# Telescopes Revision Questions

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• Created by: Lizzy Day
• Created on: 26-06-16 21:37
What are lenses?
Lenses change the direction of light rays by refraction
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What are converging lenses?
Converging lenses are convex and cause rays of light to bend towards eachother
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What is the principal axis?
The horizontal axis through the centre of a lens
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What is the lens axis?
The vertical axis through the centre of a lens
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What are axial rays?
Rays parallel to the principal axis of the lens
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What is principal focus?
The point where axial rays passing through a lens converge
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What are non-axial rays?
Rays that aren't parallel to the principal focus
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What is the focal plane?
The plane perpendicular to the principal axis of a lens on which the principal focus lies
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What does a converging lens do?
It will cause parallel non-axial rays to converge somewhere else on the focal plane
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What is the focal length?
The focal length of the lens, f, is the perpendicular distance between the lens axis and the focal plane
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What does a ray diagram show?
A ray diagram shows how light rays travel from an object through a lens. Where the rays meet is where the image is formed
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What is a real image?
A real image is formed when light rays from a point or an object are made to pass through another point in space. The light rays are actually there, and the image can be captured on a screen
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What is a virtual image?
A virtual image is formed when light rays from a point on an object appear to have come from another point in space. The light rays aren't really where the image appears to be, the image cannot be captured on a screen.
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What is the lens equation?
1/f = 1/u + 1/v
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What is an astronomical refracting telescope made of?
Two converging lenses. The objective lens converges the rays from the object to form a real image inside the telescope. The eye lens acts as a magnifying glass to form a magnified virtual image.
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How is a real image formed in this telescope?
When an object in space is viewed, it is assumed to be at infinity and the rays from each point are therefore parallel to each other. The real image is formed on the focal plane of the objective lens.
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Normal adjustment is when the distance between the objective lens and eye lens is equal to the sum of their focal lengths
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Why is a telescope set up in normal adjustment?
So that the principal focus of the objective lens is in the same position as the principal focus of the eye lens. The rays from the real image come out of the eye lens parallel. The final magnified image appears to be at infinity.
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How can you work out angular magnification?
M = angle subtended by image at eye/angle subtended by object at unaided eye
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How can you work out angular magnification with focal length?
M = fo / fe
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How do reflecting telescopes work?
Reflecting telescopes use mirrors to reflect and focus light. A parabolic concave primary mirror converges parallel axial rays from an object at its focal point, forming a real image.
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Where is the focal point in the reflecting telescope?
The focal point of the primary mirror is infront of it. An arrangement has been devised where the observer does not block any incoming light. The cassegrain arrangement.
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What are CCDs?
Astronomers use light sensitive detectors, called CCDs to capture images digitally. CCDs are silicon chips about the size of a postage stamp, divided into a grid of millions of identical picture elements.
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Why is silicon used in a CCD?
Silicon is a semiconductor so it doesn't usually have many free electrons. When light shines on a pixel, electrons are released from the silicon, with the number of electrons released being proportional to the brightness intensity of the light.
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How is an image created in a CCD?
The electrons released build up in potential wells underneath each pixel. This creates a pattern identical to the image formed on the CCD chip. Once the chip has been exposed to light for the desired time, the charge is converted to a digital signal
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What is meant by the quantum efficiency of a CCD?
The quantum efficiency of a CCD chip is the percentage of incident photons that cause an electron to be released. It is usually >70%
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What is the resolving power of a telescope?
The resolving power of a telescope is just a measure of how much detail you can see.
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What is resolution limited by?
Resolution is limited by diffraction. If a beam of light passes through a circular aperture, then a diffraction pattern of bright maxima and dark minima is formed. The central circle is the Airy disc.
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What is the requirement for two light sources to be distinguised?
