Astrophysics Flashcards
Another name for convex lenses
Converging lenses
What do convex lenses do
Focus incident light
Another name for concave lenses
Diverging lenses
What do concave lenses do
Spreads out incident light
What is the principal axis
Line passing through centre of lens at 90 degrees to its surface
Principal focus (F) in a converging lens
Point where an incident beam passes parallel to principal axis will converge
Principal focus (F) in a diverging lens
Point where light ray appears to come from, same distance from either side of lens
What is the focal length (f)
Distance between centre of lens and principal focus
How focal length affects strength of lens
Shorter focal length = stronger lens
What is a real image
Formed when light rays cross after refraction, can be formed on screens
What is a virtual images
Formed on the same side of the lens, light rays do not corss, so can’t be formed on screen
Lens formula
(1 / distance of object from centre of lens) + (1 / distance of image from centre of lens) = (1 / focal length)
What is the power of a lens
Measure of how closely a lens can focus a beam that is parallel to principal axis - to do with focal length
How focal length affects power of lens
Shorter focal length = more powerful
Power of lens value for converging and diverging lenses
Converging - positive, diverging - negative
What is power of a lens measure in
Dioptres (D)
Power of lens formula
P = (1 / u) + (1 / v) = (1 / f)
P = (1 / u) + (1 / v) = (1 / f) what is P
Power of lense
P = (1 / u) + (1 / v) = (1 / f) what is u
Distance of object from centre of lens
P = (1 / u) + (1 / v) = (1 / f) what is v
Distance of image from centre of lens
P = (1 / u) + (1 / v) = (1 / f) what is f
Focal length of lens
What are refracting telescopes comprised of
2 converging lenses
Converging lenses making up a refracting telescope
Objective lens, eyepiece lens
What is an objective lens
Collects light, makes real image of a very distance object, should have a long focal length and be large to collect as much light as possible
What is the collecting power of a telescope directly proportional to
Square of radius of objective lens
What is an eyepiece lens
Magnifies image produced by objective lens so that observer can see it, produces virtual image at infinity since light rays are parallel, reduces eye strain as observer doesn’t have to refocus every time they look between the telescope image and object in the sky
What is the normal adjustment of a refracting telescope
When distance between objective lens and eyepiece lens is sum of focal lengths, (f_o + f_e), so principal focus for the two lenses is in the same place
Ray diagram for a refracting telescope in normal adjustment
Photo 1
Another name for magnifying power
Angluar magnification
Formula for magnifying power of a telescope
M = (angle subtended by eye at image at the eye) / (angle subtended by object at unaided eye) = a / b = larger angle / smaller angle
If a and b are both under 10 degrees, what does the magnifying power of a refracting telescope formula become
f_o / f_e
When can f_o / f_e be used
When a and b are both under 10 degrees
f_o / f_e diagram
Photo 2
Most common type of reflecting telescope
Cassegrain reflecting telescope
What is a cassegrain reflecting telescope
Concave primary mirror, long focal length, small convex secondary mirror at centre, light is collected and focused on eyepiece lens
What does the secondary convex mirror in a cassegrain reflecting telescope do
Allows cassegrain to be shorted than other configurations like Newtonian which utilises plane mirror
Cassegrain reflecting mirror diagram
Photo 3
Newtonian reflecting mirror diagram
Photo 4
Description of mirrors in reflecting telescopes
Very thin (often less than 25nm thick) coating of aluminium or silver atoms stuck on a backing material
Benefits of the structure of the mirrors used in a reflecting telescope
Allow the mirrors to be very smooth and minimises distortions in the image
What is chromatic aberrations
Focal light of red light is greater than that of blue, so focus on different points (blue is refracted more), can cause a white object to produce an image with a coloured fringing (coloured edges), with effect being more noticeable for light passing throguh the edges of the lens
Type of telescope - chromatic aberrations
Caused be refraction so has little effect on reflecting telsecopes as it only occurs in eyepiece lens
What is spherical abberation
When curvature of a lens or mirror causes rays of light at edges of lens to focus on a different position to the rays at the centre of te lens, leads to image blurring and distortion
