telescopes

Question 1

how does a refracting telescope work?

Answer 1

uses lenses to bend light
the objective lens makes a small, real image while the eyepiece lens magnifies it

Question 2

name two disadvantages of refracting telescopes

Answer 2

suffer chromatic abberation
- impurities and bubbles in the glass can absorb some of the light
- large lenses are expensive and heavy, so will distort under their own weight
- high magnification requires an objective lens with a very long focal length, resulting in a very long telescope

Question 3

what is chromatic aberration?

Answer 3

different colours of light are refracted different amounts by the glass of the lenses
the principal focus (focal point) will be in slightly different places for each colour so the overall image will be blurred

Question 4

name two advantages of reflector telescopes

Answer 4

• no chromatic aberrations
• no spherical aberration
• no distortion as mirrors can be better supported
• better resolving power
• produces brighter images as the lens will absorb more light

Question 5

what is the resolving power of a telescope?

Answer 5

indication of how good it is at distinguishing two objects close together

Question 6

what is the Cassegrain system? (reflector telescope)

Answer 6

incoming light rays hit a parabolic, concave primary mirror and reflect back to a convex secondary mirror
the waves then cross at their principal focus point, I line with the centre of both mirrors and pass through the eyepiece

Question 7

what is spherical aberration?

Answer 7

if the primary mirror in a reflector telescope is not parabolic, the light rays will not converge and the image will be blurred

Question 8

what is quantum efficiency?
what is the quantum efficiency of the human eye?

Answer 8

measure of a detectors sensitivity to EM radiation
- eye has QE of 1%

Question 9

What is a CCD?
what is its quantum efficiency?

Answer 9

a type of microchip made of a silicon wafer divided into up to a million pixels
- QE of around 80%

Question 10

how does a CCD work?

Answer 10

• incident photons cause electrons to be released
• number of electrons is proportional to light intensity
• electrons are trapped in ‘potential wells’ in the CCD
• the exposure is complete, the charge from the electrons is processed to give an image

Question 11

advantages of CCD vs film

Answer 11

• CCD is very sensitive and the size of a postage stamp
• CCD has much higher quantum efficiency (80% vs 4% of film)
• CCDs are cheap and can detect radiation beyond the visible spectrum

Question 12

what is gathering power?
how does a bigger dish affect gathering power?

Answer 12

• energy or photons collected per second
• bigger dishes collect more photons per unit time so have a larger gathering power allowing them to see fainter objects