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History of Astronomical
Instruments
The early history:
From the unaided eye to
telescopes
The Human Eye
Anatomy and
Detection Characteristics
Anatomy of the Human Eye
Sensitivity of the Eye's Cones and Rods
1.2
0.8
rods
0.6
cones
0.4
0.2
Wavelength [nm]
68
0
66
0
64
0
62
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60
0
58
0
56
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54
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50
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48
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46
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0
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40
0
Relative Sensitivity
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The Human Eye as an Astronomical Instrument
The eye is a camera with:
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Focal length f = 18 mm
Aperture variable 2 – 7 mm
Fast scanning and focus adjustment
A high-resolution color sensitive center: the fovea with cone
cells
Lower resolution peripheral vision, both cones and rods
Separate day and night vision detectors:
o Cones for color vision during the day
o Rods for low-light monochromatic vision
Redundant system
Stereoscopic Rangefinding system
Powerful image processing and object identification system
connected
Empirical Starting point:
Experienced observers under ideal conditions
can just barely see stars of 6 th magnitude.
Calibration of the magnitude scale:
A star of 0 magnitude
in the visual band emits:
3.75 10-11 J m-2 s-1 nm-1
(Joules per square meter collecting area (0.38 10-4 m2),
per second collecting time (0.15 s),
and per nm filter bandpass (100nm) )
This makes 2.14 10-14 J of energy received
by the eye in one reaction time.
Each photon carries an energy of
hc/λ = 3.61 10-19 J
This means the eye receives 59000 photons
per second from the 0 magnitude star.
A 6 mag star receives a factor of 251 less
Photons, i.e. 235 photons
The eye also receives 1711 competing photons
From every square degree of the sky.
Assume light from 0.1 square degrees actually
interferes or competes with the detection of
The star, i.e. 171 competing photons.
Quantum efficiency of the eye is about 5% under optimal
conditions:
Healthy eye, perfectly dark adapted, using peripheral(rod) vision,
having enough oxygen, good nutrition (vitamin A), experienced in
the mental evaluation of faint signals.
Under these conditions, in the reaction time interval, we have:
12 photons detected from the star
competing with 9 photons from the sky.
The 12 photons have to be detected against
a total noise of sqrt (12+9) = 4.6 photons.
The 6 mag star is thus a 2.6 σ detection, which
is just a quantitiative way of saying: “barely able to see it”
Resolving Power of the Eye
Resolution (daylight viewing with fovea): 1 arcmin
Projected diameter of fovea: 100 arcmins
Sensor density:
30 106 rods / steradian = 2.7 rods/arcmin2
1.2 106 cones / steradian = 0.1 rods/arcmin2
In the fovea:
50 106 cones / steradian = 4.2 rods/arcmin2
Diameter of individual cones: 2 μm (25”)
Diameter of individual rods: 1 μm (12”)
Comparison to Diffraction Limit
Pupil diameter: 2.5 mm
Wavelength:
500 nm (green light)
Diffraction Limit:
1.22 λ/d = 0.000244 radian
= 0.84 arcmin
Under optimal bright daylight conditions, the eye
is capable of nearly diffraction-limited resolution.
At night, the pupil is larger (up to 7 mm) and the
resolution is limited by rod-cell density.
Visual Observations
• Navigation
• Calendars
• Unusual Objects (comets etc.)
Hawaiian Navigation:
From Tahiti to Hawaii
Using the North direction,
Knowledge of the lattitude,
And the predominant
direction of the
Trade Winds
Tycho Quadrant
Pre-Telescopic Observations
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Navigation
Calendar
Astrology
Planetary Motion
Copernican System
Kepler’s Laws
Why build telescopes?
• Larger aperture means more light
gathering power
– sensitivity goes like D2, where D is
diameter of main light collecting element
(e.g., primary mirror)
• Larger aperture means better angular
resolution
– resolution goes like lambda/D, where
lambda is wavelength and D is diameter of
mirror
Collection: Telescopes
• Refractor telescopes
– exclusively use lenses to collect light
– have big disadvantages: aberrations & sheer
weight of lenses
• Reflector telescopes
– use mirrors to collect light
– relatively free of aberrations
– mirror fabrication techniques steadily improving
William Herschel
Caroline Herschel
Herschel 40 ft Telescope
Optical Reflecting Telescopes
• Basic optical designs:
– Prime focus: light is brought to focus by primary
mirror, without further deflection
– Newtonian: use flat, diagonal secondary mirror to
deflect light out side of tube
– Cassegrain: use convex secondary mirror to reflect
light back through hole in primary
– Nasmyth focus: use tertiary mirror to redirect light to
external instruments
Mirror Grinding Tool
Mirror Polishing Machine
Fine Ground Mirror
Mirror Polishing
Figuring the Asphere
Crossley 36” Reflector
Yerkes 40-inch Refractor
Drawing of the Moon
(1865)
First Photograph of the
Moon (1865)
The Limitations of Ground-based
Observations
Diffraction
Seeing
Sky Backgrounds
Diffraction
Wavefront Description of Optical System
Wavefronts of Two Well Separated Stars
When are Two Wavefront Distinguishable ?
