Hubble Space Telescope

Hubble Space Telescope (1989-present)

Gemini | Apollo | Hubble Space Telescope

The Hubble Space Telescope (HST), named after Edwin Hubble, was launched in 1989. However, a flaw in its primary mirror did not allow it to perform up to expectations until a team of astronauts fixed it during a space walk in December of 1993.

Since then, the HST has returned dazzling pictures which have provided scientists with a new view of the cosmos in many different areas of astronomy, such as star formation, black holes, and the history of the universe.

The HST was one of the most expensive satellites to put in orbit. Its primary mirror is only 2.4 meters (7.87 ft) wide, but it is still able to capture images with higher resolution than any other ground telescope until recent years*. The reason for this is Earth's atmosphere. While it shields us from harmful radiation and allows us to live, it is also turbulent and heavily distorts any image that is taken through it.

Therefore, with a telescope circling Earth high above the distortion effects, HST could take sharper pictures than the twin 10 m (33 ft) telescopes atop Hawaii's Mauna Kea.

*The military declassification of the process of adaptive optics - with which most of the larger telescopes, such as the Keck atop Mauna Kea, have been fitted - allow much clearer images with higher resolution to be taken. The basic process is that a computer uses a "guide star" to monitor the turbulence in the atmosphere. Then, it controls sometimes hundreds of little motors to slightly alter the shape of the telescope's mirror to compensate for the atmospheric turbulence.

Many pictures that the HST takes are of the spectra - all of the light spread out into a rainbow - of objects. An astrophysicist proverb states that if a picture is worth a thousand words, then a spectra is worth a thousand pictures. For example, if you take a picture of a star, then you see a point of light. But, if you take a spectra of a star, then you can see the different types of chemicals it is made of, how far away it is, and other information. Most of the 140,000+ pictures that the HST has taken that have been released to the public though, are in visible light.

A famous image from HST is called the Hubble Deep Field North (left), and it was taken in 1995. It took Hubble two weeks to generate the picture, but it shows thousands of galaxies, some over ten billion years old. Pictures like this are helping astronomers to map the visible universe. Eagle Nebula (M16)

Another famous image from HST is from the Eagle Nebula (M16). It shows three pillars of gas, each about 1 light-year wide, with the largest begin 10 light-years long. It shows young protostars that are just beginning to break free of their birth nebula.

There are two web sites that are dedicated to hosting pictures taken by the HST. One is the Hubble Heritage Gallery, which releases one image a month to the public. I have that duplicated with my own twist in the Advanced version of this page. The other site is the Hubble Space Telescope Public Pictures, which has almost if not all of the pictures from HST that have been released to the public. Both sites have full descriptions of every image they have, as well as the images in varying sizes and resolutions.

Of the images the HST does take, none are in color. To produce the images that you are used to seeing, several pictures are actually taken. These pictures are taken in different filters, so that only light of certain wavelengths can reach the detector. This way, scientists can study an object in just infrared, or just optical light.

In the example to the right, three filters are used to gather red, optical green, and blue light. The images are recorded onto the detector as photon hits; the brighter the image, the more photons are detected. Thus the resulting image is also black and white.

The next step to producing the full-color images such as those in the Hubble Heritage galleries is to clean the raw pictures by removing false photon hits and any biases of the different pixels of the detector.

Finally, three (usually) images that were taken with different filters are combined in an image processing program, such as Adobe Photoshop. In this, the different filters can be assigned different colors, not necessarily in the same order as they appear in the EM spectrum and not necessarily the original color that it would appear with the human eye.

In the example above, the red light is coded blue, the optical green is red, and the blue is green. Then, delicate image manipulations are performed in order to achieve a visually pleasing image.