Telescopes COPY Flashcards
Define the Rayleigh criterion for two objects
Two objects can just be resolved when the central maximum of the diffraction pattern of one coincides with the first minimum of the other.
Why is IR astronomy useful?
- It can detect astronomical objects that are not hot enough to emit visible or UV radiation
- Passes through interstellar and intergalactic gas and dust more easily than light
What is the quantum efficiency for the eye?
About 1%
What are the bonuses of a reflecting telescope having a greater size?
- Greater collectiing power which means brighter images
- Higher resolving power which means that objects in the sky which are close together can be seen separately more easily
Why is the resolving power always worse for a radio telescope compared to a light telescope?
The wavelength of radio waves are much larger than for light waves
What is the equation for resolving power?
ø = lambda / D
D = (aperture of lens / diameter of mirror)
Draw the ray diagram for the Astronomical Telescope
What equation includes the angle subtended by the object, the distance from the object and the observer and the diameter of the object?
ø (angle subtended)= s (diameter) / r (distance)
What is the equation for angular magnification?
Angle subtended by the image at the (øi)/ angle subtended by the object at the unaided eye (øo)
or
FO/Fe
What is the resolving power of the Jodrell Bank telescope whose diameter is 75m and has radiowaves of wavelength 10m
How can radio telescopes be made lighter?
- Use wire mesh (have a reflecter with holes in it)
- As long as holes are smaller than lambda/20 in size the image is unaffected
What is chromatic aberration?
Where different wavelengths are refracted by a different amount.
Blue light is refracted more than red so it is brought to focus closer to the lens than red - it has a shorter focal length
Compare UV telescopes to light telescopes
- UV has a shorter wavelength then light and so has a better resolving power
- The UV telescope will need to be smoother
What is resolving power measured in?
Radians
What are the disadvantages of a reflecting mirror? (2)
The secondary mirror blocks some light this:
- Lowers brightness
The secondary mirror causes some diffraction of the light which:
- Decreases the sharpness
A telescope has an eyepiece of focal length 5 cm and an objective of focal length 50 cm. What angle will the moon subtend if viewed through the telescope? The diameter of the mooon = 3.5 x10^3 km. Distance of moon from earth = 384.4 x10^3 km
What are the similarities between optical and radio telescopes? (2)
- Both use a parabolic reflector to focus waves
- Both need to be sited away from pollution, humidity and away from other sources of interference
Define the focal length
The distance between the focal point and the centre of the lens
What are the differences between optical and radio telescopes? (7)
- Radio telescopes can be used during the day ans when it is cloudy
- Light telescopes produce a whole image whereas radio telescopes cannot produce an image
- Radio telescopes are longer
- Radio telescopes have one relfectin surface but light telescopes have two
- Radio have a greater surface area to improve resolving power amd due to radio signals being weaker
- Optical always has a better resolving power
- Radiowaves can penetrate dust
Define the collecting power of a radio telescope
The energy it collects per second
Draw the diffraction pattern through a circular aperture
Why do radio waves need to be focused by the reflector
Otherwise they are very weak and focusing makes them more intense.
Draw a labelled diagram showing chromatic aberration on a lens
What waves are allowed through our atmosphere?
- Visible waves
- Some UV
- Some infra-red
Why do IR telescopes need to be cooled?
IR Telescopes will detect IR from the telescope itself
It is cooled to near to absolute 0 to lower the unwanted signal
Draw the ray diagram for a reflecting telescope
What is a CCD?
- A silicon chip
- Divided into pixels
- When a photon hits a pixel, an electron is released
- No. of e- is proportional to the intensity of the light
- Charge is processed to give an image
Why is a higher quantum efficiency more favourable?
Images will be brighter
What does the phrase ‘normal adjustment’ mean for an astronomical telescope?
The rays emerging from the telescope are parallel to each other.
Compare the CCD to the eye
- The quantum efficiency is greater so images will be brighter
- CCD can detect a wider range of the EM spectrum
- CCDs have better resolution
- CCD is more convenient as data can be passed straight into a computer
- Eye is quicker
What does it mean to say that our atmosphere is transparent to certain wavelengths?
It lets in certain wavelengths
Why is a large area needed for a radio telescope?
- To increase collecting power which increases image brightness
Define Principle axis
The line passing perpendicularly through the middle of the lens
Draw a ray diagram that shows sperical aberration
A telescope is described as having a 127mm aperture and a resolving power of 0.9 arc seconds. What wavelength have the manufacturers used to calculate the resolving power?
What does chromatic aberration cause?
The image to have coloured edges
What is an advantage if IR radiation over radio telescopes?
The resolving power is greater as IR wavelengths are much shorter (but not as god as light telescopes)
Why are IR telescopes located at the top of mountains/in space?
Water vapor absorbs IR radiation
Having them high up minimises this
Define resolving power
The minimum angle in radians which can be resolved
Is a greater or smaller resolving power better?
Smaller
What is UV absorbed by in out atmosphere?
Ozone
What type of light has a shorter focal length on a refracting telescope?
Blue light
What is the collecting power of a radio telescope equal to?
The area of the telescope
What is the quantum efficiency for a CCD?
Greater than 80%
Why are telescopes put at high altitudes, in places of low humidity, well away from cities and away from sources of chemical pollution?
Pollution and humidity will reduce the intensity of the radiation.
Radio signals from mobiles can cause interference so need to be well away from cities.
What assumption is made with the astronomical telescope?
The image is upside down
What are the advantages to reflecting telescopes compared to refracting telescopes
- Mirrors can be made larger, as they can be supported from behind, unlike lenses which have to be supported from the edges
- Lenses have chromatic aborration, but mirrors dont
- Lenses and mirrors have spherical aborration but it is easier to reduce it in mirrors
- The glass of a lens can block some wavelengths of EM radiation. Impurities in the glass can also lower the intensity of the light. This doesnt happen with mirrors.
Draw the ray diagram for a parabolic reflector
How much more powerful is the largest radio telescope in the world with a diameter of 305m than the one at Jodrell Bank with a diameter of 75m?
Where does a UV telescope need to be sited?
In space so the UV cannot be absorbed by ozone
Define an Airy Disk
The bright central maximum of the diffraction pattern formed when light passes through a circular aperture
How do you reduce spherical aberration in reflecting and refracting telescopes?
Make the mirror/lens more parabolic
It is easier to make a mirror more parabolic than a lens
A telescope with a mirror of diameter 40cm is used to view objects emitting light with a typical wavelength of 500nm. Calculate the resolving power of this telescope.
Define quantum efficiency
The percentage of the number of photons falling on a device which produce a signal to the total number of photons falling on the device
What is infra-red absorbed by in our atmosphere?
Water Vapour
What can you study with X-Ray Telescopes?
Violent and energetic events in the universe
Where do X-Ray Telescopes need to be sited?
Above the atmosphere
A telescope is 1.50m long and has a magnification of 50. What are the values of the focal lengths of the objective and the eyepiece?
Draw the ray diagram for a Cassegrain telescope / reflecting telescope
describe the setup of a radio telescope
- Parabolic dish
- Reflects radio waves to an antenna at the principle focus
- The reciever has to be tuned in
Define the focal point/the principle focus
The point on the Principle axis through which all rays parallel to the Principle axis pass after being refracted by the lens
What does it mean to say that our atmosphere is opaque to certain wavelengths?
It blocks certain wavelengths
