THE HST is a reflector telescope, using mirrors to gather and focus light.  Isaac Newton was the first person to make a reflector telescope in 1668.  Newton’s telescope is 15cm long with a primary mirror only 5cm across and a mass of about 1kg. Hubble is 1320cm long, its primary mirror is 240cm across and it has a mass of 11,110 kg.  

Having the biggest mirror possible means gathering more light so that fainter things can be seen.   How much more light can HST collect compared to Newton's original? You will need to calculate the areas using the formula πr².   ANSWER

The other reason for having the biggest mirror possible in a telescope is that it can resolve finer detail.

 Newton gave a duplicate of his telescope to the Royal Society in 1672.  This is locked away in the vaults of the RS but they had the copy  above made in the 18^th century which is now on show in London.  Click on the image to enlarge it.

 

The picture above shows SOS developer, Mike Cripps, standing next to the HST backup mirror in the Smithsonian National Air and Space Museum in Washington DC.   The mirror would have been ground and silvered had it been needed.    Why not make a copy of the mirror yourself?   You'll need a few  rolls of turkey sized kitchen foil.  It makes a great background to a display of Hubble images.

HST has what is known as a Cassegrain design whereby the light picked up by the primary mirror is reflected forward onto a secondary mirror and then bounced back through a hole in the primary mirror  where it is focused onto the instrument detectors.