14: Astrophysics - Telescopes Flashcards
how do lenses change the direction of light rays?
Refraction
What do convex/converging lenses do?
focus incident light
What do concave/diverging lenses do?
spread out incident light
What is the principal axis?
the horizontal axis through the centre of a lens at 90 degrees to its surface
what is the lens axis
the vertical axis through the centre of a lens
What are axial rays
Rays parallel to the principal axis
what is the principal focus in a converging lens?
the point at which the incident beams passing parallel to the principal axis converge
what is the focal plane
the plane perpendicular to the principal axis that contains the principal focus
what is the focal length of a lens
the perpendicular distance between the lens axis and the focal plane
- the shorter the focal length, the stronger the lens
what is the principal focus in a diverging lens?
the point from which the light rays appear to come from. This is the same distance either side of the lens
when is a real image formed
when light rays cross after refraction
- real images can be formed on a screen
virtual image
- formed on the same side of the lens
- light ray do not cross, so a virtual image cannot be formed on a screen
What is the lens formula?
1/u + 1/v = 1/f = P
where u is the distance of the object from the centre of the lens
v is the distance of the image from the centre of the lens
f is the focal length of the lens
P is the power of the lens
what is the power of a lens
a measure of how closely a lens can focus a beam that is parallel to the principal axis (iow how short the focal length is)
the shorter the focal length, the _____ powerful the lens
more
Is the value of the power of a converging lens +ve or -ve?
Positive
Is the value of power in a diverging lens +ve or -ve?
negative
What is the power of a lens measured in?
Dioptres (D)
What are the two converging lenses in a refracting telescope?
- objective lens
- eyepiece lens
What is the role of the objective lens in a refracting telescope?
- collect light and create a real image of a very distant image
What is the role of the eyepiece lens in a refracting telescope?
- magnifies the image produced by the objective lens so that the observer can see it
- the lens produces a virtual image at infinity since light rays are parallel, which reduces eye strain for the observer
What properties should the objective lens of a refracting telescope have?
- long focal length and large to collect as much light as possible
What is normal adjustment for a refracting telescope?
when the distance between the objective lens and the eyepiece lens is the sum of their focal length, meaning the principal focus for the two lenses is in the same place
how do you calculate magnifying power or angular magnification of a telescope?
M = angle subtended by the image at the eye/ angle subtended by the object at the unaided eye
What does the Cassegrain reflecting telescope involve?
- a concave primary mirror with a long focal length
- a small convex secondary mirror in the centre
- the convex mirror allows the Cassegrain to be shorter than other configurations
What are important properties of mirrors used in reflecting telescopes?
- very thin coating of aluminium or silver atoms that are deposited onto a backing material
=> allows the mirrors to be as smooth as possible and minimises distortions in the image
What is chromatic aberration?
for a given lens, the focal length of red light is greater than that of blue light, which means they are focused at different points (since blue is refracted more than red).
What is the effect of chromatic aberration?
Can cause a white object to produce an image with coloured fringing, with the effect being most noticeable for light passing through the edges of the lens
What is the effect of chromatic aberration on reflecting telescopes?
Since is caused by refraction, it has very little effect on reflecting telescopes as it only occurs in the eyepiece lens
What is spherical aberration?
the curvature of the lens or mirror can cause rays of light at the edge to be focused in a different position, leading to image blurring and distortion
How can you avoid spherical aberration?
Using parabolic objective mirrors in reflecting telescopes
What is achromatic doublet?
A way of minimising spherical and chromatic aberration, consisting of a convex lens made of crown glass and a concave lens made of flint glass cemented together to bring all rays of light into focus in the same position
What are the disadvantages of refracting telescopes?
- glass must be pure and free from defects which is hard for a large-diameter lens
- large lenses can bend and distort under their own weight
- chromatic and spherical aberration both affect lenses
- incredibly heavy so difficult to manoeuvre
- large magnifications require very large diameter lenses
- lenses can only be supported from the edges
What are the advantages of reflecting telescopes?
- mirrors are a few nm thick so have excellent image quality
- mirrors are unaffected by chromatic aberration, and spherical aberration can be solved
- mirrors are not as heavy as lenses, so easy to handle and maneouvre
- chromatic aberration can be solved using an achromatic doublet
What are the similarities between radio telescopes and optical telescopes?
- both telescopes function in the same way: they intercept and focus incoming radiation to detect its intensity
- Both can be moved to focus on different sources of radiation or to track a moving source
- the parabolic dish of a radiotelescope is extremely similar to the objective mirror of a reflecting optical telescope
- can be built on the ground since both radio waves and optical light can pass easily through the atmosphere
What are the differences between radio telescopes and optical telescopes?
