Astrophysics Flashcards
What are the three lines you draw on a ray diagram?
Central ray, parallel ray and focal ray
What is the central ray?
The line through the centre of the lens
What is the parallel ray?
The ray parallel to x-axis that goes through the second focal point
What is the focal ray?
The ray through the first focal point that exits parallel to x-axis
What are the two types of telescope?
Refracting and reflecting
How do refracting telescopes work?
They use 2 convex lenses to capture light
How do reflecting telescopes work?
They use a big mirror to capture light
What are the two lenses refracting telescopes are made of?
The objective lens and the eye lens
What does the objective lens do?
Converges rays from object to form real image inside the telescope
What does they eye lens do?
Acts as a magnifying glass on real image to give a magnified virtual image that observer sees
Define the angular magnification of a refracting telescope.
Acts as a magnifying glass on real image to give a magnified virtual image that observer sees
What are the problems with refracting telescopes?
Chromatic abberation, absorb light, difficult to build, need to be long`
Define chromatic abberation.
Glass refracts different colours of light by different amounts and so the image for each colour is in a slightly different position which blurs the image
Why does the absorption of light become a problem for refracting telescopes?
Impurities and bumps in glass absorb light so faint objects can be difficult to see
Why are refracting telescopes difficult to build?
Building large lenses is difficult and expensive. Lenses are heavy and can only
be supported from edges – this means they can deform under their own weight
Why do refracting telescopes need to be long?
For large magnification object lens needs very long focal length so telescope
has to be very long (expensive and needs a large building to put it in)
Define focal point.
Point where all rays meet
What are Cassegrain telescopes made of?
They use two curved mirrors big concave and small convex) and one convex lens
What abberation do mirrors suffer from, why?
Spherical, if the shape of the mirror isn’t
quite parabolic light reflects at wrong angles and doesn’t focus on
one point only, so don’t have a sharp image
What is a CCD?
Charged Coupled Devices, silicon chips that record light
How do CCDs work?
They use the photoelectric effect – photons hit the surface of chip – release electrons that build up – places with more electrons form brighter digital image
What sort of EM radiation can CCDs detect?
They detect infra-red, visible and UV
Why are CCDs better for detecting details in images?
Human eye needs about 500 Megapixels to see a smooth image, CCDs only need a resolution of 50 Megapixels to see a smooth image
Define minimum angular resolution.
The smallest angle for which we can still see two distinct objects using a telescope
Define resolving power.
A measure of how much detail you can see
What is an Airy disc?
When you look through a telescope you get diffraction called an Airy disc
Two objects can be distinguished if?
The distance between the centre of the Airy disc of object 1 and the centre of the Airy disc of object 2 = distance of centre of disc 2 to first minimum of disc 2
How do you calculate minimum angular resolution in radians?
θ = λ / D
D = diameter of objective lens, m
What is the equation for resolving power for a radio telescope?
θ = λ / D
D = dish diameter
How do X-Ray telescopes work?
X-ray telescopes use metal mirrors to collect x-rays that graze them
How do you find the collecting power of a telescope?
The collecting power (C) of a telescope is proportional to the square of the dish diameter (the area)
What positioning do infra red telescopes need to be in?
Infra-Red is absorbed by water vapour so IR telescopes on Earth they need to be in dry places high up
What positioning do infra red/ UV/ X-ray telescopes need to be in?
Absorbed by the higher layers of the atmosphere, need to be above the atmosphere (on weather balloons or aeroplanes or in space)
What positioning do infra red/ UV/ X-ray telescopes need to be in?
Absorbed by the higher layers of the atmosphere, need to be above the atmosphere (on weather balloons or aeroplanes or in space)
What positioning do radio telescopes need to be in?
Can use radio telescopes on Earth because radio waves reach the ground
What positioning do optical telescopes need to be in?
On Earth, still and advantage to being higher up, less atmospheric ‘seeing’ - wobble from moving air
