Lenses and Telescopes Flashcards
Draw a ray diagram
What is the ray diagram for a refracting telescope in normal adjustment?
What is meant by normal adjustment?
When the distance between the objective lesne and the eye lense is the sum of both focal lengths
Hence the prinicpal focus for these two lenses is in the same place
What is the formula for angular magnification?
Think of a diagram of which this can be applied to
Diagram for the Newtonian reflecting telescope
Diagram for Cassegrain reflecting telescope
Diagram for Cassegrain reflecting telescope
Describe the material and thickness of mirrors in a reflecting telescope
The mirros in a reflecting telescope are very thin (less than 25nm)
They have a coating of aluminium or silver tat are depositied onto a backing material.
This allows mirrors to be smooth as possible and minimuse distortions in the image
What is meant by chromoatic aberration?
- Blue light refracts more than red light
- Hence when the light is refracted in a lense there will be a colour seperation of blue and red light (known as colour fringing)
- In lense terms the focal length of red light is greater than the focal length of blue light, hense they are focsed at different points
Since caused by refraction is has little effect of reflecting telescopes
(as it only occurs in the eye lense)
What is meant by spherical aberration
Curvature of a lense or mirror can caue the rays of light at the edge to be focused in a different position than those near the centre
This leads to image blurring and distortion
Effects lenses with larger diameter
Can be avoided using parabolic objective mirrors in reflecting telescopes
How can both chromatic and spherical aberrations be avoided?
By using an achromatic doublet
This consists of a convex lens made of crown glass and a concave lense made of flint glass
They are cemented together in order to bring all rays of light into focus in the same position
Disadvantages of refracting telescope
- Glass must be pure and free from defects. For large diameter lense this is difficult
- Large lenses can bend and distort under their own weight
- Chromatic and spherical aberration both effect lenses
- Refracting telescopes are incredibly heavy and therefore transportation is difficult
- Large maginifcations require very large diameter objective lenses with very long focal lengths
- Lenses can only be supported from the edges, which is an issue because of how heavy and large
Advantages of reflecting telescopes
- Mirrors that are just a few nanometres thick can be made and these give excellent image quality
- Mirrors are unaffected by chromatic abberation, also spherical abberation can be solved by using parabolic mirrors
- Mirros are not as heavy as lenses, so they are easier to handle and manoeuvre
- Chromatic abberation can effect eye lense, but this can be solved using an achormatic doublet
- Large composite primary mirrors can be made from lots of smaller mirror segments
- Large primary mirrors are easy to support from behind since you dont need to be able to see through them
Desribe purpose amd method of radio telescopes
The atomosphere absorbs pretty much all electromagnetic waves BUT NOT radio or visible light
- Hence it is possible to build ground base radio telescopes
- However they should be in an isolated location to avoid interference from nearby radio sources
- Simple radio telescopes use a parabolic dish to focus radio waves onto a reciever
Similarities between radio telescopes and optical telescopes
- Both types of telescopes function in the same way (they intercept and focus incoming radiation to detect its intensity)
- Both can be moved to focus on different sources of radiation or track a moving source
- The parabolic dish of a radio telescope is extremely similar to the objective mirror of a reflecting optical telescope
- Both can be built on the ground (ground base) since both radio and visible light rays can pass through atmosphere
Differences between radio telescopes and optical telescopes
- Radio wavelengths are much larger than visible wavelengths hence radio telescopes need to be much larger in diameter than optical telescopes in order to achieve the same quality (same resolving power)
- Construction of radio telescopes is cheaper and simpler because a wire mesh is used instead of a mirror
- Radio telescope must move across an area to build up an image, unlike optical telescopes
- Radio wave telescopes experience a large amount of man made intereference, from radio transmissions and phones, microwave ovens ect (However optical telescopes experience interfernnce from the weather conditions, light polution, stray radiation ect)
Describe the method of infrared telescopes and things that must be considered for them to work well
- Telescopes consist of large concave mirros which focus radiation onto a dector
- Since all objects emit infrared radiation as heat, the infrared telescopes must be cooled using cryogenic fluids (liquid nitrogen) to reach absolute zero
- Telescopes must also be well shielded to avoid thermal contamination from nearby objects as well as its own infrared emissions
- Infrared telescopes are used to observe cooler regions in space, however since atmosphere absorbs infrared radition, the telescopes must be launced into space and accessed remotely
Describe the location of UV telescopes and method
- Ozone layer blocks all UV radiation in rays that have a wavelength less than 300nm
- Therefore UV telescopes need to be positioned in space
- These telescopes use Cassegrain configuration to bring ultraviolet rays to a focus
- The rays are detected by solid state devices which use the photoelectric effect to convert UV photons into electrons, which pass around a circuit
- UV telescopes can be used to observe the interstellar medium and star formation regions
Describe location and method of X ray telescopes
- X rays are absorbed by atmosphere hence telescopes need to be in space
- These rays have such high energy that using mirrors like an ordinary telescope would not work as would just pass straight through
- Hence xray telescopes must be made from combination of parabolic and hyperbolic mirrors (must all be very smooth)
- Rays enter telescope, skim off the mirros and are brought into focus by CCD;s which convert light into electric pulses
What can Xray telescopes be used to observe
Active galaxies, blackholes and neutron stars
Describe method of gamma telescopes
- Do not use mirrors as all gamma rays have so much energy they would just pass through
- Instead use detector made of pixels
- As gamma photons pass through, cause a signal in each pixel they come in contact with
- Used to observe blackholes, quasars and solar flares
What are the two types of gamma ray bursts?
