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Astrophysics 🪐 > Telescopes > Flashcards

Telescopes Flashcards

1
Q

Convex/converging lens

A

Focuses incident light

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2
Q

Concave/diverging lens

A

Spreads out incident light

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3
Q

Principal axis

A

The line passing through the centre of the lens at 90° to its surface

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4
Q

Principal focus

A

Converging lens: point where incident beams, passing parallel to principal axis will converge
Diverging lens: point from which light rays appear to come from same distance on either side of lens

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5
Q

Focal length (f)

A

Distance between centre of lens and principal focus
The shorter the focal length, stronger the lens
fo + fe = length

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6
Q

Real image

A

Formed when light rays cross after refraction – form on screen

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7
Q

Virtual image

A

Formed on the same side of lines – light rays, don’t cross.
Can’t form on screen.

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8
Q

Lens/Power formula

A

1/u + 1/v = 1/f
u = distance of object from the centre of lens
V = distance of image from centre of lands.
F = focal length of lens.

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9
Q

Power of lens

A

Measure of how closely a lens can focus a beam- parallel to principal axis
Measured in dioptres (D)
Converging is positive.
Diverging is negative.

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10
Q

Astronomical telescope

A

Comprised of two converging lens

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11
Q

Objective lens

A

Role is to collect light and create a real image of a distant object.
long focal length and is large to collect as much light as possible

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12
Q

Eyepiece lens

A

Magnifies the image produced by objective lens.
Produces virtual image of infinity- reduce eyestrain.

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13
Q

Normal adjustment telescope

A
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14
Q

Angular magnification

A

M = a/b
When less than 10°
M= fo/fe

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15
Q

Reflecting telescopes

A

Two types: Newtonian , Cassegrain

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16
Q

Newtonian telescope

A
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17
Q

Newtonian telescope 2

A

Light is collected and focused on to an eyepiece lens, using a plane mirror

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18
Q

Cassegrain telescope

A
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19
Q

Cassegrain telescope 2

A

Convex mirror allows Cassegrain to be shorter than Newtonian

20
Q

Mirrors in reflecting telescopes

A

-mirrors in reflecting telescope are thin coating of ​aluminium or silver atoms​ deposited onto backing material.
-allows mirrors to be ​as smooth as possible​ and ​minimises distortions​ in the image.

21
Q

Chromatic aberration

A

– focal length of red light is greater than blue, meaning they focus on different points.
– white object produces image with coloured fringing.
– since chromatic aberration is caused by refraction. there is a little effect on reflecting telescopes as it only occurs in eyepiece

22
Q

Chromatic aberration diagram

A
23
Q

Spherical aberration

A

-Curvature of land is cause, raise our eyes to be focused at different points.
-Leads to image, blurring and distortion.
-Produced in lenses with large diameters.
– fixed by using parabolic objective mirrors

24
Q

Spherical aberration diagram

A
25
Q

Minimise spherical and chromatic aberration

A

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.

26
Q

Disadvantages of refracting telescopes

A

-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

27
Q

Advantages of reflecting telescopes

A

-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

28
Q

Radio telescopes

A

-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.

29
Q

Similarities in radio and optical telescopes

A

-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​.

30
Q

Differences between radio and optical telescopes

A

-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

31
Q

Infrared telescopes

A

-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.

32
Q

UV telescopes

A

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​.

33
Q

X-ray telescopes

A

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

34
Q

Gamma telescopes

A

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​.

35
Q

Two types of GBRs

A

➔ 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​

36
Q

Collecting power

A
  • ​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.
37
Q

Resolving power

A

​- ​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

38
Q

Minimum angular resolution

A

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

39
Q

Rayleigh criterion

A

-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.

40
Q

CCD

A

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

41
Q

Quantum efficiency

A

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

42
Q

Spectral range

A

the detectable range of wavelengths of light.

43
Q

Pixel resolution

A

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.

44
Q

Spatial resolution

A

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

45
Q

Convenience

A

How easy images are to use and form

Astrophysics 🪐 (4 decks)

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