Astrophysics (Telescopes) Flashcards
What shape is a converging lens?
Convex (thicker in the middle)
What are axial rays?
Rays parallel to the principal axis
What is the effect of a converging lens on axial rays?
The rays converge at the principal focus
What is the effect of a converging lens on non-axial rays?
Rays converge at a point on the focal plane that is not the principal focus
What is the focal length, f?
Perpendicular distance between the lens axis and the focal plane
What is a real image?
An image formed when light rays from a point on an object actually meet
It can be projected onto a screen
What is a virtual image?
An image formed when light rays from a point on an object appear to have come from another point in space
It can’t be projected onto a screen
What is the lens equation and what does each symbol represent?
1/f = 1/u + 1/v
f = focal length
u = distance between object and the lens axis
v = distance bewteen image and the lens axis (v > 0, image is real; v < 0, image is virtual)
What are the names of the lenses in a refracting telescope and what kind of lens are they?
Objective lens and eye lens
Both are converging (convex) lenses
What is the length of the telescope (set up in normal adjustment) in terms of focal lengths?
Telescope length = fo + fe
What does normal adjustment mean in terms of the position of the principal foci?
The principal focus of the objective lens is in the same position as the principal focus of the eye lens
What can be assumed about the rays coming from one point on an object in space and why?
The rays from one point on the object are parallel
As the object is so far away it can be assumed to be at infinity
What is the role of the objective lens?
Converges the parallel rays from the object to form a real image on its focal plane inside the telescope
What is the role of the eye lens?
Acts as a magnifying glass on the real image to form a magnified virtual image which the observer views
What does normal adjustment mea in terms of the final image?
The telescope produces a final magnified virtual image
Image appears to be at infinity
This is because the light rays leave the telescope parallel
What is the equation for angular magnification in terms of the angles subtended?
M = angle subtended by image at eye / angle subtended by object at unaided eye
What is the equation for angular magnification (M) in terms of focal lengths of the lenses?
M = fo / fe
Which lense in a refracting telescope normally has a longer focal length?
fo is typically larger than fe to give a large magnification M to view objects in space
What is the shape and function of a Cassegrain telescope’s primary mirror?
Parabollic and concave
It reflects axial rays from an object causing them to converge at its principal focus
Forms a real image in front of the mirror
What is the shape and function of a Cassegrain telescope’s secondary mirror?
Convex
Reflects light out through a hole in the primary mirror
How is an image seen using a Cassegrain reflecting telescope?
The light enters the eyepeice lens
A virtual image can be viewed at infinity
What are 2 advantages of a large diameter telescope that give a higher image quality?
Greater collecting power
Greater resolving power
Is it easier to build large diameter reflectors or refractors and why?
Easier to build large diameter reflectors as mirrors can be supported from behind
Lenses in refractors can only be supported at the edges
Large lenses can distort or break under their own weight
What effect does chromatic aberration have on an image?
Image has multi-coloured edges
What causes chromatic aberration?
Different wavelengths of light are refracted by different amounts and have different focal lengths
Red light is refracted less than blue light
Red light has a longer focal length than blue light
Does chromatic aberration affect refractors or reflectors and why?
Only affects refractors as they use refraction and lenses
Does not affect reflectors as they use reflection and mirrors
What effect does spherical aberration have on images?
Blurred images
What causes spherical aberration?
Rays further from the principal axis are brought to a focus closer to the lens/ mirror
What types of mirrors and lenses are affected by spherical aberration?
Spherical mirros and spherical lenses (including convex lenses)
How can spherical aberration be avoided when using a concave mirror?
By using a parabolic mirror
All axial rayjs converge at the same principal focus
What are the 2 problems associated with the secondary mirror of a reflecting telescope?
Causes less light to reahc the primary mirror
Light is diffracted as it passes around the “spider” (framework) holding it in place
Why are reflectors lighter than refractors and why is this an advantage?
Mirrors are lighter than lenses
Reflectors are more manoeuverable for tracking objects
Which two parts of the EM spectrum can be detected the most easily by ground-based telescopes?
Visible light
Radio waves
Which molecule in the Earth’s atmosphere absorbs UV rays?