Two sources can be distinguished if the centre of the Airy disc from one source is atleast as far away as the first minimum of the other source.
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What is the Rayleigh criterion?
0 = /\/D
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What are the disadvantages/difficulties in refracting telescopes?
Chromatic aberration (different wavelengths of light), bubbles/impurities scatter light, large lenses get distorted, for a large magnification, they must be large, which is expensive.
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What are the disadvantages/difficulties in reflecting telescopes?
Spherical aberration, if the shape of the mirror isnt quite right, incoming light can be blocked by secondary mirror and mirror supports, reduced image clarity.
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Parabolic dish, working in the same way as an objective mirror. A wire mesh can be used instead of a mirror as the long wavelength radio wave won't notice the gaps.
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What is a pre-amplifier?
A preamplifier amplifies the weak radio signals without adding too much noise to the signal. The signal is then amplified further by a second amplifier before being passed through a tuner to filter out any unwanted wavelengths.
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What is the process of focusing in radio telescopes?
Focus signal onto antenna signal is amplified, filtered and analysed to produce a digital image.
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What is the resolving power of a radio telescope?
For a radio telescope to have the same resolving power as an optical one, its dish would need to be 1 million x bigger. Its r.power is worse than the unaided eye.
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How do astronomers get around this bad resolving power?
Astronomers link lots of telescopes together using computer programming, their data can be combines to form a single image. This is equivalent to one huge dish the size of the separation of the telescopes.
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What are the benefits of radio telescopes?
Using wire and mesh makes construction easier, dish doesn't need to be perfect, cheaper, the wavelengths are less affected by imperfections in the shape
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What are IR and UV telescopes?
They are very similar to optical reflecting telescopes. They use the same parabolic mirror set-up to focus the radiation onto a detector. In both cases, CCDs or special photographic paper are used as the radiation detectors.
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What do the shapes of the mirrors have to be like in UV and IR telescopes?
The longer the wavelength, the less it is affected by imperfections in the mirror. IR mirrors can be sloppy, but UV mirrors must be very precise.
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What is the problem with IR telescopes?
They produce their own IR radiation due to their temperature. They need to be cooled to very low temperatures using liquid helium, or refrigeration units.
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What is the structure of an X-ray telescope?
X-rays only reflect if they just graze a mirror's surface. By having a series of nested mirrors, you can gradually alter the direction of X-rays enough to bring them into focus on a detector. A grazing telescope.
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How are X-rays detected?
The X-rays can be detected using a modified Geiger counter or a fine wire mesh. Modern X-ray telescopes such as the XMM_Newton telescope use highly sensitive X-ray CCD cameras.
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What is a problem with doing astronomy on Earth?
Trying to look through the atmosphere. Our atmosphere only lets certain wavelengths of em radiation through and is opaque to the others
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What telescopes can be used on Earth?
optical and radio telescopes can be used on the surface as the atmosphere is transparent to these wavelengths
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Where can infrared radiation be observed?
A few wavelengths of IR radiation can reach the Earth's surface, but most are absorbed by water vapour in the atmosphere. On Earth, the best way to observe I-R radiation is up high( and dry), e.g. Mauna Kea Volcano in Hawaii.
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Where can ultraviolet radiation be observed?
Most UV rays, X-rays and some IR radiation are absorbed higher up in the atmosphere, being on a mountain would not help. So, high altitude weather balloons or aeroplanes are used.
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What is the ideal situation for observing radiation?
To get the telescope above the atmosphere all together, by launching it into space and setting it in orbit around the Earth.
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What is the resolving power of a non-optical telescope limited by?
The Rayleigh criterion (depends on the wavelength of radiation and diameter of objective mirror) The quality of the detector, e.g. how fine the wire is or how many pixels on the CCD.
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What is the collecting power of a telescope?
It is proportional to its collecting area. A bigger dish or mirror collects more energy from an object in a given time. This gives a more intense image, so the telescope can observe fainter objects.
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## Other cards in this set

### Card 2

#### Front

What are converging lenses?

#### Back

Converging lenses are convex and cause rays of light to bend towards eachother

### Card 3

#### Front

What is the principal axis?

### Card 4

#### Front

What is the lens axis?

### Card 5

#### Front

What are axial rays?