- Since radio wavelengths are much larger than visible wavelengths, radio telescopes have to be much larger in diameter to achieve the same resolving power
- construction of radio telescopes is cheaper and simpler because a wire mesh is used instead of a mirror
- A radio telescope must move across an area to build up an image
- Radio telescopes experience a large amount of man-made interference from radio transmissions, phones, microwave ovens
- Optical telescopes experience interference from weather, light pollution and stray radiation
How do infrared telescopes work?
- They use infrared radiation to create images of astronomical objects
- They consist of large concave mirrors which focus radiation onto a detector
What are the limitations of infrared telescopes?
- Telescopes must be cooled using cryogenic fluids to almost absolute zero since all objects emit infrared radiation as heat
- Must be well shielded to avoid thermal contamination from nearby objects as well as its own infrared emissions
How are infrared telescopes used?
- Used to observe cooler regions in space
- the atmosphere absorbs most infrared radiation so these telescopes must be launched into space
What do ultraviolet telescopes do?
Use ultraviolet radiation to create images of astronomical objects
Where to UV telescopes have to be positioned?
In space, as the ozone layer blocks all UV rays that have a wavelength less than 300nm
how do UV telescopes work?
- use the Cassegrain configuration to bring UV rays to a focus
- Rays are detected by solid state devices which use the photoelectric effect to convert UV photons into electrons, which pass around a circuit
What are UV telescopes used to observe?
Interstellar medium and star formation regions
What do X-ray telescopes do?
Use X-rays to create images of astronomical objects
Where do X-ray telescopes need to be positioned?
In space, as all X-rays are absorbed by the atmosphere
What do X-ray telescopes need to be made out of?
A combination of very smooth parabolic and hyperbolic mirrors, as rays would pass through normal mirrors.
How do X-ray telescopes work?
Rays enter the telescope, skim off the mirrors and are brought into focus on CCDs which convert light into electrical pulses.
What are X-ray telescopes used to observe?
High energy events and areas of space such as active galaxies, black holes and neutron stars
What do gamma telescopes do?
Use gamma radiation to create images of astronomical objects
What are gamma telescopes made of?
- Not mirrors because rays have so much energy they would just pass straight through
- Use a detector made of layers of pixels
- As gamma rays pass through they cause a signal in each pixel they come into contact with
What are gamma telescopes used to observe?
Gamma ray bursts (GRB)
Quasars
Black holes
Solar flares
What are the two types of GRBs?
- Short-lived (last between 0.01-1s, associated with merging neutron stars, or a neutron star falling into a black hole)
- Long-lived (last between 10-1000s, associated with a type II supernova)
What is collecting power?
A measure of the ability of a lens or mirror to collect incident EM radiation
The greater the collecting power…
… the greater the size of the objective lens/mirror …. + … the brighter the images produced by the telescope
What is resolving power?
the ability of a telescope to produce separate images of close-together objects
How do you resolve an image?
the angle between the straight lines from Earth to each object must be at least the minimum angular resolution (θ), where θ is in radians.
θ = λ/D
where λ is wavelength and D is the diameter of the objective lens/mirror
What is the Rayleigh Criterion?
Two objects will be not be resolved if any part of the central maximum of either of the images falls within the first minimum diffraction ring of the other
What are charged-coupled devices (CCDs)
an array of light-sensitive pixels, which become charged when they are exposed to light by the photoelectric effect
What features of a CCD can you compare with the human eye?
- Quantum efficiency: the percentage of incident photons which cause an electron to be released
- Spectral range: the detectable range of wavelengths of light
- pixel resolution: the total number of pixels used to form the image on a screen
- spatial resolution : the minimum distance two objects must be apart in order to be distinguishable
- Convenience: how easy images are to form and use
How does the quantum efficiency of a CCD compare with the human eye?
CCD approx 80%
Human eye 4-5%
How does the spectral range compare of a CCD compare with the human eye
CCD: Infrared, UV, visible
Human eye: Only visible
How does the pixel resolution of a CCD compare with the human eye?
CCD: ~50 megapixels
Human eye: ~500 megapixels
How does the spatial resolution of a CCD compare with the human eye?
CCD: ~ 10µm
Human eye: ~100µm
How does the convenience of a CCD compare with the human eye?
CCD: Needs to be set up, but images produced are digital
Human eye: Simpler to use as there is no need for extra equipment
Which is more useful for detecting finer details and producing images that can be shared and stored? CCD or human eye?
CCD
