Technical tour de force Hubble celebrates 25 years

The Hubble Space Telescope was launched 25 years ago on April 24, 1990. Plans to send a reflecting telescope into space date back to 1946. The first serious plans to build one also took shape in 1968, and Hubble’s green light came in 1977.

The space telescope with a mirror diameter of 2.4 meters has been orbiting the Earth for a quarter of a century, initially in 97 minutes at 598 kilometers altitude. Now slightly faster, in 95.6 minutes at 555 kilometers altitude. Because the telescope is placed outside the Earth’s atmosphere, there are no problems with disturbance of the incoming light, both in the visible part of the spectrum and invisible to the human eye. This made it possible to see more of the universe with an optical telescope than ever before. But not only that. Hubble also made it possible to estimate the age of the universe more accurately: scientists estimated it to be between thirteen and fourteen billion years old. A lot more accurate than before with between ten and twenty billion years. The telescope also played an important role in discovering dark energy and proved that every galaxy has a black hole at its core.

Sending a telescope into space, however relatively limited in size, is no mean feat. The initial plan was to launch the more than 11,000 kilogram device in 1983, but due to an accumulation of problems and the disaster with the space shuttle Challenger in 1986, this was postponed until 1990. During the launch, the project had already had about 2 $0.5 billion, well over the estimated $400 million. In the end, NASA itself also couldn’t bear the costs anymore and asked the European space agency ESA to also pay for it and to supply the first generation of the instruments and solar cells.

It will not have escaped anyone’s notice that the Hubble has provided beautiful images of the universe. This is possible because Hubble has the necessary instruments on board: two cameras, two image spectrographs and different aiming sensors. In addition, two mirrors, the large concave mirror with a diameter of 2.4 meters and a second, convex mirror that returns the light to a hole in the center of the large mirror at the focal point. Various instruments can collect the light there. Shortly after the launch, NASA found that the large mirror had a small deviation. The mirror was ground 2.2 microns too flat at the edges, causing spherical aberration. This mainly caused problems with weak light sources, which meant that much research could no longer be done. During a second space shuttle mission in December 1993, astronauts were able to fit a corrective mirror. In total there would be four maintenance missions, the last of which was in 2009. Construction of the mirror began in 1979 and is supported with a honeycomb structure to keep everything as light as possible. The mirror was finished at the end of 1981.

The COSTAR corrective optical system, Corrective Optics Space Telescope Axial Replacement, was removed during the last space mission in 2009 because all instruments had been replaced in the meantime with new or different variants, with a built-in correction for the spherical aberration. COSTAR was replaced by a Cosmic Origins Spectrograph, COS, which can measure ultraviolet radiation from weak point sources. This makes it possible to study the formation and evolution of galaxies and other large structures in the universe.

For the big overview, Hubble has the Wide Field Camera. The third incarnation of this has already been installed, the WFC3. The camera takes pictures in the spectrum visible to us. WFC3 and its predecessors have repeatedly provided iconic images of the universe. The WFC3 has two different channels, each of which can capture different wavelengths. The ultraviolet wavelength channel is used to study nearby galaxies or those in which many stars are forming. The infrared channel is used to capture light from distant galaxies and thus immediately capture a piece of the history of the universe.

Installed in 2002, the Advanced Camera for Surveys, ACS, can capture visible light and infrared like WFC3. Currently, the WFC3 has taken over much of the precision work from this camera, which replaced the Faint Object Camera. The FOC was Hubble’s first ‘zoom lens’ for twelve years.

In addition to capturing images of objects, an important research tool is also the separation of light in the basic wavelengths with the Space Telescope Imaging Spectrograph, STIS. In this way the chemical composition, density and temperature of objects can be determined. In this way we can still find out what kind of elements objects consist of. But spectrography is also used, for example, to discover exoplanets. In addition, STIS is also important in the discovery of black holes. The light from stars and gas orbiting the center of a galaxy is redder as it moves away from the telescope and bluer as it approaches it, or red-and-blue shift. The location of a black hole can be determined by looking at reddish material on one side and bluish on the other, revealing that this material orbits an object very quickly.

Absorption in the atmosphere by wavelength (source: Wikipedia)

The three aiming sensors are there to turn and keep the telescope in the right direction and take measurements at the same time. Two sensors keep the telescope in the right direction and one can take measurements. They use stars to focus and lock themselves on. Because the orbits of the stars are known, the sensors can keep the telescope constantly in the right direction. The precision of the instruments is comparable to keeping a laser focused on a dime 800 kilometers away for 24 hours. Because of their great precision, much more has been known about the exact locations of celestial bodies since Hubble.

The resolution of the telescope has been discussed before. Partly because there are no atmospheric disturbances, the telescope can see a lot more. For example, an atmospheric disturbance from Earth creates the familiar star shape of stars and planets. A good telescope on Earth can hardly tell the difference between two very close stars. The ‘resolution’ is therefore determined in arcminutes and arcseconds. A degree of arc consists of 60 arc minutes or 3600 arc seconds. A telescope on Earth can’t see much more than 1 arcsecond, so if two stars are closer than an arcsecond, they are indistinguishable. The Hubble telescope can see 0.1 arcsecond. The human eye sees about 60 arcseconds.

HRT; Hubble, orbits the Earth. The Webb telescope in fixed orbit around the sun 1.42 million kilometers from Earth.

What comes after Hubble now? Even more detail. And that requires a bigger mirror. All of this has already been put into operation with the 6.5-meter James Webb Space Telescope. If all goes well, this telescope will be launched at the end of 2018. The Webb will mainly investigate longer wavelengths in order to look even further back into the past. Compared to Hubble, the telescope is ‘lightweight’. Webb will also be much further from Earth at a million and a half miles away. It will also not revolve around the earth, but is at a fixed point from the earth in orbit around the sun.

Data bundle users: some images are very large!

1. How far into the past can different telescopes look 2. Pillars of Creation, part of the Eagle Nebula 3. Emission Nebula NGC 6357 4. Infrared image of Mystic Mountain in the Carina Nebula 5. Cluster MCS J0416.1-2403 consisting of nearly 200 images 6. Butterfly Nebula NGC 6302 7. Mystic Mountain, photo released in celebration of the 20th anniversary of Hubble, part of the Carina Nebula 8. Images of Saturn’s aurora

Data bundle users: some images are very large!