3.9.1 Telescopes Flashcards
What do convex lenses do to incident light?
They focus the light
What are convex lenses also know as ?
Converging lenses
What do concave lenses do to incident light?
They spread out light
What is the principle axis?
The line passing through the centre of the lens perpendicular to its surface
What is the principle focus for a convex lens?
the point where incident rays of light will converge
What is the principle focus for a concave lens?
The point where incident rays of light appear to be coming from
What is the focal length?
The distance between the principle focus and the centre of the lens
The shorter the focal length ….
the stronger the lens
What is the real image?
The image formed when light rays cross after refraction
What is unique about the real image?
It can be formed on a screen
What is the virtual image?
The image formed on the same side of the lens
With regards to the virtual image, what happens to the light rays?
They don’t cross
What can’t be done with the virtual image?
It can’t be formed on a screen
What is the lens formula?
1/u + 1/v = 1/f
In the lens formula, what is u?
Distance of the object from the centre of the lens
In the lens formula, what is v?
Distance of the image from the centre of the lens
In the lens formula, what is f?
The focal length of the lens
What is the unit for the power of a lens?
Dioptres (D)
What is the power of a lens a measure of?
A measure of how closely a lens can focus a beam
What is the value of the power of a convex lens?
Positive
What is the value of the power of a concave lens?
Negative
Regarding a refracting telescope in normal adjustment, which lens collects light?
The objective lens
Regarding a refracting telescope in normal adjustment, why does the objective lens collect light?
To create a real image of a very distant object
Regarding a refracting telescope in normal adjustment, why does the objective lens have a long focal length?
To collect as much light as possible
Regarding a refracting telescope in normal adjustment, what does the eyepiece lens do?
It magnifies the image produced by the objective lens
Regarding a refracting telescope in normal adjustment, why is the eyepiece lens useful?
It produces a virtual image at infinity that reduces eyestrain for the observer
Normal adjustment for a refracting telescope is the sum of what?
The two focal lengths
or fe+fo
Regarding a refracting telescope in normal adjustment, principle focus for the two lengths are always where?
In the same place
What is the equation for angular magnification? (magnifying power)
α/β (Larger angle over smaller angle)
What happens when α and β are both less than 10 degrees?
α/β = fo/fe
Regarding a Casegrain reflecting telescope, what is the primary mirror?
A parabolic/concave objective mirror
Regarding a Casegrain reflecting telescope, what is the secondary mirror?
A convex mirror
Regarding a Casegrain reflecting telescope, where are the principle foci? (2)
After the secondary mirror and there is a virtual one behind the mirror
Regarding a Casegrain reflecting telescope, what lens is used to focus light?
A convex lens
What is a merit of reflecting mirrors? (2)
They are very thin
They have a coating of aluminium or silver atoms
What is an advantage of mirrors having a coating of aluminium or silver atoms?
They are smooth, minimising distortion
What do reflecting telescopes prevent? (2)
Chromatic aberation
Spherical aberation
What is chromatic aberation?
when red light has a greater focal length than blue light
In chromatic aberation, why does red light have a greater focal length than blue light?
because blue light is refracted more
How can you tell if an image is being effected by chromatic aberation?
the images are produced with coloured fringing
What is spherical aberation?
when rays of light are focused in different positions
How does spherical aberation occur?
The curvature of the lens or mirror can cause rays of light at the edge to be focused in different positions
What types of lenses does spherical aberation effect?
Lenses with large diameters
How can you tell if an image has been effected by spherical aberation?
The image will be blurred or distorted
How can spherical aberation be completely avoided?
By using parabolic objective mirrors
Which is lighter a lens or a mirror?
A mirror
What are disadvantages of refracting mirrors?
Glass must not have defects
Large lenses can bend or distort under their own weight
They’re heavy
They’re not easy to move
Large magnifications require large lenses
Can only be supported from the edges
What is an advantage of using reflecting telescopes?
Eliminates chromatic or spherical aberation
They’re thin
They’re not heavy
Large ones can be made of smaller ones
They can be supported from behind
What are similarities of radio and reflecting telescopes?
They focus incoming radiation and focus it
They can be moved to focus on different objects
They both have a parabolic shape
What are differences between radio and optical telescopes?
Radio telescopes have a very large diameter, to collect radio waves (due to their large wavelength)
Radio telescopes are much cheaper to build
Radio telescopes have to move to focus on objects
Radio experiences a large amount of man-made interference
Radio has a better collecting power
Radio must have a much greater diameter to achieve the same resolving power as optical telescopes
What are infrared telescopes made up of?
Large concave mirrors focussing radiation onto a detector
How must infrared telescopes be kept? (2)
Cooled using cryogenic fluids to almost absolute 0
Shielded to prevent thermal contamination
What areas of space do infrared telescopes observe?
Cooler regions of space
Where can infrared telescopes also be stationed?
In space and remote controlled from Earth
Where must UV positioned?
In space
Why can’t UV telescopes be positioned on Earth?
Because the ozone layer blocks all UV rays less than 300nm
What configuration do ultraviolet telescopes use?
Cassegrain
How do gamma telescopes create images?
Don’t use mirrors, use a detector made of pixels
What do gamma telescopes observe? (4)
Gamma ray bursts
Quasars
Blackholes
Solar flares
Where do X-ray telescopes have to be stationed?
in space
Why do X-ray telescopes have to be stationed in space?
Because X-rays are absorbed by the atmosphere
What are X-ray telescopes comprised of?
A combination of parabolic and hyperbolic mirrors
What do X-ray telescopes usually observe? (4)
High energy areas of space:
Active galaxies
Blackholes
Neutron stars
What is collecting power of a lens/mirror?
The ability to collect incident EM radiation
What is collecting power inversely proportional to?
the area of the objective lens
The greater the collecting power…..
The brighter the image produced
What is resolving power?
The ability of a telescopes to produce separate images of close together objects
or a measure of of how much detail you can see
What is needed for an image to be resolved?
The angle between the straight lines from Earth to each object must be at least the minimum angle resolution
According to Rayleigh’s criterion what causes images not to be resolved?
If any part of the central maximum of either image falls within the first minimum diffraction
What is collecting power proportional to?
Diameter squared
What does CCD stand for?
Charged-coupled devices
What is quantum efficiency?
the percentage of incident photons which cause an electron to be released
What is pixel resolution?
The number of pixels used to form an image on a screen
Regarding pixel resolution, what can a large amount of small pixels do that a small amount of large pixels can’t?
A lot of small pixels can resolve an image more clearly than a small amount of
large pixels.
What is spatial resolution?
The minimum distance that two objects must be apart in order to be distinguishable
What is spatial resolution used to do?
Observe small objects
What is convenience?
How easy images are to form and use
What is the quantum efficiency of a CCD?
Roughly 80%
What is the quantum efficiency of the human eye?
4-5%
What is the spectral range of a CCD?
IR, UV and visible
What is the spectral range of the human eye?
Only visible
What is the pixel resolution of a CCD?
Varies but is roughly 50 megapixels
What is the pixel resolution of the human eye?
roughly 500 mega pixels
What is the spatial resolution of a CCD?
10 micro metres
What is the spatial resolution of the human eye?
100 micro metres
What is the convenience of a CCD?
Needs to be set up but image produced are digital
What is the convenience of the human eye?
Simpler to use, no need for extra equipment
What are CCD’s more useful for?
detecting finer details and producing images which can be shared and stored
