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A LEVEL PHYSICS > Astro telescopes > Flashcards

Astro telescopes Flashcards

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1
Q

What is a lens

A

A piece of glass that refracts incident radiation

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2
Q

Types of telescopes in astronomy

A

Optical - Use visible light to produce images

Non optical - Use non visible parts of em spectrum to produce images
(more commonly used in astronomy)

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3
Q

Main types of optical telescopes used in astronomy

A

Refracting - Image produced when radiation refracts through glass
Expensive, easily distort and difficult to manufacture

Reflecting - Image produced when image is reflected off glass
Cheaper and easier to manufacture

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4
Q

Makeup of refracting and reflecting telescopes

A

Refracting - two converging lenses in normal adjustment

Reflecting - two mirrors and a converging lens (Cassegrain arrangement)

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5
Q

How many times is light refracted through a lens

A

Twice upon reaching each surface of lens - however treat as if refracted once at centre

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6
Q

Diagram defs

A

Principle axis - Straight line through lens perpendicular to lens

Principal focus/focal point - point where rays parallel to principal acid are focused to.

Focal length - distance from centre of lens to principal focus

Focal plane - plane on each side perpendicular to principal axis containing principal focus

Where lines cross an image is formed

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7
Q

Rules for drawing ray diagrams

A

Rays parallel to lhs principal axis (axial rays) refracted through rhs principle focus - works both ways - if a ray emerges parallel to axis then it must have come from lhs principle focus.

Convex lens as pointy arrows up and down

Rays passing through the centre of the lens (origin) pass straight through

Non axial rays more likely as only 1° of orientation gives axial whereas 359 give non axial.

Non axial parallel rays refracted to same point on focal plane - can be determined by ray going through centre of lens - undisturbed

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8
Q

Why do rays passing through the centre not change direction

A

Lens is thin and it’s surfaced are parallel to each other at the axis

(Symmetry of lens means deviation at first point is cancelled by deviation at second point)

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9
Q

Examples of convex lenses

A

Lens in phone camera and eye

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10
Q

Real Vs virtual image

A

Real formed when rays from object pass through another point in space

Virtual when rays from point on object appear to have come from a point in space - can’t be projected on a screen

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11
Q

Conditions for converging lens to form real/virtual image

A

If object between 1 and 2 focal lengths away - magnified, real and inverted. Same size when at 2f, diminished beyond 2f

If closer then virtual and upright
Closer to lens smaller virtual image. Max at focal distance

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12
Q

Lens eq + derivation

A

1/f = 1/u + 1/v

Draw diagram then pairs of similar triangles arrive at v/u = v-f/f

Note if v is negative, image is virtual

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13
Q

Optical astronomical telescope that uses a converging lens

A

Astronomical refracting telescope

(Refractors)

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14
Q

Makeup of astronomical telescope

A

Objective and eyepiece lens in normal adjustment, objective used to form real image from parallel rays, eyepiece magnifies image - virtual image formed at inf

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15
Q

Normal adjustment meaning

A

Final image at infintity

Distance between lens = fo+fe

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16
Q

Drawing refracting telescope diagram

A

Fo»fe

Draw non axial ray through centre to focal plane. Draw parallel ray (as from inf) through lhs f emerging parallel to axis. 3rd ray parallel and meeting at intersection.

Draw construction line from poi through centre of eyepiece lens. Refract non axial rays through eyepiece lens to leave lens parallel to construction line - dashed lines to show virtual image at infinity.

Detector e.g. eye refracts rays to produce image

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17
Q

Angular magnification + consequences

A

Angle subtended by image/angle subtended by object

Angle subtended by object can be used to calculate diameter of object (e.g. star) or distance between object and telescope
S=rø

As fo must be greater than fe, this makes refracting telescopes difficult to construct (must be v long for large magnification)and sag easily (they can only be supported at the edges as radiation needs to pass through the glass)

Total length of telescope= fo+fe

Note magnification and angular magnification are the same value for telescope, only referred to as angular when derived from change in ø

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18
Q

Deriving M=fo/fe (in normal adjustment)

A

Consider a and b then height of real image formed at focal plane. Eliminate h and use sma

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19
Q

Aberration for refracting telescopes + other drawbacks

A

Chromatic - image blurred due to different colours varying in image position. As most celestial objects emit white light. Shorter wavelength greater refraction (greater angular deflection after refraction.) Focal length varies for different wavelengths

Can only be supported from edges - lead to lenses being distorted

Impurities/bubbles in lens can absorb and scatter some radiation - telescopes struggle to detect optically faint objects

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20
Q

Cassegrain arrangement + notes

A

Parabolic (primary) mirror first mirror radiation hits, convex mirror is secondary. +Eyepiece lens.
Concave mirror converges axial rays from an object onto a principle focus. If mirror isn’t perfectly parabolic (slightly spherical), multiple foci will form after reflection - spherical aberration. (Focus produced if rays cross over). Image blurred if multiple foci form.

