Telescopes Flashcards

Question

Minimise spherical and chromatic aberration

Answer 1

Use achromatic doublet - consists of convex lens made of crown glass and concave lens made of flint glass cemented together - allows light to focus in the same position.

Answer 2

-glass must be pure and free from defects ~hard to achieve in large diameters - large lenses can bend and distort under own weight due to heaviness -chromatic and spherical aberration both affect lenses. -refracting telescopes are incredibly heavy and difficult to manoeuvre. -large magnifications require large diameter objective lenses with very long focal lengths. -lenses only supported from edges

Answer 3

-mirrors that are few nanometres thick can be made and give excellent image quality. -mirrors unaffected by chromatic aberration and spherical aberration be solved using parabolic mirrors. -mirrors not as heavy as lenses, so easier to handle -large composite primary mirrors can be made from lots of smaller mirror segments -large primary mirrors easy to support from behind since you do not need to see through them

Answer 4

-radio telescopes use radio waves to create images of astronomical objects. -the atmosphere is transparent to large range of radio wavelengths (it does not absorb them) so it’s possible to build radio telescopes that are ground-based. -they should be in isolated locations to avoid interference from nearby radio sources. -the simplest radio telescope uses a parabolic dish to focus radio waves onto a receiver.

Answer 5

-both telescopes function in the same way: they intercept and focus incoming radiation to detect its intensity -both telescopes can be moved to focus on different sources of radiation, or to track moving source. -the parabolic dish of a radio telescope is similar to the objective mirror of a optical telescope. -both telescopes can be built on the ground since both they can pass easily through the atmosphere.

Answer 6

-radio wavelengths larger than visible wavelengths, radio telescopes have to be larger in diameter than optical telescopes to achieve same quality to have same resolving power. -construction of radio telescopes is cheaper and simpler because wire mesh is used instead of mirror. -radio telescopes experience man-made interference from radio transmissions, phones, microwaves etc. - -optical telescopes experience interference from the weather conditions, light pollution etc

Answer 7

-Infrared telescopes use IR to create images of astronomical objects. -telescopes consist of large concave mirrors which focus radiation onto detector. -all objects emit infrared radiation as heat, infrared telescopes must be cooled using cryogenic fluids (liquid nitrogen or hydrogen) to almost absolute zero. -must be well shielded to avoid thermal contamination from nearby objects and its own infrared emissions. - used to observe cooler regions in space. - atmosphere absorbs most IR so telescopes must be launched into space and accessed from ground.

Answer 8

UV telescopes use UV radiation to create images of astronomical objects. Ozone layer blocks all UV rays that have a wavelength of less than 300nm, meaning UV telescopes need to be in space. Telescopes use Cassegrain configuration to bring UV rays to a focus. Rays detected by solid state devices which use photoelectric effect to convert UV photons into electrons, which then pass around circuit. UV telescopes used to observe interstellar medium and star formation regions.

Answer 9

X-ray telescopes use X-rays to create images of astronomical objects. X-rays are absorbed by atmosphere, telescopes need to be in space to collect data. These rays have high energy that using mirrors like optical telescope wouldn’t work as they pass through. Means X-ray telescopes made from combination of parabolic and hyperbolic mirrors which must be smooth. Rays enter telescope, skim off mirrors, and brought into focus on CCDs - convert light into electrical pulses. X-rays high-energy, used to observe high-energy events and areas of space such as galaxies, black holes

Answer 10

Gamma telescopes use GR to create images of astronomical objects. Telescopes don’t use mirrors as GR have high energy so pass through. Instead, they use detector made of layers of pixels. As gamma photons pass through, they cause signal in each pixel These telescopes observe things like gamma ray bursts (GRBs), black holes and solar flares.

Answer 11

➔ Short-lived - last anywhere between 0.01 and 1 second, and thought to be associated with merging neutron stars or a neutron star falling into black hole. ➔ Long-lived - these can last between 10 and 1000 seconds, and they are associated with Type II supernova

Answer 12

- a measure of ability of a lens or mirror to collect incident EM radiation. The collecting power increases with the size of the objective lens/mirror. Its directly proportional to area of objective lens. collecting power ∝ (objective diameter)2 The greater the collecting power, the brighter the images produced by the telescope.

Answer 13

- the ability of telescope to produce separate images of close-together objects. For image to be resolved, angle between straight lines from Earth to each object must be at least the minimum angular resolution

Answer 14

θ = Dλ Where λ is the wavelength of radiation and D is the diameter of the objective lens or objective mirror. 0 is in radians

Answer 15

-states that two objects will not be resolved if part of central maximum of either of images falls within first minimum diffraction ring of other. -as light enters telescope, its diffracted in a target-like shape called an ‘airy disc’ -the central maximum is the bright white circle in the centre, and each of the dark rings around it are the minimum rings.

Answer 16

an array of light-sensitive pixels, which become charged when they are exposed to light by the photoelectric effect.

Answer 17

the percentage of incident photons which cause an electron to be released.

Answer 18

the detectable range of wavelengths of light.

Answer 19

the total number of pixels used to form the image on a screen. A lot of small pixels will be able to resolve an image more clearly than a small amount of large pixels.

Answer 20

the minimum distance two objects must be apart in order to be distinguishable. This is used to observe small details.

Answer 21

How easy images are to use and form