Astrophysics - telescopes Flashcards
What is an axial ray?
Ray parallel to the principle axis
What does a converging lens do to a beam of axial rays?
Focuses an incident beam onto the principal focus
What does a converging lens do to a beam of non-axial rays?
Focuses an incident beam at the focal length (some point along it)
What happens to rays that pass through the centre of a lens?
They pass through undeviated
What is one astronomical unit?
The earth too sun distance
What is the equation for the angle subtended by an object?
α (in radians) = diameter of object / distance between object and observer
(due too the arc length equation r*α = arc length)
How many arcseconds are in 1 arcminute, arcminutes in 1 degree and arcseconds in 1 degree?
60’’ = 1’
60’ = 1 degree
3600’’ = 1 degree
What is the equation for magnification (angular size)?
M = α / B = f0 / fe
α = angular size viewed through telescope
B = angular size viewed by the naked eye
f0 = focal length of objective lens
fe = focal length of eyepiece
What is chromatic aberration?
For non monochromatic light, the different wavelengths all have different refractive indexes and refract a different amount. The longer wavelengths refract more and locus closer too the lens.
What is spherical aberration?
The curvature of a lens can cause rays of light at the edge (furthest from principle axis) to be focused closer to the lens than those near the centre, leading to image blurring and distortion.
What are the disadvantages of refracting telescopes?
Large magnifications require large objective lenses and very long focal lengths.
Large-diameter lenses are heavy and tend to distort under their own weight as well as hard to manufacture with sufficient clarity and free from defects.
Heavy and difficult to manoeuvre quickly.
Very large.
Suffer from chromatic aberration and spherical aberration.
What are the advantages of reflecting telescopes?
Large single mirrors can be made light
Mirror surfaces can be made a few nanometres thick, giving excellent image properties
Mirrors use only the front surface for reflection, so removing many of the problems associated with lenses
No chromatic aberration, and no spherical aberration when using parabolic mirrors
Relatively light mirrors allow rapid response to astronomical events
Smaller segmented mirrors can be used to form a large composite objective mirror
What is the collecting power of a telescope?
The telescopes ability to collect incident radiation
What effect does the collecting power of a telescope have?
A larger collecting power increases the brightness/ intensity of the image
What is collecting power directly proportional too?
The square of the diameter of the aperture
What is the resolving power of a telescope?
The ability to produce separate images of closely spaced (measured in angular separation) objects
Aka the ability to resolve images
Define (in words) the Rayleigh Criterion, how does this link to resolving power?
Rayleigh Criterion- The minimum angular resolution for two objects to be resolved
Smaller Rayleigh Criterion, Higher resolving power
When does the Rayleigh Criterion occur?
When the central maximum of the first airy disk (circular rings caused by 3D diffraction around an object) coincides with the first minimum of the second objects airy disk pattern
What is the resolving power of a telescope directly proportional to?
D - the diameter of the aperture
Why do radio telescopes require large diameters?
For a high collecting power
Radio waves are on the order of metres so too keep the Rayleigh Criterion low, the Diameter must be very large (and form a clear picture)
When does spherical aberration of mirrors not occur?
If the mirror is perfectly parabolic
Describe spherical aberration of a mirror (in a reflecting telescope)
The rays further away from the principal axis reflect off of the mirror and focus at the principal axis closer to the mirror
The rays closer to the principal axis reflect off of the mirror and focus at the principal axis further away from the mirror
(Referring too the concave primary mirror in most cases)
How does a radio inferometer improve resolution?
Inferometers are placed at a path difference a whole number of wavelengths from the antenna so the signals interfere constructively amplifying it
Resolution is equal too the angular distance between successive maxima, so the further the distance apart, the higher the resolution
What is the equation for the resolution of a radio inferometer?
λ / L
How does a CCD work?
Charge coupled devices are divided into pixels
Incident light interacts eradicates an electron using the photoelectric effect
This effects the charge in each pixel which is directly proportional too the brightness
What is an advantage of CCD’s over other devices?
A digital image is produced by information being stored on a file of the required information to reproduce it so it can be shared easily and processed remotely.
Remote sharing and processing is useful for spaced telescopes.
What is quantum efficiency?
Measure of a photon detectors sensitivity
What is the equation for quantum efficiency?
(number of photons detected / number of photons incident ) x100
What does a higher quantum efficiency mean?
The time needed to acquire an image of the same intensity relative to other imaging devices is much smaller