Once primary mirror reflects to principal focus, convex mirror placed just in front of principle focus. Image would be formed at primary focus but interacts w mirror first (if there was a detector at point of principle focus, it would block out incident radiation, draw to prove it).

Secondary mirror reflects light through hole in concave mirror - hole can be in shadow of secondary mirror as it doesn’t receive incident radiation as convex mirror blocks it out.
Forms real image beyond primary mirror, eyepiece lens used to magnify.

U can also get refracting telescope issues as well bc of this e.g. chromatic aberration.

Rays refracted parallel from eyepiece lens, forming virtual image at infinity

Secondary mirror + support can block out incoming light and diffract some reflected light around it reducing image clarity

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21
Q

Pros of Cassegrain telescopes

A

Large mirrors cheaper to build than large lenses. Can also be supported from underneath so don’t distort as much as lenses

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22
Q

Rayleigh criterion for “diffraction limit to resolution”

A

States that two images are just resolvable when the CENTRE of the diffraction pattern of one is directly over the first minimum of the other diffraction pattern

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23
Q

Collecting power

A

Amount of radiation received by telescope per second

Proportional to area (I=P/A) therefore d^2

Larger collecting power can produce images of fainter objects in universe

Greater area of dish, greater collecting power

24
Q

Resolving power

A

Measure of how much detail can be observed by telescope. Ability to see different objects as separate (resolution).

Resolving power also limited by quality of detector e.g. ccd or eye - no point having a telescope w greater resolution than the detector as finer detail will be lost when registering image

25
Q

Minimum angular resolution

A

Smallest angular separation at which the instrument can distinguish two points/objects. Ideally telescope has low minimum angular resolution.

Ø= 1.22lamda/D where D is diameter of dish
Theta in radians

Lower minimum greater resolving power

26
Q

What type of em radiation makes the majority

A

Radio

27
Q

Majority of radiation emitted by celestial bodies

A

Non optical

28
Q

Structure of radio telescopes

A

Primary concave dish (made of mesh wire), reflects waves to a focal point (antenna placed to detect then information transmitted for analysis)

Pre amplifier amplifies weak radio signals without adding too much noise before sending off

29
Q

Pros of radio telescopes

A

Ground based observations limited by atmosphere, however radio and visible radiation least limited - allows radio telescopes to exist on ground

Primary dish can be made of wire mesh instead of glass, as radiowave doesn’t have to be optically reflected (like visible in an optical telescope) Since radiowaves have longer λ so dish doesn’t have to be made as precisely. Wire mesh reduces weight and cost of production

No need for eyepiece lens as non optical

Manoeuvrable - allow source of the waves to be tracked, sources move across sky due to earth rotating and not source moving. Dish can be turned by motors to compensate for rotation

Can be used during day and night as there’s no interference from visible

30
Q

Cons of radio telescopes

A

Man made interference - radio transmissions, phones, radar etc can make detecting radio emitters difficult. Difficult to deal with interference from orbiting satellites, but effect of things e.g. microwave ovens minimised by building away from centre of population.

Large λ so large minimum angular resolution, poor resolving power. Hard to identify sources. Radio telescope needs to be abt 1mil times bigger to have same resolving power as optical telescope. (ratio of λ )

31
Q

Overcoming resolving power of radio telescopes

A

Very poor, worse than unaided eye, so astronomers link lots of telescopes together - combine data from many telescopes to form a single image. Effective diameter is distance between telescopes. Gives thousands x better resolving power.

Linking telescopes also helps to remove local interference from individual detectors, enhancing common signal from distant sources

Radio telescope has to scan across a source to build an image

32
Q

Why radio teles are easier to manufacture

A

Primary dish from wire mesh - cheaper + lighter than glass

Longer wavelength of radiation, less affected by imperfections - less spherical aberration

33
Q

Condition for radiowaves to be reflected by and not diffract through the mesh

A

Mesh spacing must be smalling than λ /20.

λ very large so this is good

34
Q

Infrared telescopes

A

Large concave reflector which focuses infrared radiation onto a detector at a focal point. Infrared has longer λ so less affected by imperfections. CCDs or special photographic paper used as detector

35
Q

Pros of infrared teles

A

Long λ so less affected by spherical aberration. Objects in space that aren’t hot enough to emit light emit infrared, can therefore provide images that can’t be seen using optical telescopes.

36
Q

Caveats for infrared teles

A

Produce their own IR radiation due to their temperature so need to be cooled to very low temps using liquid helium or refrigeration units to prevent IR swamping IR from space. (Otherwise would produce interference)

Water vapour in the atmosphere absorbs IR radiation so telescope needs to be sited where atmosphere is as dry and high as possible (mountain in hawaii, water vapour there has less effect than at lower level)

Can be put on satellites in orbit of Earth - not affected by water vapour however must be cooled to a few degrees above 0K to detect IR from weak sources

37
Q

UV telescopes

A

Parabolic reflector onto CCD/special photographic film detector. Must be carried on satellites as UV absorbed by atmosphere. UV also absorbed by glass so tele uses mirrors to focus incoming UV radiation onto detector. UV emitted by high temp stars, so useful to map hot gas clouds and study hot objects in space e.g. glowing comets, supernovae and quasars.

