Lecture 1: telescopes

Question 1

the first telescopes

Answer 1

galileo galilei made his own telescope in 1609 using refractive lenses

-magnification of up to 30x
-first observation of Jupiter’s moons

Question 2

types of refracting telescopes

Answer 2

galilean and keplerian

Question 3

galilean telescope

Answer 3

convex and concave lenses

Question 4

keplerian telescope

Answer 4

pair of convex lenses
inverted image

Question 5

chormatic aberration

Answer 5

dispersion
no single focal plane
blue halo

Question 6

achromatic lens

Answer 6

solution to chromatic aberration

combination of two lenses with different dispersion

brings red and blue to focus in the same plane

order of magnitude reduction in chromatic aberration

Question 7

apochromat

Answer 7

one step further than achromatic lens in fixing chromatic aberration

combination of 3 lenses

beings red, green and blue to focus in the same plane

Question 8

magnification is defined by

Answer 8

apparent increase in angular size

Question 9

yerkes refractor

Answer 9

long focal length designed for high magnification
largest refracting telescope in existence
40” or 1m objective

Question 10

limitations to yerkes refractor

Answer 10

residual chromatic aberration
long focal length - poor light gathering
lens sags under own weight, distorting the image
absorption in thick lens
cost/size of optics/dome

Question 11

reflecting telescopes

Answer 11

spherical concave mirror collects light from astronomical object

free from chromatic aberration

mirrors simpler to make - only one optical surface

Question 12

spherical aberration

Answer 12

rays from edge come to focus nearer to mirror than rays form the centre

Question 13

spherical aberration is eliminated by

Answer 13

using a parabolic mirror

Question 14

cassegrain reflector

Answer 14

adopted by the majority of the world’s largest telescopes

parabolic primary, hyperbolic secondary

focus easily accessible - ideal for mounting large instrumentation

possible to have longer focal length, focus adjust by moving secondary

Question 15

optical astronomy

Answer 15

astronomical object
atmosphere
telescope
instrumentation

Question 16

light gathering

Answer 16

S=Fn Tna A Tni Tnd Qn

s prop. A prop. D^2

Question 17

diffraction: star is a

Answer 17

point source at infinity

Question 18

diffraction: plane-parallel waves at telescope are brought to

Answer 18

focus to form an image of the star

Question 19

diffraction at aperture is caused by

Answer 19

aperture not covering entire wavefront radiated by star

Question 20

angular resolution

Answer 20

a=1.22 lambda/ D

Question 21

resolution improves with

Answer 21

scale

Question 22

hooker telescope

Answer 22

largest telescope from 1917 to 1948

instrumental to many scientific advancements including: star classification system, observation of cepheid variables and evidence for dark matter from rotational velocity measurements

Question 23

telescope site considerations

Answer 23

latitude
cloud cover
light pollution
‘seeing’ - still air (limited turbulence)
height above sea level (more turbulence lower in the atmosphere)
humidity
political stability

Question 24

major telescope sites

Answer 24

chile
hawaii
canary islands

Question 25

atmospheric turbulence

Answer 25

pockets of air with different temperatures, densities and refractive indices corrugated wavefronts dominant cause of aberations for large-scale telescopes

Question 26

adaptive optics

Answer 26

use laser to create artificial reference star 'star' is around 90km in sky measures turbulence to give feedback to adaptive optics system

Question 27

gemini observatory

Answer 27

two relectors, fitted with IR and optical detectors fitted with adaptive optics employs laser guide star

Question 28

ESO very large telescope

Answer 28

four telescopes, each with 8.2m reflector UV, visible and IR instrumentation light coherently combined to form 'very large telescope' adaptive optics installed

Question 29

hubble space telesocpe

Answer 29

near infrared, visible, UV eliminates effects of atmospheric turbulence

Question 30

JWST

Answer 30

segmented mirror IR