Lecture 1: telescopes Flashcards
the first telescopes
galileo galilei made his own telescope in 1609 using refractive lenses
-magnification of up to 30x
-first observation of Jupiter’s moons
types of refracting telescopes
galilean and keplerian
galilean telescope
convex and concave lenses
keplerian telescope
pair of convex lenses
inverted image
chormatic aberration
dispersion
no single focal plane
blue halo
achromatic lens
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
apochromat
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
magnification is defined by
apparent increase in angular size
yerkes refractor
long focal length designed for high magnification
largest refracting telescope in existence
40” or 1m objective
limitations to yerkes refractor
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
reflecting telescopes
spherical concave mirror collects light from astronomical object
free from chromatic aberration
mirrors simpler to make - only one optical surface
spherical aberration
rays from edge come to focus nearer to mirror than rays form the centre
spherical aberration is eliminated by
using a parabolic mirror
cassegrain reflector
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
optical astronomy
astronomical object
atmosphere
telescope
instrumentation
light gathering
S=Fn Tna A Tni Tnd Qn
s prop. A prop. D^2
diffraction: star is a
point source at infinity
diffraction: plane-parallel waves at telescope are brought to
focus to form an image of the star
diffraction at aperture is caused by
aperture not covering entire wavefront radiated by star
angular resolution
a=1.22 lambda/ D
resolution improves with
scale
hooker telescope
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
telescope site considerations
latitude
cloud cover
light pollution
‘seeing’ - still air (limited turbulence)
height above sea level (more turbulence lower in the atmosphere)
humidity
political stability
major telescope sites
chile
hawaii
canary islands
atmospheric turbulence
pockets of air with different temperatures, densities and refractive indices
corrugated wavefronts
dominant cause of aberations for large-scale telescopes
adaptive optics
use laser to create artificial reference star
‘star’ is around 90km in sky
measures turbulence to give feedback to adaptive optics system
gemini observatory
two relectors, fitted with IR and optical detectors
fitted with adaptive optics
employs laser guide star
ESO very large telescope
four telescopes, each with 8.2m reflector
UV, visible and IR instrumentation
light coherently combined to form ‘very large telescope’
adaptive optics installed
hubble space telesocpe
near infrared, visible, UV
eliminates effects of atmospheric turbulence
JWST
segmented mirror
IR