When is spherical abbertation most pronounced
In lenses with a large diameter
How to avoid spherical abberations
Using parabolic objective mirrors in reflecting telescopes
What are achromatic doublets used for
To minimise spherical and chromatic abberations in lenses
What is an achromatic doublet made up of
Convex lens made of crown glass and a concave lens made of flint glass that have been cemented together
What does an achromatic doublet do
Brings all rays of light into focus in the same position
Achromatic doublet diagram
Photo 5
Disadvantages of refracting telescopes
Pure, weight, abberations, construction, size, support
Pure as a disadvantage of refracting telescopes
Glass must be pure and free from defects, very difficult for such large diameter lenses
Weight as a disadvantage of refracting telescopes
Lenses can bend and distort under own weight
Abberations as a disadvantage of refracting telescopes
Chromatic and spherical abberations affect lenses
Construction as a disadvantage of refracting telescopes
Incredibly heavy so difficult to manoeuvre
Size as a disadvantage of refracting telescopes
Large magnification requires large diameter objective lense and long focal lengths
Support as a disadvantage of refracting telescopes
Can only be supported from edges - large and heavy
Advantages of reflecting telescopes
Thin, abberations, lighter, achromatic doublets, mirror segments, support from behind
Thin as an advantage of reflecting telescopes
Only need to be a few nanometers thick and still give excellent image quality
Abberations as an advantage of reflecting telescopes
Mirrors are unaffected by chromatic abberations, spherical abberations can be solved by using parabolic mirrors
Lighter as an advantage of reflecting telescopes
Not as heavy as lenses so easier to handle and manoeuvre
Achromatic doublets as an advantage of reflecting telescopes
Can solve chromatic abberations (from eyepeice lens) bis achromatic doublets
Mirror segments as an advantage of reflecting telescopes
Large composite primary mirrors can be made from lots of smaller mirror segments
Support from behind as an advantage of reflecting telescopes
Large primary mirrors are easy to support from behind due to not needing to see through them
Which sort of telescopes are preferred
Reflecting over refracting
What do radio telescopes do
Create images of astronomical objects using radio waves
Why is possible to build ground based radio telescopes
Atmosphere is transparent to radio waves so it doesn’t absorb them
Why should radio telescopes be in isolated locations
To avoid interference from nearby radio sources
Basic principle of radio telescopes
Use parabolic dish to focus radio waves on receiver
Similarities between radio and optical telescopes
Function, movement, parabolic, ground
Function as a similarity between radio and optical telescopes
Both intercept and focus incoming radiation to detect its intensity
Movement as a similarity between radio and optical telescopes
Can be moved to focus on different sources or to track moving sources of radiation
Parabolic as a similarity between radio and optical telescopes
Parabolic dish of radio telescope is similar to objective mirror of a reflecting optical telescope
Ground as a similarity between radio and optical telescopes
Can be ground-based as radio waves and optical light can pass through atmosphere easily
Differences between radio and optical telescopes
Size, cost, building up an image, interference
Size as a difference between radio and optical telescopes
Radio wavelengths are much larger than visible wavelengths so radio telescopes have to have a bigger diameter to achieve the same resolving power as an optical telescope, due to the larger diameter, radio telescopes have a much larger collecting power
Cost as a difference between radio and optical telescopes
Radio telescopes are cheaper and simpler to build because a wire mesh is used instead of a mirror, given the mesh size is less than (wavelength) / 20, radio waves will be reflected (not refracted)
Building up an image as a difference between radio and optical telescopes
Radio telescopes have to move across an area to build up an image, unlike optical telescopes
Interference as a difference between radio and optical telescopes
Radio telescopes experience a large amount of man-made interference from radio transmissions, phone, microwave ovens etc., optical telescopes experience interference from weather conditions, light pollution, stray radiation etc.
