Astrophysics Telescopes Flashcards
How do you know if an image is virtual
When an object is on the same side of the focal point from the lens (this means the image is not inverted)
How do you know if an image is real
When the object is opposite to the focal point (this means the image is inverted)
What does a convex light do?
Focuses the incident light
What does a concave light do?
Spreads out the incident light
What is the principal axis
The straight line passing through the centre of the lens 90 degrees to its focus
What is the principal focus on convex and concave lens
Convex lens: the point where the incident light rays converge on the principal axis
Concave: the point where light rays should have appeared from behind the lens on the principal axis
What is the focal length?
The horizontal distance between the centre of the lens and the principal focus
What are real images
When light rays cross after refraction, real images can be formed on screen
What are virtual images
Formed on the same side of the lens, the light rays do not cross so a virtual image appears on the screen
What is the power of a lens>
A measure of how closely a lens can focus a beam that is parallel to the principal axis
- converging lens: positive
- diverging lens: negative
What is a refracting telescope? 1 mark
A telescope with two convex lenses
- objective lens and eyepiece lens
What is the role of the objective lens in a refracting telescope?
To collect light and create a real image of distant object.
It has a long focal length and is large to collect as much light as possible
What does the eye piece lens of a refracting telescope do?
Magnifies the image of the objective lens
lens produces a virtual image at infinity
reduces eye strain
What is normal adjustment for a refracting telescope
When the distance between the objective lens and eye piece lens is the sum of their focal length (fo + fe)
What is the formula for angle of magnification in a refracting telescope?
M = alpha / beta = fo/fe
Alpha is always bigger than beta,
What is the cassegrain reflecting telescope?
A telescope comprising of a small convex mirror and large concave mirror and an eye piece lens
Why are the mirrors in cassegrain reflecting scopes very thin and coated by silver atoms?
Allows mirrors to be as smooth as possible and minimise distortions in the image
What is chromatic aberration?
The focal length is of red light is greater than blue light which means they focus at different points (blue is refracted more than red)
- this causes a white object to produce an image with coloured fringing
- chromatic aberration is caused by refraction
What is spherical aberrration?
The curvature of the lens or mirror can cause light rays at the edge to be focused at different position, this leads to image blurring
- occurs mostly in lenses with large diameters which can be avoided using parabolic objective mirrors.
What is Achromatic doublet
A method to reduce chromatic and spherical aberration, this is done by cementing together a convex lens and concave lens to allow all rays of light to focus in the same position
What are the disadvantages of refracting telescopes?
- glass must be pure from defects
- large lenses can bend under their own weight
- chromatic and spherical aberrations
-refracting telescopes are heavy - large magnifications require large diameters
- lenses must be support only from the edges
Advantages of reflecting telescopes
- mirrors are few nanometers thick and give excellent image quality
- unaffected by by chromatic aberrations
- mirrors not as heavy
- easier to support from behind
What is collecting power
A measure of the ability of a lens or mirror to collect incident EM radiation
What is collecting power proportional to?
Pc = pi/4 * d^2
What is normal adjustment?
When focal planes of objective lens and eye piece lens overlap
Why are reflector telescopes better due to being the same size as refractor telescopes
For the same size you get a high magnification due to the mirror creating a larger length
Why do you want a small angular resolution
This allows you to resolve objects that are very close to each other
Diffraction of dark fringe (degree/radians) = wavelength / Diameter of aperture
How do we know if an image is fully resolved or just resolved
Fully resolved: when fixed dark fringes touch
Just resolved: when central brightness is touching
When drawing spherical aberration which light ray creates a focal point closest to the lens
the furthest ray from the centre
explain the structure of CCD
- HIGH QE
- incident photons help release electrons
- electrons released dependent on intensity
What in the atmosphere is responsible for absorbing infrared radiation
water vapour
Why does water vapour affect the reading of stars temperatures
As water vapour absorb longer wavelengths (infrared)
Peak wavelength would be shifted to the left
This makes stars appear hotter
Lens formula
1/u + 1/v = 1/f
u = distance of object from centre of lens
v = distance of image from centre of lens
f = focal length of lens
What are radio telescopes 1 mark
Telescopes that use radio waves to create images of astronomical images
Characteristics of radio telescopes
Atmosphere is transparent enough to absorb radio waves
Mostly ground based
Must be isolated to avoid interference from radio sources
Uses a parabolic dish
Why do radio telescopes require a large diameter
As radio waves are large, larger diameter is needed to achieve the same resolving power as normal telescopes
What are infrared telescopes?
Use infrared radiation to create images of astronomical images
Characteristics of infrared telescopes
Large concave mirrors
Must be cooled to almost absolute zero due to all object emitting infrared radiation (do not pick up radiation on the dish itself)
Must be well shielded to avoid thermal contamination
Must be accessed from space
What are ultraviolet telescopes
Telescopes that use ultraviolet radiation to create images of astronomical objects
Why can’t UV telescopes be used on the ground
Ozone layer absorbs UV radiation
Characteristics of UV telescopes
Must be in space
Uses cassegrain configuration
rays are detected by solid state devices which use photoelectric effect to convert UV photons to electrons to pass around a circuit
X-ray telescopes?
Telescopes that use x-rays to create images of astronomical images
Characteristics of X-ray telescopes
Must be used in space
- must use the combination of parabolic and hyperbolic mirrors which are extremely smooth due to high energy received
- brought into focus by CCDs which convert light into electrical pulses
What are X-ray telescopes used to observe
High energy events like active galaxies, black holes and neutron stars
What are gamma ray telescopes
Telescopes that use gamma radiation to create images of astronomical objects
Characteristics of gamma telescopes
Gamma rays pass through them so they’re detected via a detector made of layers of pixels
What do gamma rays observe
gamma ray bursts, quasars, black holes and solar flares
What are the two types of gamma ray bursts (GRB)
Short lived - (0.01-1sec) merging neutron stars, formation of black holes, neutron stars falling into black holes
Long lived (10-1000sec) - type 2 supernova (death of massive stars)
The greater the collecting power the…
…brighter the image
According to Rayleigh Criterion, when is two objects not resolved?
If any part of the central maximum is either of the images fall within the first minimum diffraction ring
What are charged-coupled devices?
An array of light sensitive pixels which become charged when they’re exposed to light by the photoelectric effect
What is quantum efficiency
Percentage of incident photons which can an electron to be released
What features of CCD can be compared to the human eye?
Quantum efficiency
Spectral range - detectable range of wavelengths
Pixel resolution - total no. of pixels
Spatial resolution ( min distance to resolve two images)
Convenience
What is CCD quantum efficiency compared to the human eye?
around 80 percent
eye is around 4-5