Comparing UV image w optical/infrared image gives useful info about hot spots in object

38
Q

X ray telescopes

A

X rays don’t really reflect, are usually absorbed/pass straight through. Can be made to reflect by “grazing” a mirror, by using a series of nested mirrors, can be made to come to a focus on a detector (grazing telescope)

Tend to use geiger counters or ccds as detectors

X rays absorbed by atmosphere, need to be in space
Diffraction insignificant due to short λ

39
Q

What can X ray telescopes detect

A

X ray pulsars - stars emitting x ray beams that sweep around sky as they spim

X and gamma ray bursters billions of light yeats away which emit bursts of gamma rays

40
Q

Atmosphere absorption notes

A

Atmosphere only allows certain wavelengths through, transparent to visible and radio, opaque to others. However some spectral windows where atmosphere is transparent to infrared for certain wavelengths. between 3-5um and 7-14um

41
Q

Problem w space based IR telescopes

A

Expensive, difficult to maintain conditions in space - can’t have coolant replenished. Instead place on aeroplane/weather balloon

42
Q

Detectors for non optical telescopes

A

CCD (or special photographic film)

43
Q

UV telescope cons

A

Short wavelength - heavily affected by imperfections, mirror needs to be very parabolic. UV also absorbed by ozone - telescope needs to be in space as ozone above atmosphere. Strap to high altitude aeroplanes

44
Q

Telescope note

A

Many operate beyond their expected time, and mods can be made to correct faults via direct intervention, or by changing the mission objective

45
Q

Main reason for putting a telescope in space

A

Absorption of em waves by atmosphere, light pollution and interference. Effect atmosphere has on light as it passes through

46
Q

CCD

A

Array of light sensitive pixels which become charged when exposed to radiation. After exposure, array connected to circuit which transfers charge collected by each pixel to output electrode connected to capacitor. Voltage of output electrode read electronically then capacitor is discharged before next pulse. Output electrode produces a stream of pulses, with amplitude in proportion to energy received by each pixel. Pulses are stored and used to create an image

47
Q

How CCD works

A

Photoelectric effect - photons incident on silicon chip, if E>Φ then electron liberated from pixel, amount of charge builds up to form image. Dividers trap charge so it’s retained on specific part. When exposure is complete electrodes used to shuffle electrons along array so charge from each well measured. Some photons his ccd but don’t cause photoemission

Number of electrons trapped in well proportional to incident photons, ie to intensity of radiation. So pattern in array is same as image pattern of photons

48
Q

Quantum efficiency notes

A

number of photons registered/total incident photons
x 100

QE of CCD approx 80%
QE of eye 1-2%
QE of grains of photographic film 4%

49
Q

Resolution of ccd

A

Depends on size of each pixel, typically 10um. Two images on a ccd need to be separated by at least one unilluminated pixel to be seen separately. minimum spacial resolution of eye 100um

(light sensitive receptors of eye about 5um, although cells aren’t evenly distributed on retina, also away from centre of retina, cells joined to each nerve fibre so effective size is greater than 5um)

Brackets is yap

50
Q

Further advantages of ccd

A

Can be used to record changes of an image - can record sequence of fast changing celestial images which can be seen by the eye but not followed, and can’t be recorded on film.

Wavelength sensitivity (100nm to 1100nm) greater than eye (350nm to 650nm) hence can be used with suitable filters to obtain infrared images.

QE is the same from 400nm to 800nm, reducing to zero below 100nm and at 1100nm

Low light intensity, loss of colour vision from eye, not a problem for ccd

However ccds in astro need to have a large number of pixels in a small area compared to ccds in electronic cameras.

ccds often cooled to low temps otherwise random emissions of electrons causes a “dark current” which doesn’t depend on light intensity

If you leave it on for too long, charge could build up too much and spill over dividers - bleeding - causing image to be smeared

51
Q

CCD vs eye again

A

CCD QE 70% eye 1%
CCD detects all wavelengths eye only visible
CCD spatial resolution 10um eye 100um
CCD higher resolving power
CCD you can copy and share images more easily - sharing info
Eye easier to use

52
Q

Spaital resolution

A

Minimum distance between objects which allows them to be seen as separate.

depends on number of pixels per cm^2

53
Q

Reflecting vs refracting

A

Problems of refractors:
Can suffer spherical aberration and chromatic aberration.
Reflecting are lighter. Reflecting are shorter. Mirrors do not suffer
from chromatic aberration.
Problems of reflectors:
Spider/secondary mirror block some of the light/reduce image
brightness/cause diffraction effects.

54
Q

When commenting on whether things can be seen w teles

A

Comment on whether a “detailed view” can be obtained rather than just seeing it

55
Q

Positioning of optical telescopes

A

Light pollution requires them to be away from centre of population/High up to avoid obscurity from clouds

Radio telescopes need to be located from other radio radiation

A LEVEL PHYSICS (17 decks)

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