- Short lives: 0.01 and 1 second, associated with merging neutron star
- Long lived: 10 to 1000 seconds, associated with type II supernova
What is meant by collecting power
A measure of ability of a lense or mirror to collect incident EM radiation
The collecting power increases with size of objective lens/mirror
Directly proportional to area of objective lens
What is meant by resolving power
The ability of a telescope to produce sepertate images of close together objects
For an image to be resolved, angle between straight lines from object to earth must be at least minimum angular resolution
What is meant by the Rayleigh criterion?
Criterian for determining the minimum angulat seperation at which two point light sources can be resolved, meaning when can they be distinguished by as seperate entities.
It states two objects will NOT be resolved if any part of the central maximum of either image falls within the first minimum diffraction ring of the other.
Rayleigh criterion equation?
Since n = 1 because we are concerned about when central max is near first minimum and theta = sin theta for small angles these are very small angles indeed
Explain what is meant by CCD’s and how they work
Charged coupled devices are an array of light sensitive pixels which become charged when they are exposed to light by the photoelectric effect
- Photons hit silicon in a pixel, this creates free electrons that are confined to the pixel causing charge to accumulate on it
- This charge can be measured and used to create a digital signal
- Signal describes where light hits and to what intensity
What is meant by quantum efficiency?
The percentage of incident photons which cause an electron to be released
What is meant by spectral range
The detectable range of wavelengths of light
What is meant by pixel resolution
The total number of pixels used to form the image on a screen
A lot of small pixels will be able to resolve an image more clearly than a small amount of large pixels
What is meant by spatial resolution
The minimum distance two objects must be apart in order to be distinguishable
compare CCD to human eye
What is meant by luminosity?
The total amount of energy emitted in the form of electromagnetic radiation each second
Also known as the power output of the star (W)
What is meant by intensity
The power recieved from a star (its luminosty) per unit area
(Also known as brightness)
What is meant by apparent magnitude
Brightness is subjective (dependant on where your observing)
Apparent magntiude of an object is how bright the object appears to be,
Depends on luminosity and distance from earth
Explain Hippacros scale and its log refinement
Classified based on apparent magnitude
Brightness stars given apparent magnitude of 1
Dimmest stars given apparent magnitude of 6
It is refined using log scales
A magntiude 1 star has intensity 100 times greater than a magnitude 6 star
(5 magntiudes difference corresponds to difference in intensity of 100 times)
Hence a difference of 1 corresponds to difference of 2.52 times (100 to power of 1/5)
Then to find difference of 1 to 3 it would be 2.52 to power of 2
Equation for ratio of intensities using log refinement constant (2.51)
Explain how the sign of apparent magntiude corresponds to brightness on the refined log scale
More negative = BRIGHTER
More positive = DIMMER
Invented by hupparachus which sounds like hippocrite
What is meant by absolute magnitude and equation
Absolute magntiude is based on power output of the object
- It does not depend on distance from earth
- Defined as what apparent magntiude would be if it were 10 parsecs away from Earth
What is meant by angle of parallax? How do you find it? What is an astronomical unit?
Distance from sun to earth is 1 astronomical unit = 1.5 x 10 ^11
Hence you can calculate angle of parallex if you know distance d and radius r using tan
Angle is very small and is measured in arcseconds
(use small angle approximation)
What is meant by a parsec?
A star is exactly one parsec away (pc) if the angle of parallax is = 1 arcsecond = 1/3600 degrees
How do you convert parsec to metres?
What is meant by a light year?
All electromagnetic waves travel at the speed of light
Distance that electromagnetic waves travel through a vacuum in one year is called a light year (ly)
If we see light that is 10 light years away, we are seeing something from 10 years ago
How do you convert light years into parsec and metres