Atmospheric Turbulence
Characteristics of Good Sites
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Geographic latitude 15° - 35°
Near the coast or isolated mountain
Away from large cities
High mountain
Reasonable logistics
Modern Observatories
The ESO-VLT Observatory at Paranal, Chile
UH 0.6-m
Pu`u Poliahu
UH 2.2-m
UH 0.6-m
The first telescopes on Mauna Kea (1964-1970)
Local Seeing
Flow Pattern Around a Building
Incoming neutral
flow should enter
the building to
contribute to
flushing, the height
of the turbulent
ground layer
determines the
minimum height of
the apertures.
Thermal
exchanges with the
ground by recirculation inside
the cavity zone is
the main source of
thermal turbulence
in the wake.
Mirror Seeing
When a mirror is warmer that the air in an undisturbed enclosure, a
convective equilibrium (full cascade) is reached after 10-15mn. The limit on
the convective cell size is set by the mirror diameter
LOCAL TURBULENCE
Mirror Seeing
The contribution to seeing due to turbulence over the mirror is
given by:
The warm mirror seeing varies slowly with the thickness of the convective
layer: reduce height by 3 orders of magnitude to divide mirror seeing by 4,
from 0.5 to 0.12 arcsec/K
Mirror Seeing
The thickness of the
boundary layer over a
flat plate increases
with the distance to
the edge in the and
with the flow
velocity.
When a mirror is warmer that the air in a flushed enclosure, the convective
cells cannot reach equilibrium. The flushing velocity must be large enough
so as to decrease significantly (down to 10-30cm) the thickness turbulence
over the whole diameter of the mirror.
Thermal Emission Analysis
VLT Unit Telescope
*>15.0°C
14.0
12.0
10.0
8.0
6.0
4.0
2.0
*<1.8°C
UT3 Enclosure
• 19 Feb. 1999
• 0h34 Local
Time
• Wind summit:
ENE, 4m/s
• Air Temp
summit: 13.8C
Gemini South Dome
Night Sky Emission Lines at Optical Wavelengths
Sky Background in J, H, and K Bands
Sky Background in L and M Band
V-band sky brightness variations
H-band OH Emission Lines
Camera Construction Techniques 1.
The photo below shows a scientific CCD camera in use at the Isaac Newton Group. It is
approximately 50cm long, weighs about 10Kg and contains a single cryogenically cooled CCD.
The camera is general purpose detector with a universal face-plate for attachment to various
telescope ports.
Mounting clamp
Pre-amplifier Pressure Vessel Vacuum pump port
Camera mounting
Face-plate.
Liquid Nitrogen
fill port
Camera Construction Techniques 4.
A cutaway diagram of the same camera is shown below.
Thermally
Insulating
Pillars
Electrical feed-through
Vacuum Space
Pressure vessel
Pump Port
Telescope beam
Face-plate
CCD
Focal Plane
of Telescope
Optical window
...
CCD Mounting Block Thermal coupling
Boil-off
Nitrogen can
Activated charcoal ‘Getter’
Camera Construction Techniques 5.
The camera with the face-plate removed is shown below
CCD
Retaining
clamp
Temperature servo circuit board
Aluminised Mylar
sheet
Gold plated copper
mounting block
Top of LN2
can
Platinum resistance
thermometer
Pressure
Vessel
‘Spider’.
The CCD mounting
block is stood off from
the spider using
insulating pillars.
Location points (x3)
for insulating pillars
that reference the CCD
to the camera face-plate
Signal wires to CCD
Camera Construction Techniques 6.
A ‘Radiation Shield’ is then screwed down onto the spider , covering the cold components but
not obstructing the CCD view. This shield is highly polished and cooled to an intermediate temperature
by a copper braid that connects it to the LN2 can.
Radiation Shield
Camera Construction Techniques 7.
Some CCDs cameras are embedded into optical instruments as dedicated detectors.
The CCD shown below is mounted in a spider assembly and placed at the focus of a
Schmidt camera.
CCD Signal connector (x3)
Copper rod or ‘cold finger’
used to cool the CCD. It is
connected to an LN2 can.
‘Spider’ Vane
CCD Clamp plate
Gold plated
copper CCD
mounting
block.
FOS 1 Spectrograph
CCD Package