Telescopes Flashcards
Convex/converging lens
focuses incident light
Concave/diverging lens
spreads out incident light
Principal axis
The line passing through the centre of the lens, perpendicular to its surface
Focal Length (f)
The distance between the centre of the lens and the principal focus.
Real Image
Formed when light rays cross after being refracted by a lens. Real images can be projected onto a screen.
Virtual image
formed on the same side of the lens. The light rays do not cross, so a virtual image cannot be formed on a screen.
Reflecting Telescope
A telescope which uses mirrors to focus incident light onto an eyepiece lens.
Refracting Telescope
A telescope which uses lenses to focus incident light.
Resolving Power
The ability of a telescope to produce separate images of close-together objects.
Rayleigh Criterion
This states that two objects will not be resolved if any part of the central maximum of either of the images falls within the first minimum diffraction ring of the other.
Quantum Efficiency
The percentage of photons incident on a CCD which causes an electron to be released
Primary Mirror
The mirror equivalent of an objective lens. The primary mirror collects light and focuses it onto a secondary mirror in a reflecting telescope.
Normal Adjustment
When the distance between the objective and eyepiece lenses in a refracting telescope is equal to the sum of their focal lengths.
Objective Lens
The lens in a telescope that collects light and creates a real image. Objective lenses should have long focal lengths and be large in order to collect as much light
as possible.
Chromatic Aberration
An effect caused by the different focal lengths of different wavelengths of light that leads to different colours being focused at different points.
Spherical Aberration
When light is focused in different places due to the curvature of a
lens or mirror, causing image blurring. This can be resolved in
reflecting telescopes by using a parabolic mirror
State 3 advantages of reflecting telescopes
- There is very little chromatic aberration
- Simpler to increase the size of the objective since mirrors can be supported from behind and are
lighter than lenses - Using parabolic mirrors stops spherical aberration , mirrors are unaffected by chromatic
aberration
Explain the structure, positioning and uses of a
single dish radio telescope
Structure: Large parabolic dish that focuses radiation
onto a receiver
Positioning: can be ground-based but must be in
isolated locations
Uses: observing things such as galaxies, stars and black
holes
Explain the structure, positioning and
uses of an infrared telescope
Structure: Large concave mirror focusing light onto a
detector. Must be cooled with cryogenic fluids to avoid
interference
Positioning: Must be in space as infrared light is
blocked by the atmosphere
Uses: observing cooler regions in space (from a few
tens to 100K)
Explain the structure, positioning and
uses of an ultraviolet telescope
Structure: Cassegrain configuration that focuses
radiation onto solid state devices
Positioning: Must be in space as ultraviolet light is
blocked by the ozone layer
Uses: observing the interstellar medium and star
formation regions
Compare the quantum efficiency of a
CCD to the eye
Quantum efficiency: the percentage of incident photons that
liberate an electron in the photoelectric effect. This can be
upwards of 80% for a CCD, compared to 4-5% for the human
eye.
Compare the resolution of a CCD to the eye
CCDs have a spatial resolution of 10 micrometres, the minimum
resolvable distance for the human eye is around 100 micrometres
so CCDs are better for capturing fine detail.
Compare the convenience of a CCD to
the eye
The CCD is more convenient for accessing data remotely
What is the principal focus?
A point on the axis which is the samedistance from the optical centre as the focal length. This is where light rays travelling parallel to the principal axis prior to refraction converge.
Cassegrain Reflecting Telescope
Newtonian Reflecting Telescope:
Ray diagram for a refracting telescope in normal adjustment:
How does a CCD work? (6)
- photons hit the silicon in a pixel, this creates free electrons that are confined to to the pixel
- this causes charge to accumulate
- this charge can be measured and used to create digital signal
- signal describes where light hits and intensity
- the charge on each pixel will vary depending on how many photons hit it
- this allows a digital image of an object to be created
What is a CCD ?
an array of light-sensitive pixels, which become charged when they are exposed to light by the photoelectric effect.
Why does virtual image not captured on screen?
Light rays aren’t really where the image appears to be, so no image captured on screen
What is resolution limited by?
Diffraction
How is diffraction formed? (3)
- A beam of light passes trough a circular aperture, spreading out
- light interferes construcively and destructively
- Diffraction pattern of bright maxima and dark minima is formed
What is the central circle called when diffraction is formed?
Airy disc
How can Chromatic Aberration be corrected?
by using an achromatic doublet
what does achromatic doublet consist of?
consists of two lenses, one converging, made of crown glass, which is fixed to a diverging lens made from a different type of glass called flint glass.
How do the lenses in an achomatic doublet reduce Chromatic Aberration?
lenses refract the light in opposite directions and allows the red light to be brought to a focus at the same point as the blue light.
Similarities between radio telescopes and optical telescopes (3)
- Both telescopes function the same way: they intercept and focus incomingradiation to detect its intensity.
