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Telescopes
Telescopes
refracting
focal plane
focal length, f
optical axis
image
lens (diameter, D)
focal plane


focal length, f
optical axis
reflecting
prime focus
Primary mirror
(paraboloid + hyperboloid)
flat mirror
Newtonian focus
Secondary mirror
(hyperboloid)
Diffraction and resolution
I
n=2
Airy disk
n=1
n=1
n=2
distance
I
Airy disk
1.22 /D distance
I
1.22 /D
distance
Rayleigh Criterion: A
point source is said to
be resolved if the
closest peak of any
other Airy disk falls at
least as far away as
its own first minimum.
aberrations
chromatic aberration (refracting)
spherical aberration (spherical mirror)
centre focus
edge focus
coma (lense and parabolic mirror)
instrument
detector
Typical instruments:
• Camera
• Spectrograph
• Polarimeter
• Photometer
Detectors:
• Charge-coupled devices
(CCDs)
• Photographic plates
ground
HST
HST repaired
HST repaired +
ground
without
with
adaptive optics
spectrograph
slit in telescope
focal plane
beam
from
telescope
collimator
mirror
CCD

diffraction
grating
For normal incidence:
imaging lens
n = order number
 = wavelength
d = groove spacing
infrared - ionized H
visible (emission
nebula illuminated by stars)
globular cluster M79
visible star light
UV - massive
stars
Radio
interferometry

a
L
signals
combined

The effective
baseline,
L
= a cos ZD
(where a is the
distance between
the telescopes, and
ZD is the zenith
distance of the
source.)
350 m radio telescope
in Arecibo
Very Large Array in
Sorocco - 36 km
Very Long Baseline
Interferometry
galaxy with radio lobes
X- ray
grazing telescope
CCD
Charge Coupled Device
Chandra
XMM - Newton