CAJ 8: The Hubble Telescope - Optics (part 3)

How is it possible that a telescope can view stars that are more hundred or even thousand light-years away? What optical device is capable of picturing galaxies that are so far away that we cannot even conceive the enormous distances?

It is not only the fact that it does not have to look through our polluted atmosphere, but also the "eyes" of Hubble. The eyes of Hubble have also a real name that is the Optical Telescope Assembly. This system is designed to offer the widest possible field of view. The system consists of two main mirrors, apertures and supporting trusses.

As the light strikes the telescope, it enters the tube, which prevents stray light to enter the telescope. After it entered the tube it passes the first optical glass. Than it strikes the first mirror that is shaped like an upside down bowl. This mirror is called the concave. Because of the shape of the mirror, it diverts the light and reflects it to the centre of the optical glass in which the second convex shaped mirror can be found. This mirror directs the light again to the first mirror. The first mirror has a hole in the Center where the light can enter and can reach its focal point right where the science instruments are placed. This basic model is called the

Ritchey-Chretien Cassegrain. 

The main mirror measures 2.4 meters in diameter. The smaller mirror that redirects the light towards the science instruments is only 0.3 meters in diameter. The focal plane where the light gets picked up is roughly the size of a dinner plate. 

The mirrors are developed in a very special way. They are treated with abrasives so that the surface of the mirrors is perfectly smooth. The mirrors are designed so that they do not deviate from a perfect curve by more than 1/800,000th of an inch. To help you understand this I use an example. The mirrors are so smooth that if the mirror were as big as the Earth in Diameter, the biggest bump would be 6 inches tall. 
The mirrors are made of ultra-low expansion glass and are kept constantly at the same temperature. This prevents the glass from cracking or warping. The surfaces are coated with a very thin layer of aluminum and a similarly thin layer of protecting magnesium-fluoride. The magnesium-fluoride allows the mirrors to be even more reflective to ultraviolet light. 

In the first post I already mentioned the first problems of the telescope with the mirrors and blurry images. This was because of the primary mirror. After the telescope was launched and the first images were sent to Earth, it became apparent that something is wrong with the telescope as the images are all very blurry. This was because the primary mirror had a flaw called spherical aberration. The outer edge of the mirror was four microns flatter than intended. Four microns equal approximately one-fiftieth of a single human hair.

During the first servicing mission this problem was solved by putting small corrective mirrors on the primary mirror. After this the images became much sharper and the telescope could start to function properly. During the fourth servicing mission the corrective mirrors were replaced by an instrument called Cosmic Origins Spectrograph. This device breaks down the light reaching the telescope. By analyzing these light waves, scientists can determine the density and chemical components of a planet. This technology was a major development and improved the telescope's sensitivity up to 10 times, especially the sensitivity to ultra-violet light.