As stated earlier, it is not possible for scientists to directly visit the overwhelming majority of the observable Universe. The only method that can be used is to observe light that has been emitted from these distant places using telescopes.
Note - As of 11th of February 2016 this is no longer the case, as the detection of Gravitational Waves has opened an entirely new method of observing the wider Universe.
The First Telescope
Telescopes are devices which allow an observer to resolve objects at distances greater than would be possible with the naked eye.
The first telescopes were invented in the Netherlands in the 17th century CE by Hans Lippershey, a spectacle maker. These early telescopes were used for monitoring ships at a great distance, and it wasn't until Galileo built his own telescope in the 1609 that the first telescope was used for astronomical purposes. The image below shows the telescope Galileo used to discover the five largest moons of Jupiter (hence their designation as Galilean Moons), which is on display at the Museo Galileo in Florence, Italy:-
Types of Telescope
There are Three main types of optical telescope used today:-
1. Refracting telescope - uses lenses.
2. Reflecting telescope - uses mirrors.
3. Catadioptric telescope - uses both lenses and mirrors.
The above diagram shows the path Light takes as it passes through a refracting telescope. All refracting telescopes consist of two lenses:-
1. A large Objective Lens - At front of telescope, main function to gather light, the larger the lens, the fainter (and therefore more distant) the object that can be seen.
A highly curved Eyepiece Lens - At rear of telescope, main function to provide magnification to the image.
The above diagram shows the path light takes as it passes through a reflecting telescope. All reflecting telescopes consist of two curved mirrors, which work in a similar setup to the lenses within a refracting telescope. As with the refracting telescope, the larger the objective mirror, the greater the light-gathering ability of the telescope.
The above diagram shows the path light takes as it passes through a catadioptric telescope. This design is a combination of both a refracting and reflecting telescope, giving the benefits of both, with fewer of the drawbacks. The cost of this type is much higher, however, which limits their use to mostly high-end scientific use.
The video below shows a summary of the telescope:-
Optical Telescope Case Study : The Hubble Space Telescope
The Hubble Space Telescope is a 2.4 m diameter reflecting optical telescope that was placed in Low-Earth Orbit in 1990. This telescope is still functional today, though is due to be replaced soon.
The H.S.T. observes the Universe in the visible and near-infrared part of the Electromagnetic spectrum, and has produced some of the most detailed images of space ever taken.
The link below will open the National Geographic's 'Top 10' Hubble Space Telescope image collection:-
The H.S.T. can take such high quality images due to two factors. Firstly, its very large objective mirror allows a large amount of light to be gathered, allowing very dim and distant objects to be imaged clearly.
The main reason, however, is the H.S.T.'s location. By being in space, beyond the distortion caused by the atmosphere, the Hubble Space Telescope can capture detail that would not have been capable from the Earth's surface.
Modern Optics (Not Assessable)
Modern Earth-based telescopes are beginning to rival the Hubble's imaging capability, however. These new telescopes overcome the problem of atmospheric distortion by using a system called Adaptive Optics:-
The above diagram shows how the adaptive optics system works. Light entering the telescope is distorted due to the atmosphere. The wavefront sensor detects these distortions and rapidly changes the shape of the objective mirror to correct for these distortions. This allows most of the atmospheric distortion to be removed from the light that falls on the high-resolution camera, giving a very clear image.
The image below shows Uranus imaged with and without Adaptive Optics:-