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What is a real image?
When light rays from an object are made to pass through another point in space.
What is a virtual image?
When light rays from space appear to have come from another point in space - the light rays are NOT where the image appears to be.
Define the variables in 1/f = 1/u + 1/v
f = focal length
u = distance between object and lens
v = distance between image and lens (v is positive if image is real, and negative if virtual)
Describe the structure of a refracting telescope.
2 converging lenses.
Define Normal adjustment.
A refracting telescope uses normal adjustment - the magnified image appears to be at infinity. f0 > fe (where 0 means focal length of objective lens, and e means focal length of eye lens)
Give equations for Magnification.
M = θi/θo = f0/fe
Where i = angle subtended by image.
and O = angle subtended by object at the eye
Describe the structure of a reflecting telescope.
2 mirrors, and a converging lens
primary mirror = concave
secondary mirror = convex
Explain why the secondary mirror is needed for reflecting (cassegrain) telescopes.
The principle focus of the light rays after reflecting off of the first mirror is in front of the mirror, so a secondary mirror is placed so that the observer doesn’t block out any light. (otherwise the observer would have to look through the same direction that light is coming into the telescope)
What is the resolving power of a telescope?
How much detail you can see.
What is the Minimum Angular Resolution?
The smallest angular separation at which the instrument can distinguish 2 points.
State the relationship between the Minimum Angular Resolution and the Resolving Power.
The smaller the MAR, the greater the RP
Give on behaviour of waves that limit the resolution of a telescope.
Diffraction. (e.g a beam of light passing through a circular aperture of a telescope causes a diffraction pattern called an airy disc)
Give the requirement needed to distinguish between 2 light sources.
The centre of the airy disk (the maxima on the diffraction pattern) one on of the light sources is at least as far away as the first minima of the other source.
Explain what is meant by the Rayleigh Criterion
It identifies the minimum subtended angle between 2 objects that can be resolved. This minimum angle occurs when the first maxima of one diffraction pattern coincides with the first minima from the other object’s diffraction pattern.
What does D and θ mean in the equation for the Rayleigh Criterion
D = Diameter of objective lens/mirror
θ = MAR in radians
To see very fine details, do the objective lenses/mirrors need to be very large or small in diameter?
Very large
Explain disadvantages of refracting telescopes
Chromatic Abberations - glass refracts the different colours of light by different amounts - blurs the image
Also may suffer from Spherical Abberations if the lens is not parabolic enough
Large Magnification requires very large focal lengths, so they are very long
Large lenses increases the likelihood of defects and impurities in the glass
Weight - Telescopes are very heavy and distort under their own weight
Explain the advantages of Refracting Telescopes.
Lower Maintenance compared to reflectors
Less sensitive to temperature changes
Explain the disadvantages of Reflecting Telescopes
Spherical Abberations - if the mirror isn’t parabolic enough so light rays dont all converge at the exact same point - blurs image
Secondary mirror blocks light entry onto first mirror, so quality of image decreases
Mirrors are exposed to air - need regular maintenance
Secondary Mirror also causes some diffraction
What are CCD’s?
They are very sensitive image detectors
Explain how a CCD works
Contain a silicone chip divided into pixels
Photons hit the chip, and electrons are released
The electrons get trapped in a potential well in the CCD
The electrons alter the charge on each pixel
The image produced is identical to the electron pattern
When exposure is complete the charge is processed to create a digital image
What is quantum efficiency?
The proportion of photons incident compared to the photons that are actually detected
Compare CCD’s to the human eye.
CCD’s have a QE of 80%, whereas eyes have a QE of 1%
CCD’s can detected Infrared, Visible and UV light, whereas eyes can only detect visible light
CCD’s have a spatial resolution of 10micrometers, whereas eyes have a SR of 100micrometers (so CCD’s better for capturing the fine details)
CCD’s can be linked to a computer for capture of images and analysis
Describe the features of a radio telescope
Parabolic dish that works in the same way as the objective mirror of a reflecting telescope.
Antenna is used as a detector at the focal point instead of an eye/camera
Explain a downside to Radio Telescopes.
For a radio telescope to have the same resolving power as an optical telescope, the dish would need to be a million times bigger.
Explain how they get over the downside of radio telescopes having such a low resolving power.
They link lots of telescopes together - and combining them to form a single image (equivalent to having one big dish)
Give some advantages of Radio Telescopes
Dish does not have to be as smooth as a mirror, as to avoid spherical aberrations it only has to have a precision of λ/20
It has a greater collecting power, as the dishes are so big, so they produce brighter images
Where can we station Optical and Radio telescopes and why?
On the earth, as as the atmosphere is transparent to these wavelengths
Where must IR telescopes be positioned?
At high-altitudes in dry places - as water vapour absorbs the IR
Where are UV and X-ray telescopes positioned, and why?
High altitude weather balloons or aeroplanes, absorbed very high up in the atmosphere - but ideally we want the telescopes above the atmosphere - launch into orbit in space.
Give 2 similarities between IR/UV telescopes and Optical Reflecting telescopes.
They both use a parabolic mirror
They both use CCD’s
Give an advantage that IR telescopes have (manufacturing wise) over Optical Reflecting telescopes
IR has a longer wavelength than light, so it is less affected by imperfections in the mirror, and don’t need to be as perfectly shaped as optical telescopes
Give a disadvantage of UV telescopes (manufacturing wise) compared to optical telescopes.
They have shorter wavelengths, so need to be more precisely made.
Give another disadvantage of IR telescopes.
They produce their own IR radiation due to their temperature, so they need to be cooled using liquid helium or fridge units
Why are X-ray telescopes constructed differently to other telescopes?
X-ray radiation doesn’t usually reflect, it either gets absorbed or passes straight through it. They do reflect if they GRAZE a mirrors surface
What is the name of the telescope used to capture X-rays and how does it work?
A Grazing Telescope. A series of nested mirrors gradually focus the X-rays enough to a detector. They can be detected using a modified Geiger counter or a fine wire mesh.
X-rays have a much smaller collecting power than other telescopes
What 2 factors is the resolving power of a telescope dependent on?
The Rayleigh Criterion
The quality of the detector (i.e CCD)
What is the Collecting Power proportional to?
The Diameter2 (The area)
A bigger dish collects more energy from an object in a given time, giving a more intense image and allowing the telescope to observe fainter objects
What is a Star’s luminosity?
The total energy emitted per second (where the energy is in the for of EM radiation)
Give an equation for the Intensity of a star
I = L/4πd2 (intensity it the brightness)
What 2 factors does brightness depend on?
Luminosity
Distance from us
What is the Hipparchus scale?
a scale that gives us a value of apparent magnitude, going from 1 (brightest stars) to 6 (dimmest stars)
What is the scale factor between magnitude 1 stars and magnitude 2 stars?
2.51x
Give an equation for the brightness ratio between 2 stars.
IA/IB = 2.51mB-mA
Where mB and mA are the apparent magnitudes
If the angle of parallax to a star is 1arcsecond (1/36000) then what distance is the star from the earth?
1 parsec
What is the absolute magnitude (M) of a star?
It is what the apparent magnitude WOULD be, if the star was 10pc away from earth.
What is different between m (apparent) and M (absolute)?
M does not depend on distance, as all stars are given a standardised distance of 10pc.
Give an equation linking m and M, and state what makes this equation special.
m - M = 5log(d/10)
d is distance measured in parsecs (and 10 = 10pc)
What is 1 AU
The mean distance between the earth and the sun
What is a lightyear?
The distance that light/EM waves travel in one year.
What is Stefan’s law?
P = σAT4
