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Basics of Telescopes
Peter Gillingham, Anglo Australian Observatory
Optics
•
Simple telescope - magnification, pupil, brightness, resolution
•
Achromatic refractors
•
Reflecting: Newtonian, Cassegrain, Ritchey-Chrétien
•
Schmidt
•
Prime focus corrector lenses
•
Atmospheric dispersion correction
•
Seeing
Mechanics
•
Supports for monolithic and segmented mirrors
•
Mountings - equatorial, alt-azimuth
•
Drives - gears, rollers, encoding, vibration
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
1
Simple astronomical visual telescope 5 x 25
10°
2°
telescope objective
focal length 100 mm
clear diameter 25 mm
eyepiece
focal length 20 mm
eye
pupil diameter
(angular) magnification: 100 mm/20 mm = 5 x
exit pupil diameter:
25 mm/5 = 5 mm
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
2
Chromatic aberration of simple lens
25 mm dia.
100 mm focal length
longitudinal
chromatic
aberration
1 mm dia.
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
3
Spherical aberration for a simple lens
(100 mm f/4)
with optimum curvatures
90 µm dia.
images stepping through focus
with equal curvatures
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
4
Achromatic doublet
(100 mm f/4)
BK7
F2
images stepping through focus
10 µm dia.
50 µm dia.
best focus at 550 nm
best compromise focus
for 400, 550, 700 nm
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
5
Imaging by spherical mirror
(100 mm f/4)
diameter of circle of least confusion: 13 µm
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
6
Images with parabolic and spherical concave mirrors
200 mm diameter 1600 mm focal length (f/8) wavelength 550 nm
parabolic mirror
squares are
30 µm on a
side
(~ 3.9 arcsec)
spherical mirror
0.2 mm inside
focus
spherical mirror
0.2 mm outside
focus
parabolic mirror
Airy function
Strehl ratio 0.7
PSF (point
spread function)
profiles at best
focus
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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Diffraction limited resolution
Rayleigh criterion with f/8 beam at 550 nm wavelength
1.2
1.12 f/d
= 4.9 µm
relative intensity
1
0.8
sum of
intensities
0.6
0.4
0.2
0
0
5
10
15
distance (micron)
20
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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8
Cassegrain and Ritchey Chrétien systems
Primary conic
constant
Classical Cassegrain:
- 1.0
Secondary conic
constant
Classical Cassegrain:
- 5.7818
Ritchey Chrétien:
- 1.1717
Ritchey Chrétien:
- 8.3086
Rays for on-axis star for AAT (3.9 metre diameter f/8)
Circle diameter
0.748 mm
= 5 arcsec
Circle diameter
0.748 mm
= 5 arcsec
Spot diagrams on axis, 14 arcmin, and 20 arcmin off axis
left: for classical Cassegrain optics, right: for Ritchey Chrétien optics
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
9
Schmidt telescope 1.2 m f/2.5, as for UKST
achromatic corrector with
ashericity exaggerated x100
circle dia.
58.4µm
= (4 arcsec)
Spot diagrams with left: single element corrector; right: achromatic corrector
wavelengths 450, 600, 900 nm
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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Prime focus corrector (AAT 1 degree triplet)
circle dia
66 µm
=1.0 arcsec
Corrector layout
all spherical UBK7
circle dia.
2.5 mm ~ 40 arcsec
Images 0, 0.35°, and 0.5 ° off axis: left without corrector, right with corrector.
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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Atmospheric dispersion correction
CaF2/LLF6
LLF6/CaF2
prisms with dispersions cancelling
(set for zenith)
Dispersion
from 370 nm
to 1000 nm at
ZD 61°
CaF2/LLF6
LLF6/CaF2
prisms with dispersions adding
(set for large ZD)
3.3 arcsec
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
Combined
image after
compensation
by prisms
12
Seeing - blurring of images by
air temperature inhomogeneity
relative intensity
1
0.8
0.6
0.5 arcsec
FWHM
0.4
0.2
0
-0.2
-2
-1
0
2
1
radius (arcsec)
Light passing through a sphere of air
1°C cooler than the surrounding air,
then through a perfect lens.
Profile through resulting image.
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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Axial supports for telescope mirrors
3 point support
9 point whiffle tree support
array of pneumatic pads
behind AAT primary
36 point whiffle tree,
as on Keck segment
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
14
Deflections of mirrors
4 metre diameter mirror
supported on 3 points
range of heights: 5.4 µm
45 cm diameter mirror supported
on 9 point whiffle tree
range of heights: 35 nm
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
15
Principle of whiffle (or swingle) tree
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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Equatorial (or polar) telescope mounting
declination axis
polar axis
tube axis (optical axis)
S
N
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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Alt-azimuth telescope mounting
tube axis
elevation or altitude axis
(horizontal)
azimuth axis (vertical)
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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Spur gear drive, as on AAT
3.6 m dia. gear wheel
provision for
eliminating backlash
in drive
gear train
giving very
high ratio
between
motor and
polar axis
absolute and
incremental optical
rotary encoders
absolute encoder resolution
1 arcsec
incremental encoder
resolution 1/20 arcsec
drive motors
detail of mesh between
main gear wheel and
drive pinion
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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Susceptibility of telescopes to vibration
10 m
lever arm
oscillatory
force at 2Hz
moment of inertia :
1000 tonne m2
natural frequency of
vibration around
elevation axis : 2 Hz
damping : 0.01 x critical (more
than for structural steel)
Amplitude of oscillatory force needed to maintain angular
vibration through ± 0.5 arcsec = 0.77N = 78 gram
Basics of telescopes Peter Gillingham Observational
Techniques Workshop April 2001
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