- Both radio telescopes and optical telescopes can be moved to focus on different sources
of radiation, or to track a moving source. - Both optical and radio telescopes can be built on the ground since both radio waves and
optical light can pass easily through the atmosphere.
Differences between radio telescopes and optical telescopes (3)
- Since radio wavelengths are much larger than visible wavelengths, radio telescopes
have to be much larger in diameter than optical telescopes in order to achieve the same
quality image/have the same resolving power. - Construction of radio telescopes is cheaper and simpler because a wire mesh is used
instead of a mirror. - As radio waves have a larger objective diameter, they also have a larger collecting
power
Disadvantage of reflecting telescopes (1)
it can suffer from spherical aberration
Disadvantages of refracting telescopes(5)
- Large lenses can bend and distort under their own weight due to how heavy they are.
- Chromatic and spherical aberration both affect lenses.
- incredibly heavy and therefore can be difficult to manoeuvre.
- Glass must be pure and free from defects.Achieving this for a large diameter lens is very
difficult. - Large magnifications require very large diameter objective lenses with very long focal
lengths. leading to large, expensive buildings being needed to house them.
If object is further than the focal length away from lens, the image is …
real and inverted
If object is closer than the focal length away from lens, the image is …
virtual
benefits of radio telescopes (2)
- Construction of radio telescopes is cheaper and simpler because a wire mesh is used
instead of a mirror. - Dish doesn’t have to be anywhere near as perfect as the mirrors and lenses used in optical telescopes. This is because the longer the wavelength of radiation being detected, the less affected by imperfections in the shape of the dish or mirror collecting it.
What has the same effect as increasing the diameter of the telescope, which leads to an improved minimum angular resolution and collecting power ?
linking together several radio dishes into an array, and combine the signals that they receive.
What is the equation for quantum efficiency?
Quantum efficiency = conducting electrons released per second/ incident photons per second
What light can a CCD detect? (3)
- Visible light
- UV light
- Infrared
One unique problem of IR telescopes
they can produce their own IR radiation from their heat, so must cooled to very low temperatures using liquid helium
why can’t IR telescopes be put in certain locations ?
Water vapour in the Earth’s atmosphere absorbs some of the incoming IR radiation
Where are IR telescopes usually built?
usually built very high up on mountains or in very dry areas like deserts to avoid water vapour absorbing IR radiation
Advantages of IR telescopes (3)
- As IR radiation has a longer wavelength, the mirrors used in IR telescopes don’t need to be as perfectly shaped as those in visible telescopes.
- IR astronomy allows for observations to be made that cannot be made with other parts of the E.M. spectrum.
- IR telescopes can be much smaller than radio telescopes to have the same resolving power.
Disadvantages of IR telescopes (3)
- IR telescopes have to be kept at very cool temperatures, which can be very difficult and costly, especially if they are in hot locations such as deserts.
- Can only be used in very specific types of place (high altitude and dry).
- IR telescopes have lower resolving powers than those of optical telescopes of the same size.
how are images formed in IR telescopes?
Incoming infrared waves are reflected by a paraboloid primary mirror onto a smaller secondary mirror which focusses the rays onto a detector which records the image.
how are images formed in U-V telescopes?
Incoming rays are reflected by a paraboloid mirror either directly onto a detector or onto a secondary mirror which directs them onto detector.
Advantages of U-V telescopes (3)
- Can detect objects not visible at other wavelengths.
- U-V telescopes have higher resolving powers than optical telescopes of the same.
- U-V telescopes are much smaller than radio telescopes for the same resolving power.
Disadvantages of U-V telescopes (2)
- U-V telescopes need to be placed in orbit to make observations.
- U-V telescopes must have mirrors with even smaller impurities than acceptable for optical telescopes.
Why do UV telescopes need to be positioned in space?
ozone layer blocks all ultraviolet rays that have a wavelength of less than 300nm.
Advantages of Gamma Ray Telescopes (3)
- Can detect objects and phenomenon that cannot be detected in other wavelengths.
- Are smaller than most other telescopes as they don’t need to focus light.
- Some observations can be made from Earth’s surface.
Disadvantages of Gamma Ray Telescopes (3)
- Cannot focus gamma rays into images with same level of clarity as other forms of telescope
- Many telescopes have to be placed in orbit.
- Gamma rays come from all direction and can penetrate satellites into detector, so satellites need to be shielded or incorporate sensors to detect and account for rays coming in from other directions.
What are the two limiting factors affecting gamma rays ?
- Rayleigh Criterion = θ≈λ/D = lower θ means better resolving power
- Quality of detector= how many pixels there are on CCD, or for wire mesh X-ray detector, how fine the wire mesh is
How can you alter the direction of X-rays enough to bring them to a focus on a detector and what is this type of telescope called ? (2)
- By having a series of nested mirrors
- Grazing telescope
