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

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The Human Eye as an Astronomical Instrument The eye is a camera with:

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

• 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

• 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

Pu`u Poliahu

UH 0.6-m

UH 0.6-m

UH 2.2-m

The first telescopes on Mauna Kea (1964-1970)

Local Seeing

Flow Pattern Around a BuildingIncoming 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 re-circulation 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 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

The contribution to seeing due to turbulence over the mirror is given by:

Mirror Seeing

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.

The thickness of the boundary layer

over a flat plate increases with the

distance to the edge in the and

with the flow velocity.

Thermal Emission Analysis

VLT Unit Telescope

UT3 Enclosure• 19 Feb. 1999• 0h34 Local

Time• Wind summit:

ENE, 4m/s• Air Temp

summit: 13.8C

*>15.0°C

*<1.8°C

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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.

Pre-amplifierPressure Vessel Vacuum pump port

Liquid Nitrogen fill port

Camera mountingFace-plate.

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 varioustelescope ports.

Mounting clamp

Camera Construction Techniques 4.

...

A cutaway diagram of the same camera is shown below.

Thermally Electrical feed-through Vacuum Space Pressure vessel Pump PortInsulatingPillars

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Optical window CCD CCD Mounting Block Thermal coupling Nitrogen can Activated charcoal ‘Getter’

Boil-off

Face-plate

Camera Construction Techniques 5.

CCDTemperature servo circuit board

Platinum resistancethermometer

Gold plated copper mounting block

Top of LN2can

PressureVessel

Signal wires to CCDLocation points (x3)for insulating pillarsthat reference the CCDto the camera face-plate

Aluminised Mylarsheet

Retainingclamp

The camera with the face-plate removed is shown below

‘Spider’.The CCD mountingblock is stood off from the spider usinginsulating pillars.

Camera Construction Techniques 6.

A ‘Radiation Shield’ is then screwed down onto the spider , covering the cold components butnot obstructing the CCD view. This shield is highly polished and cooled to an intermediate temperatureby a copper braid that connects it to the LN2 can.

Radiation Shield

Camera Construction Techniques 7.

‘Spider’ Vane

CCD Clamp plate

CCD Signal connector (x3)Copper rod or ‘cold finger’used to cool the CCD. It isconnected to an LN2 can.

Gold plated copper CCD mounting block.

CCD Package

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.

FOS 1 Spectrograph