Astrophysics Part 1: Telescopes Flashcards
What is the axis of symmetry called?
Principle axis
What is the principal focus?
A point on the axis which is the same distance from the optical centre as the focal length. This is where the light rays travelling parallel to the principle axis prior to refraction converge.
Define focal length.
The distance between the centre of the lens and the principle focus. (Lens axis to the focal plane)
Draw the ray diagram for a refracting telescope in normal adjustment.
Check notes, cgp, pmt
Give the formula for angular magnification in normal adjustment.
M = angle subtended by image at eye / angle subtended by object at unaided eye
State the equation that relates M to the focal length for objective and eyepiece lenses.
M = b / a
M = b / a = h/fe x fo/h = fo/fe
Equation for focal length using u and v. Define u and v
1/f = 1/u + 1/v
U = distance between object and lens axis
v = distance between image and lens axis (positive if image is real, negative if it is virtual)
What isthe cassegrain arrangement and draw it.
It is a reflecting telescope which uses a parabolic concave primary mirror and convex secondary mirror. (There’s also an eye piece)
Check drawing on handouts pmt or cgp.
What is spherical aberration and how can it be minimised? What kind of telescopes are affected?
Spherical aberration - both refractors and reflectors suffer from spherical aberration.
In refracting and reflecting - lenses/ mirrors that are ground into a spherical surface, refract rays by different amounts meaning the rays do not pass through the same focal point leading to a blurring of the image we see.
Minimised by using a lens with a parabolic shape however very expensive to make
What is chromatic aberration and what type of telescopes does it effect?
Reflecting telescopes don’t get affected by chromatic aberration only refractive do.
Chromatic aberration - glass refracts different colours of light by different amounts (blue more than red). Results in coloured fringing This means that the different wavelengths are focused at different points.
What are the advantages of reflecting telescopes over refracting telescopes?
Reflectors can be much larger than refractors as they can be supported from behind, whereas a lens must be supported at the edge and large lenses are likely to break under its own weight. The large the diameter of the telescope the more light it is able to collect.
Reflectors do not suffer from chromatic aberration. Parabolic lens are very expensive not so much for mirrors
What re some disadvantages of reflecting telescopes?
The secondary mirror and the ‘ spider ‘ frame work holding it in place both diffract the light as it passes, leading to a poorer quality image.
This is some refraction and therefore chromatic aberration eventually, in the eyepiece used to view the final image.
Don’t say any non-optical differences such as cost it wont give u any marks.
Don’t think that the secondary mirror will block the light leading to an incomplete image which it doesn’t
Define resolving power.
An indication of how good something is at separating ( resolving ) two objects close to one another.
What are the three main non-optical telescopes?
I-R, UV, x-ray
Explain the structure, positioning and uses of a single dish radio-telescope.
Structure - parabolic dish that focuses radiation onto a receiver
Positioning- can be ground-based but must be in isolated locations
Uses - observing things such as galaxies, stars and black holes
Define the Rayleigh Criterion. Draw a diagram to explain
Two objects will be just resolved if the central maximum of the diffraction pattern of one image coincides with the first minimum of the other. Look at pmt or notes for diagram
What is minimum angular resolution of telescope? Also what is the equation
Also explain how diamter is proportional to the resolving power
The smallest angular separation at which an instrument can distinguish two points.
θ = λ/D
D = diameter of the Aperture
θ = minimum angular resolution of the instrument in radaians
If two objects are greater than the value of theta you will be able to see two objects. So this means that the smaller the angle the greater the resolving power. The greater the diameter the smaller the minimum angle resolution and the greater the resolving power. THEREFORE , the resolving power is proportional to the diameter of the aperture.
Why do radio telescopes need to be larger than optical telescopes?
Since radio waves have a much larger wavelength than visible light, in order to achieve the same resolving power as an optical telescope, the objective diameter must be larger in accordance with θ = λ/D. Astronomers get around this problem by linking lots of telescopes together.
What are the advantages an disadvantages of radio telescopes compared to optical telescopes?
Instead of a mirror a wire mesh can be used since the long wavelengths of radio waves don’t notice the gaps. Makes it much cheaper.
The shape of the dish has to have a precision of about λ/20 to avoid spherical aberration. So dish does not have to be anywhere near perfect as a mirror.
radio telescopes have to scan across the radio source to build up an image unlike optical telescopes.
Explain the structure, positioning and uses of an infrared telescope.
Structure - parabolic mirror focusing light onto a detector. Must be cooled with cryogenic fluids to avoid interference (as they themselves produce heat).
Positioning - must be in space as infrared light is blocked by the atmosphere. Or high altitude and dry areas on Earth. (Won’t work for UV or x-ray tho)
Uses - observing cooler regions in space.
Explain the structure, positioning and uses of an ultraviolet telescope.
Structure - cassegrain configuration that focuses radiation onto solid state devices.
Positioning - Must be in space as UV is blocked by the atmosphere
Uses - observing interstellar medium and star formation regions
Explain the structure, positioning and uses of an x-ray telescope
Structure - combination of hyperbolic and parabolic mirror to focus radiation onto a CCD.
Or by using grazing mirrors to gradually alter the direction of the x-ray wave to bring them to focus onto a detector. (Grazing telescope)
Positioning - must be in space as x -rays are blocked by the atmosphere.
Uses - observing high energy events and areas such as active galaxies, black holes and neutron stars.
Explain how the collecting power of a telescope is proportional to its collecting area.
The collecting power increases with the size of the objective lens/mirror.
A = πd^2/4, collecting power is therefore directly proportional to the diameter^2
What is a CCD and how does it work?
Charged-coupled devices are silicon chips divided into pixels, when a photon (several regions of the EM spectrum) is incident on a pixel, electrons are released from the silicon atoms via the photoelectric effect, the electrons are confined to the pixel so it accumulates charge, the position and magnitude of which is used to create a digital image.
Remember that number of electrons liberated is proportional to the intensity of the incident light.
Compare the quantum efficiency of a CCD to the eye,
Quantum efficiency: the percentage of incident photons that liberate an electron in the photoelectric effect. This can be upwards of 70% for a CCD, compared to 1-2% for the human eye
QE = number photons detected / number of photons incident x100
Compare the resolution of a CCD to the eye
CCDs have a spacial resolution of 10 micrometers, the minimum resolvable distance for the human eye is around 100 micrometers so CCDs are better for capturing the fine detail.
Spacial resolution is how far apart two objects need to be to distinguish them.
Compare the convenience of a CCD to the eye
The CCD is more convenient for accessing data remotely. It is easier to analyse CCD data on computers, and CCDs have a wider spectral range, allowing them to perceive wavelength that cannot be detected by the human eye. That being said, looking down a telescope is not an inconvenient task.
How do I convert 0.56 degrees into arc seconds
2016 arc sec
Define collecting power
A measure of the ability of a lens or mirror to collect incident EM radiation Collecting power is directly proportional to the area of the objective lens/primary mirror.
The greater the collecting power, the brighter the images produced by the telescopes.
What is the normal adjustment diagram and draw it
Check pmt or notes