What do infrared telescopes do
Create images of astronomical objects using infrared radiation
Basic principle of infrared telescopes
Large concave mirror which focuses radiation on a detector
How is infrared radiation emitted
As heat
How to overcome the heat of infrared radiation for infrared telescopes
Cool the telescope using cryogenic fluids (liquid nitrogen or hydrogen) to almost absolute zero
How to prevent interference to infrared telescoper
Telescope must also be well shielded to avoid thermal contamination from nearby objects as well as its own infrared emission
What are infrared telescopes used for
To observe cooler regions in space
Issue with ground based infrared telescopes
Atmosphere absorbs most infrared radiation so telescopes must be launched into space and accessed remotely from the ground
What do ultraviolet telescopes do
Create images of astronomical objects using ultraviolet radiation
Where do ultraviolet telescopes need to be placed and why
Space, ozone blocks all ultraviolet rays with a wavelengths of less than 300nm
Basic principle of ultraviolet telescopes
Utilise cassegrain configuration to bring rays to focus, rays are detected by solid state devices which use photoelectric effect to convert UV photons into electrons which pass around circuit
What can ultraviolet telescopes be used for
To observe interstellar medium and star formation regions
What do x-ray telescopes do
Create images of astronomical objects using x-rays
Where do x-ray telescopes need to be placed and why
Space, atmosphere absorbs all x-rays
Why do normal mirrors not work for x-rays
Rays have such high energy that they would pass straight through mirrors in a normal optical telescope
X-ray telescope basic principle
Made from a combination of parabolic and hyperbolic mirrors - must all be extremely smooth, rays enter telescope, skim off the mirrors and are brought to focus on CCDs which convert light into electrical pulses
What can x-ray telescopes be used for
Can be used to observe high-energy events and areas of space such as active galaxies, black holes and neutron stars
What do gamma telescopes do
Create images of astronomical objects using gamma radiation
Why don’t gamma telescopes use mirrors
Gamma rays have so much energy that they would pass straight through a mirror
Gamma telescope basic principles
Used detectors made of layers of pixels, as gamma photons pass through they cause a signal in each pixel that they come into contact with
What can gamma telescopes be used for
Gamma ray bursts (GRBs), quasars, black holes, solar flares
How many types of gamma ray bursts (GRBs) are there
2
What are the 2 types of gamma ray bursts (GRBs)
Short-lived, long-lived
Short-lived gamma ray bursts
Last anywhere between 0.01 and 1 second, associated with merging neutron stars (forming a black hole), or a neutron star falling into a black hole
Long-lived gamma ray bursts
Can last between 10 and 1000 seconds, associated with a type 2 supernova (death of a massive star)
What is collecting power
Measure of ability of lens/mirror to collect incident electromagnetic radiation
Collecting power increases with size of lens/mirror elaboration
Collecting power is directly proportional to to the area of the objective lens
Area of objective lense formula
((pi)d^2) / 4
Collecting power is directly proportional to to the area of the objective lens implies that
Collecting power is directly proportional to to the diameter squared of the objective lens
How does the collecting power impact the image produced by a telescope
The greater the collecting power is, the brighter the images are
What is resolving power
Ability of telescope to produce separate images of close-together objects
What conditions need to be meet for an image to be resolved
Angle betweent the straight lines from Earth to each object must be at least the minimum angular resolution (theta), where theta is in radians
Minimum angular resolution formula
theta = wavelength / d
theta = wavelength / d what is theta
Minimum angular resolution
theta = wavelength / d what is d
Diameter of objective lens or mirror
What does the rayleigh criterion state
2 objects will not be resolved if any part of the central maximum of the image falls withing the first minimum diffraction ring of the other
What is an airy disc
Circular diffraction pattern, occurs when light enters telescope
Central maximum of airy disc
Bright white circle at centre
Minimum diffraction rings of airy disc
Dark rings around central maximum
Maximum diffraction rings of airy disc
Light rings around central maximum
What do CCD’s stand for
Charge-coupled devices
What are charge-coupled devices
Array of light sensitive pixels, become charged when exposed to light by the photoelectric effect
What features of CCDs can be compared to the human eye
Quantum efficiency, Spectral range, pixel resolution, spatial resolution, convenience
What is quantum efficiency
Percentage of incident photons which cause an electron to be released
What is spectral range
Detectable range of wavelengths of light
What is pixel resolution
Total number of pixels used to form an image on a screen (lots of small pixels are better than a few large pixels)
What is spatial resolution
Minimum distance between 2 objects to be distinguishable (used to observe small details)
What is convenience (comparison of CCDs and human eye)
How easy images are to form and use
Quantum efficiency - CCD vs human eye
CCD - 80%, Eye - 4-5%
Spectral range - CCD vs human eye
CCD - infrared, UV and visible, Eye - Only visible light
Pixel resolution - CCD vs human eye
CCD - varies, about 50 megapixels, Eye - about 500 megapixels
Spatial resolution - CCD vs human eye
CCD - 10 micrometers, Eye - 100 micrometers
Convenience - CCD vs human eye
CCD - needs to be set up, produces digital images, Eye - simpler to use, no need for extra equipment
Advantages of using CCDs
More useful for detecting finer details and producing images which can be shared and stored
