specification points Flashcards
draw a ray diagram to show the image formation in normal adjustment
and an equation for the angular magnification
M = angle subtended by image at eye/ angle subtended by object at unaided eye
= 𝜶 / β
what is the definition for the normal adjustment
The setting for a refracting telescope in which the light emerges parallel from the eyepiece lens and the image is viewed at infinity
what is the focal length of the lens
The distance between the principal focus of a lens and its optical centre.
give an equation for magnification with reference to the focal length of the eyepiece and objective lense
M = f₀ / fₑ
f₀ = focal length of objective lense
fₑ = focal length of eyepiece lense
draw a diagram showing the cassegrain arrangment for a reflecting telescope
list some disadvantages of refracting telescopes
Mounting of the lens and support can only be made using the edge of the lens
Using glass of sufficient clarity and purity and free from defects to make large-diameter telescopes is extremely difficult
Large-diameter lenses are heavy and tend to distort under their own weight
Suffer from chromatic aberration and spherical aberration
Heavy and difficult to manoeuvre quickly
Difficult to mount heavy observing equipment and associated electronics
Large magnifications require large objective lenses and very long focal lengths
list some advantages of reflecting telescopes
Large single mirrors can be made, which are light and easily supportable from behind
Mirror surfaces can be made just 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, can still occur in normal reflecting telescopes
Relatively light mirrors allow rapid response to astronomical events
Smaller segmented mirrors can be used to form a large composite objective mirror
what is Spherical aberration
The distortion of an image due to imperfections in the mirror or lens causing differing focal lengths
what is chromatic aberation
An optical defect that causes light of different colours to be focused at different focal points.
list some similarities between radio telescopes and optical telescopes
Radio telescopes can be ground-based because the atmosphere is transparent to a large range of radio wavelengths. Optical telescopes can also be ground based
Radio dish antennae need large structures to support them and to steer them. Depending on the optical telescope, the structure can vary, but to attain a large magnification, they do need to be supported by large structures too.
list some differences between radio telescopes and optical telescopes
Radio telescopes can operate during the day and night and are situated away from artificial sources of radio interference. But optical telescopes are mainly used only at night
Optical telescopes collect visible light and magnify it for viewing, while radio telescopes collect invisible radio waves, amplify them, and record them for study.
optical telescopes are commonly used in satellites whereas radio telescopes are mainly used on earth
radio telescopes usually have a much lower resolution than a similarly sized optical telescope
Radio telescopes can detect faint galaxies which no optical telescope can, they can work even in cloudy conditions and they can work during the day and night
radio telescopes mainly have low resolving powers
what is the minimum angular resolution of a telescope
The minimum angle, θ, that an instrument can distinguish between two small objects for a particular wavelength of light or other electromagnetic radiation, as determined by the Rayleigh criterion: θ ≈ λ / D
what is the equation for thr rayleigh criterion and what does it mean
θ ≈ λ / D
This states that two point objects can be resolved by an optical instrument if their angular separation is at least λ/D, where λ is the wavelength of the radiation and D is the diameter of the objective mirror or lens
what is collecting power and what is it proportional to
A measure of a telescope’s ability to collect incident electromagnetic radiation, and which is directly proportional to the square of the diameter of its objective.
collecting power ∝ (objective diameter)²
compare the eye and CCD as detectors in terms of:
quantum efficiency, resolution and convenience of use
Whereas for CCDs, most photons will cause an electron to liberated (high quantum efficiency), the human eye has a much lower quantum efficiency for low-light regions, as well as losing colour, meaning CCDs are much more effective in low-light scenarios
A simple comparison between the number of pixels/light sensitive cells per unit area suggests that CCDs and the human eye have similar resolutions
(convenience is quite obvious i think, our eyes are just there init)
CCDs also allow for remote viewing, long exposure times, detection of waves outside the visible region and computer analysis
what is a CCD
Charge-coupled device (CCD): A semiconductor device in which light is converted directly into digital information, commonly used in cameras and in conjunction with telescopes for digital imaging.
what is quantum efficiency
( detected photons / incident photons ) x 100
in a CCD
definition for apparent magnitude, m
The apparent brightness of a star expressed on the magnitude scale.
what is absolute magnitude, M
The apparent magnitude a star would have if it were placed at a standard distance of 10 parsec from the Earth.
What is the Hipparcus scale
A scale describing the apparent magnitude (relative brightness) first devised by Hipparchus of Nicaea (190–20BC).
what is the brightest and dimmest apparent magnitude of a star that can be seen with the naked eye
brightest stars that can be seen with the naked eye as magnitude 1.0 and the faintest as magnitude 6.0
how does magnitude difference correspond to brightness/intensity
A magnitude difference of 1 corresponds to a brightness ratio of 2.51
what is luminousity
The total energy radiated by a star each second (also called power); units J s^−1 or W
what is the brightness and its equation
The amount of energy radiated per second per square metre (also called intensity or radiation flux); unit Wm−2.
b = L / 4𝝿r²
parsec (pc) definition
The astronomical distance at which the angle subtended by the mean distance of the Earth–Sun system, i.e.1 AU, is one arcsecond
in other words, the distance at which an object lies if its measured parallax angle is 1 arcsecond; 1 pc = 3.262 ly or 2.06×105 AU.