrometers in diameter, and cut through the plate at an angle, so a photon entering a channel at the top of the plate with high enough energy will hit the wall of the channel and create a cascade of electrons that amplify the signal, which is subsequently read by a detector at the back of the plate. The infrared channel of the WFC3 and all three channels of the NICMOS use a photodiode array as their detector. Photodiodes are the sensors used in solar cells, where the electric current generated by incoming light is restricted in order to build up voltage. In the WFC3 and NICMOS sensors, the incoming infrared light strikes the diode and creates a current that is recorded and processed to produce images. Although infrared radiation is not visible to the naked eye, we experience it as heat, and since the Hubble’s mirrors are protected by heating elements, this provided a design challenge for the installation of the NICMOS. Without a cryogenic cooler protecting the sensors, this heat would distort its images. The cooler, and consequently the NICMOS, has been essentially non-functional since the end of 2008, and so when the WFC3 was installed in the last servicing mission in 2009, it became the main infrared camera on board.
Now that we know how to take pictures in space, the next question is: how can we move the telescope in space and make sure it’s pointed where we want it? The Hubble’s answer to this question lies in its pointing control system, which is made up of three components: the Fine Guidance Sensors (FGS) to locate its target, reaction wheels to spin the telescope, and gyroscopes to measure its angular momentum. The high-resolution images that the Hubble is known for wouldn’t be possible without the precision control of this system.
The FGS are highly accurate interferometers, and may be the most impressive instruments on the Hubble, due to their extreme precision. These instruments make such exact measurements, that once the telescope is pointed at its target, it doesn’t deviate more than 0.007 arcseconds for more than 95% of the time. For some perspective, this level of stability and precision is described by NASA as being comparable to keeping a laser beam focused on a dime that is 400 miles away. In order to keep the telescope in position, two of the sensors “lock onto” pre-selected guide stars (the third is free to make scientific measurements on positions and motions of objects in its field of view). “Locking onto” stars is accomplished through the measurements taken by the two interferometers in each sensor. Interferometers are devices that measure interference patterns in a beam of light that has been split by a prism and directed into photomultiplier tubes which turn the light into an amplified electric signal. The phase difference between the signals is then processed to determine the angle of the incident light relative to the base of the prism, and this angle can be used to determine the positions of the stars emitting the light. Scientific measurements recorded by the FGS have determined the positions of stars with 10 times the accuracy of Earth-bound interferometers.The physical positioning of the telescope is technologically relatively simple and is accomplished by its reaction wheels, with the help of the gyroscopes. There are no rockets or engines on the Hubble Space Telescope, so other than rotational maneuvers afforded by the reaction wheels, its motion is subject to its natural orbit around the Earth. Reaction wheels are the positioning devices in every unmanned spacecraft. They are flywheels spun by electric motors at very high speeds. The law of conservation of angular momentum causes the spacecraft to spin about its center of mass in the opposite direction, and braking the wheel brakes the rotation of the spacecraft. The on-board gyroscopes are rate gyroscopes, which means their function is to sense the angular momentum of the Hubble as it spins. Although there are six of them, only two are needed at any given time for normal operation. Inside the gyroscope is a wheel spinning at a constant 19,200 rpm inside a sealed gas-filled cylinder (the gimbal). This cylinder is suspended in a thick fluid and attached to a torsion spring. As the telescope rotates, the inertia of the spinning wheel in the gyro causes it to resist this change in its motion and thereby exert a measurable force on the spring.
But wait, there’s more!
We’ve only learned a little about the technology behind the scenes of the amazing images of the Hubble Space Telescope, but there’s much more to be learned about its history, operations, communication, computers, and solar devices, not to mention its incredible discoveries. With the Hubble space telescope, astro-physicists narrowed down the age of the universe and determine its rate of expansion. They also discovered dark energy and got our first glimpses of exo-planets. Those of us old enough to remember the early days of the Hubble recall that shortly after its deployment, it was discovered that the primary mirror was ground incorrectly, producing blurrier images than promised. The correction and all further service missions were accomplished by sending astronauts on long complex spacewalks at the site of the orbiting telescope.Now, Hubble is reaching the end of its lifespan; with the retirement of the space shuttle, there will be no more servicing missions and it is unclear how much longer the systems we’ve described will last. It was designed to be brought down to Earth with a spacecraft, but now it will be allowed to reenter Earth’s atmosphere on its own sometime after 2019 and will de-orbit with the help of an unmanned robotic mission that will guide it into the ocean. It will be succeeded by the James Webb Space Telescope, which has been specifically designed to study the birth and evolution of galaxies, and is projected to launch in 2018.