Telescopes Flashcards
What are astronomical telescopes ?
Refracting telescopes.
This has two converging lenses: the objective and the eyepiece.
The objective is closest to the object and has the longest focal length, f0.
The eyepiece is closest to the eye and has the shortest focal length, fe.
The phrase normal adjustment is used to mean the rays emerging from the telescope are parallel to eachother
The assumption is that the image is upside down.
What can the length and magnification of telescope tell you
Length = fo + fe
Magnification = fo/fe
or
magnification = θi/θo
How to calculate degrees from radians
(degrees / pi)*180
180 degrees per Pi
What are parabolic reflectors
A parabolic reflector does reflect all rays parallel to the principal axis through the
focal point.
reflecting telescopes vs refracting
- Mirrors can be made larger than lenses because they can be supported from
behind, unlike lenses which can only be supported at the edges. Also if lenses
are made too large, their weight can distort their shape, which causes a
change in the direction of the rays refracted through them. - a greater area means a greater collecting power (more energy collected per second) which means
brighter images. Also, a greater diameter means a higher resolving power
which means that objects in the sky which are close together can be seen
separately more easily. - Lenses have chromatic aberration- different wavelengths are refracted by
different amounts. Mirrors do not have this effect as they do not refract light. - Lenses suffer from spherical aberration
- The glass of the lens can block some wavelengths of electromagnetic
radiation. Impurities in the glass can also lower the intensity of light going
through the lens. These would not happen with a mirror
However, a disadvantage of the reflecting telescope is that the secondary
mirror blocks some light. This lowers the brightness of the image. It also
decreases the sharpness of the image as there is diffraction around the
mirror. However, the mirror does not cause a black ring in the centre of the
image!
What is an Airy Disc
An Airy Disc is the bright central maximum of the diffraction pattern formed
when light passes through a circular aperture.
what is quantum efficiency
Quantum Efficiency is 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.
The quantum efficiency for a CCD is greater than 80% whereas the quantum
efficiency for the eye is about 1%
CCD Vs Eye
As the quantum efficiency of the CCD is much greater than the eye then the
images from a CCD will be brighter than for an eye.
A CCD can detect a wider range of the EM spectrum than the eye. It can
detect infra red, visible and UV whereas the eye can only detect visible
wavelengths.
CCDs have a better resolution than the eye and so can capture finer detail.
The digital information from a CCD can be passed straight into a computer
which can then analyse the data quickly and accurately. In addition, you get
an image of the object you are looking at. This makes it much more
convenient than the eye.
Using your eye is quicker than using a CCD as there is no equipment to set
up- you just look through the telescope.
what is the collecting power of a radio telescope.
This is the energy it collects per second and is proportional to the area of
the telescope. Area = π d^2/4 where d is the diameter of the telescope
What is resolving power
The resolving power of a radio telescope can be calculated by using θ = λ/ d
and will always be worse than for a light telescope ( a greater value of θ) as the
wavelength of radio waves are so much larger than for light wav
How smooth does a radio dish have to be ?
The dish does not have to be as perfect as mirror for a light telescope. As
long as the surface is within about 1/20 of a wavelength the focusing will be
unaffected by imperfections.
Also, the reflector does not need to be solid. Large dishes can sag under their own
weight, so wire mesh is sometimes used to make them lighter (not to cut down
wind resistance). Provided the holes are less than λ/20 in size diffraction does not
degrade the image and radio waves will not pass through mesh with gaps less than
1/20 of the radio wavelength. (Like satellite TV dishes)
What Electromagnecic waves are absorbed by the atmosphere and why
Some UV radiation is absorbed by ozone in the Earth’s atmosphere, while
some infra- red is absorbed by water vapour in the atmosphere.
Similarities
1 Radio telescopes are similar to optical reflecting telescopes in that they use a
parabolic reflector to focus waves.
2 Both telescopes need to sited away from pollution and humidity away from
sources of interference
Differences
1 Two advantages that radio telescopes have over optical telescopes are that
they can be used in daylight and when there is cloud cover.
2 Whereas light telescopes produce a whole image e.g of a galaxy a radio
telescope measures the intensity in the narrow beam parallel to the telescope
axis and cannot produce an image. To build up a “picture” the telescope must
change direction, scanning the sky in lines.
3 Radio telescopes are longer than optical telescopes.
5 Radio telescopes have one reflecting surface but light telescopes have two.
6 Radio telescopes have a much greater surface area than reflecting optical
telescopes. They need to be larger to get a reasonable resolving power as
radio wavelengths are thousands of times larger than those of visible light.
However, from θ = λ/ d it is clear that although the resolving power can be
improved by increasing the diameter of the telescope, a radio telescope will
always have a worse resolving power than an optical telescope.
7 Radio telescopes also need a large surface area because radio signals are
weaker than optical signals.
8 Light is also scattered by dust and gases in space as it travels from the star to
Earth. However, radio waves can penetrate dust, so we can use them to
observe objects which are hidden from light telescopes by the dust.
Where are UV,IR and X-Ray telescopes positioned
UV - Usually in space(absorbed by ozone)
IR - high mountains or space (absorbed by water vapour)
X-Ray - above atmosphere
