Astrophysics Flashcards
How to draw a ray diagram if an astronomical telescope forming an image in normal adjustment
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For an astronomical refracting telescope, what is required to view object from space
f_o have to be much greater than f_e
Define the two equations to calculate the angular magnitude
M = angle subtended by image at eye / angle subtended by object at unaided eye M = f_o / f_e
What is the angle subtended by the image at the eye
The angle between the top of the image, the centre of the eye and the bottom of the image formed by the telescope
What is the angle subtended by the images at the unaided eye
The angle between the top of the object the centre of the eye and the bottom of the object.
What is the Cassegrain arrangement
Cassegrain arrangement is used to create a reflecting telescope using a primary and secondary mirror.
Give three comparisons between the eye and the Charge Coupled Devices as detectors
- the quantum efficiency of a CCD
How to draw a ray diagram showing the paths of rays through the telescope up to the eyepiece
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Give three comparisons between the eye and the Charge Coupled Devices as detectors
- the quantum efficiency of a CCD is around 80% ad the quantum efficiency of a human eye is 1%, so CCDs detect far more of the light that falls on them than the eye.
- The eye can produce a higher resolution picture than CCDs, but due to spatial resolution, the CCDs are better for capturing fine details.
- CCDs are less convenient to use as using a human eye to look through a telescope is simpler than settings up a CCD. However, CCDs can produce images that can be stored, copied and shared globally.
What is the minimum angular resolution
The minimum angular resolution is the smallest angular separation at which the instrument can distinguish two points.
what is the equation used to calculate the minimum angular resolution using the Rayleigh criterion?
Minimum angular resolution = Wavelength / Diameter of the aperture
Advantages and disadvantages of refracting telescopes
- Glass refracts different colours of light by different amounts. This shifts of colours are called the chromatic aberration
- Any bubble or impurities in the glass absorb and scatter some of the light, so very faint objects are not seen.
- For a large magnification, f_o needs to be very long, which make it expensive
Advantages and disadvantages of reflecting telescopes
- Large mirrors are much cheaper to make large lenses.
- Mirror don’t suffer from chromatic aberration but they can have spherical aberration. If the shapes of mirrors are not parabolic, then the rays might not meet at the same point.
3 Some incoming light might be blocked by the secondary mirror, which decreases the image clarity.
What formula is used to calculate the distance to the star
distance to star = radius of the earth’s orbit/angle of parallax in radians
How does single dish radio telescopes work
It has a parabolic dish made out of wired mesh. An antenna is used as the detector at the principal focus.
A preamplifier is used to amplify the radio waves. A second amplifier is used and it is passed through a tuner to get rid of any unwanted waves and false-colour images will be produced by a computer using the data
Radio waves have a long wavelength, which means that its resolving power is worse than the unaided eye.
People group radio telescopes together using computer programming to combine their data together to form a single image.
How do Inferred telescopes work
They use the same parabolic mirror to set-up to focus the radiation onto a detector and CCDs are used as the detector.
Inferred waves have a longer wavelength, which means that mirrors don’t have to be perfectly shaped
I-R telescopes produce their own inferred waves due to temperature, which means that they need to be cooled to a very low temperature
How do UltraViolet telescopes work
They use the same parabolic mirror to set-up to focus the radiation onto a detector and CCDs are used as the detector
what is a light year
It is the distance electromagnetic waves travel through a vacuum in one year.
How are optical, radio, inferred, ultraviolet and x-ray telescopes positioned?
Our atmosphere only let certain wavelength of the electromagnetic radiation
We can use optical and radio telescopes on the surface of the Earth because the atmosphere doesn’t affect these wavelengths.
Most of the inferred waves are absorbed by water vapour, which means that it is best to set I-R telescopes in high and dry places.
Most ultraviolet, x-rays are absorbed higher up in the atmosphere, which is why they can be strapped to high altitude planes weather balloons or aeroplanes.
The ideal situation is to get your telescopes into space, where it doesn’t get affected by the atmosphere.
What is the collecting power
It is the amount of energy it can collect in a given time
What is the collecting power proportional to
The collection area
What part of optical, radio, inferred, ultraviolet and x-ray telescopes determine its collecting power
For radio, optical, U-V and I-R telescopes it is the area of the objective mirror or dish.
For x-ray telescopes, it is the size of the opening through which x-rays can enter the telescope
What is the resolving power
Resolving power is the ability for telescopes to distinguish small or two close objects.
What two factors does the resolving power depend on
The Raleigh criterion
Telescoped that detect shorter wavelength will have higher resolution power if the diameter of the disk remains the same
Quality of the detector
The resolving power of the telescope is limited by the resolving power of the detector. For CCDs, it is how many pixels there are and for mesh X-Ray detectors, it is how fine the wire mesh is.