Ozone (O3)
Which molecule in the Earth’s atmosphere absorbs I-R rays?
Water (H2O)
What are three ways a telescope can be put into space?
On a satellite in orbit
On a plane
On a hot air balloon
What is one advantage of locating an optical telescope at high altitude on the ground
Reduces the effects of atmospheric absorption and distortion
What is one advantage of locating an optical telescope in a dry environment?
Reduces cloud coverage
What are 2 advantages of locating a telescope away from populated areas?
Reduces light pollution (for optical telescopes)
Reduces radio interference (for radio telescopes?
Why can the reflecting dish of a radio telescope be made from a mesh rather than solid metal
Radio waves are reflecetd by (not diffracted through) the mesh if the mesh size is smaller than λ/20
Radio waves have a long enough wavelength for this
Why is using a mesh better than using a solid dish?
Lower weight (more manoeuverable and easier to support)
Cheaper (requires less material)
Which type of telescope requires the most precisely shaped parabolic reflecting dish?
U-V as it has the shortest wavelength so is more likely to be affected by imperfections greater than λ/20
Why do I-R telescopes need a cooling system?
They need ot be kept cool so they don’t produce their own I-R radiation (which could interfere with the I-R they are detecting)
Why can’t x-ray telescopes make use of a parabolic reflecting dish?
The x-rays would pass through or be absorbed
How do x-ray telescopes allow the rays to be focused on a point?
They use grazing mirrors
The angle of incidence is large enough for the rays to be reflected
A series of these mirrors are required to gradually focus the rays
What is collecitng power?
The energy collected per second
What is collecting power directly proportional to?
Area and dish diameter2
Why is it good to have a high collecting power?
Gives a more intense image
Telescope can observe fainter objects and objects at greater distances
What is resolving power a measure of?
A measure of how much detail you can see
(Higher resolving power = more detail)
What is minimum angular resolution?
The smallest angular separation at which the instrument can distinguish two points
Smaller minimum angular resolution = more detail
What is the equation for minimum amgular resolution and what does each symbol mean?
θ ≈ λ / D
D is the diameter of the aperture in metres
θ is the minimum angular resolution in radians
λ is the wavelength in metres
What limits the resolution of a telescope?
The diffraction of light as it passes through the opening of the telescope
What is the Rayleigh Criterion?
Two objects will be just resolved if the centre of the diffractioin pattern (the Airy disc) of one image coincides with the first minimum of the other
For a given wavelength, will a smaller or larger diameter telescope produce a more detailed image and why?
Larger diameter will result in a smaller minimum angular resolution (as θ ≈ λ / D)
Higher resolving power means a more detailed image
What factors can be discussed when comparing telescopes?
Collecting power
Resolving power
The shape of the telescope
The loatioin of the telescope
(if optical) comparing reflectors or refractors
What is a CCD?
Charge couples device
A detector used in telescopes
Made of a silicon chip split into pixels
How does a CCD work?
Incident photons cause electrons to be released from each of the pixels
Number of electrons is proportional to intensity of light
Processed electronically to give a digital image
What is quantum efficiency?
Percentage of incident photons which cause an electron to be released
quantum efficiency = (number of electrons released per second / number of photons absorbed per second) x 100
Which has a better quantum efficiency, a CCD or the human eye?
A CCD
A CCD has a quantum efficiency of 70-80% and a human eye has one of 4-5% (1% in low light)
Why is a higher quantum efficiency better?
HIgher intensity image obtained for a given exposure time
Allowing dimmer objects ot be viewed
What determines the resolution of a CCD
Related to the number of pixels per unit area and their size
Smaller pixels = better resolution = clearer image
What determines the resolutions of the human eye?
The spacing between the light-sensitive cells on the eye’s retina
Which has a better resolution, a CCD or the human eye?
If the pixel size is small enough, a CCD will have better resolution than the human eye
What advantages (other than resolution and quantum efficiency) are there for using CCDs as telescope detectors rather than the human eye?
Images are stored and analysed digitally so can be accessed remotely
A larger rage of wavelengths can be detected (including beyond the visible spectrum)
Can be exposed for longer times to observe dimmer objects
Number of images captured in a time period can be easily adjusted
