Astro A2 - Telescopes Flashcards
What is the principal focus
Where the light rays which were refracted by a lens cross
What is the principal axis
Line parallel to light rays before entering the lens
What is the focal length
Length between the lens and the principal focus
Why is a lens depicted as a very thin lens
So that a light ray travelling through the middle of the lens can be depicted as a straight line
What occurs when an object is further than one focal length away from a lens
Real image, inverted on the other side of the object, magnified
What occurs when an object is one focal length away from a lens
An image is not formed
What occurs when an object is less than one focal length away from a lens
Virtual image, upright behind the object, magnified
What is in a refracting telescope
Two lenses, an objective lens and an eyepiece lens
What is the objective lens
The lens that gathers all the light from the object at infinity
What is the eyepiece lens
The lens which forms a magnified virtual image at infinity
What does it mean for a telescope to be in “normal adjustment”
Distance between both lenses = objective focal length + eyepiece focal length
How to find the angle subtended by an object
Theta = h/d, where h is object height and d is distance away
What will occur when a circular mirror reflects light
There are many focal points on the principal axis
What is the shape of a mirror to have a single focal point on the principal axis from parallel light
Parabolic mirror
Describe the Cassegrain telescope
Light is first reflected off a parabolic mirror. This travels to a second convex reflector at the focal point. This directs the light out of a small hole in the parabolic mirror, and into an eyepiece
What is chromatic aberration
Light with longer wavelength refract less, so causes different focal points for different wavelengths. Only an issue for diffractive telescopes. Causes blurred edges
What is spherical aberration
Aberration caused by spherical lenses
What are the main advantages of reflective telescopes
Reflectors can be made much larger than refractors because a mirror can be supported from behind, whereas a lens must be supported at the edge. A large lens is likely to break under its own weight
What are the similarities of an optical telescope and a radio telescope
Parabolic surfaces to reflect
What are the differences of an optical telescope and a radio telescope
No need for a secondary reflector as the aerial can be placed at the focal point
What are some issues with the radio telescope
Poor resolving power, man-made interference is a big issue. Interference can be minimised by building telescopes away from centres of population
What are advantages of a radio telescope
The reflective dish can be made of a mesh with size smaller than λ/20, due to radio’s large wavelength
Where and why are most telescopes placed
Into space due to absorption of waves by the atmosphere, light pollution, interference at ground level, atmosphere changes direction of light
Where are infrared telescopes used
4200m above sea level for finding exoplanets
Where are ultraviolet telescopes used
Low earth orbit for analysing quasars
Where are x-ray telescopes used
Earth orbit to discover supermassive black holes
What are the advantages of large diameter telescopes
Greater collecting power so images are brighter, better resolving power so images are clearer
What is collecting power
The rate at which useful energy is received by a telescope. Proportional to area
How to increase collecting power
Larger diameter, increase exposure time, use sensitive detectors
What is resolving power
Same as the minimum angular resolution
Describe the diffraction pattern by a circular aperture
A large central maximum called airy disc which is twice as wide as the further maxima. Propagates outwards with maxima and minima
What is resolution
How much detail the telescope can show
What is the Rayleigh Criterion
‘Two objects will be just resolved if the centre of the diffraction pattern of one image coincides with the first minimum of the other’
What is the equation for minimum angular separation by the Rayleigh Criterion
θ = λ/D, where D is the diameter of the objective, θ in radians
The lower the minimum angular separation…
The better the resolution
What affects the real value of the resolving power
Refraction of light, the spider holding the secondary mirror in a Cassegrain mirror
What is 1 arcminute
1/60 of a degree
What is 1 arcsecond
1/3600 of a degree
What is a CCD
A charge-coupled device is a semiconductor which light is converted directly into digital information
Describe the structure of a CCD
A silicon chip made of millions of pixels, connected to electrodes.
How does a CCD work
Photons striking the silicon liberate electrons which are then trapped in potential wells of the pixels. When sufficient electrons have been liberated for a full exposure, the electrodes are used to shuffle the electrons to be measured and create the image
What is the response of a CCD
Linear response, so that very faint parts of a relatively bright image can be obtained
What is quantum efficiency
Number of photons detected/ Number of photons incident
What is the quantum efficiency of a CCD
70-80%
What is the quantum efficiency of an eye
1% at 550nm
What is the resolution, compared between CCD and the eye
CCDs and the human eye have roughly similar resolution
Why are CCDs more convenient than using a telescope or the eye
Allows for remote viewing, direct analysis, long exposure times and detection of wavelength beyond the visible spectrum
What is an airy disc
The central maximum of spherical diffraction of light
What is the response of an eye
Logarithmic
