Telescopes

Telescopes must be placed in specific locations depending on the wavelength of light they observe. Telescopes that observe in the gamma region must be placed in space, as gamma rays cannot penetrate the Earth's atmosphere. The atmosphere is % opaque to gamma rays, as shown in the image below. Telescopes that observe in the UV region must also be placed in space, as the Earth's atmosphere absorbs most UV rays.

Telescopes that observe in the visible region need to be placed in remote areas to avoid light pollution, which interferes with these optical telescopes.

Telescopes that observe in the IR region can be placed on Earth but must be placed in dry areas (e.g., deserts) where there is less water in the atmosphere, as water molecules in the atmosphere absorb IR radiation.

Radio telescopes, like optical telescopes, need to be placed in remote areas to avoid interference from electronic devices (e.g. mobile phones). Radio telescopes are parabolic in shape to prevent spherical aberration and can be constructed with a mesh design to reduce cost and weight.

When a wave passes an obstacle / through an aperture, it is diffracted. When monochromatic light is passed through a narrow slit an interference pattern is seen, if this is rotated about its centre, a circular pattern is seen what the pattern from a circular aperture looks like. The large central maximum is known as the Airy Disc.

The resolution of a telescope tells us how much detail can be seen. If two objects are very close together, the telescope can resolve them (see them as separate points).

A telescope can resolve an image if the central maximum of the diffraction pattern of one image coincides with the first minimum of the other image.

This is also known as the Rayleigh Criterion.

Formula:

Therefore, larger diameter telescopes have a lower value of the Rayleigh criterion, resulting in better resolution and thus clearer images.

To know if a telescope can clearly observe a distant object, such as a crater on Pluto. We need to determine if the size of the objects is greater than the minimum angular resolution of the telescope.

For example, to determine if a crater on Pluto with a diameter of can be observed by a telescope onboard a spacecraft that is away, we need to calculate the minimum angular resolution of the telescope and then use this and the equation for an arc of a circle, , to determine the minimum size of object that can be detected by the telescope at this distance. The telescope onboard has a diameter observing at the wavelength of .

The telescope on board can distinguish between objects that are apart.

At a distance of the telescope can observe objects that are apart. The crater is much larger, so it can be seen by the telescope.

When making observations, telescopes use Charge-Coupled Devices (CCDs) rather than the human eye. CCDs can make observations across all parts of the EM spectrum -> they are not restricted to wavelengths in the visible region, like the eye. CCDs also allow for longer observation times, collecting more light, which results in better images and provides digital images that can be stored and examined at a later date.

Charge-Coupled Devices (CCDs) are a crucial component of the detection systems of many modern telescopes, owing to their high quantum efficiency.

CCDs typically have a quantum efficiency , whereas the eye has a quantum efficiency of %.

A CCD is composed of a silicon chip that is divided into individual elements, known as pixels. When a photon hits a pixel, an electron is released—the greater the intensity of light, the more electrons that are emitted. The image produced is identical to the electron pattern.