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Astronomical Observational Techniques and Instrumentation

RIT Course Number 1060-771 Professor Don Figer

Telescopes

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Aims and outline for this lecture

• describe most important system parameters for telescopes• review telescope design forms

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Backyard Telescope

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Telescope System

• Opto-mechanical and thermal control

• Acquisition & guiding

• Telemetry and sensing

• Instrumentation and instrument interfaces (ports)

• Software for telescope and instrument control

• Technical support and maintenance

• Data storage and transfer

• Software pipelines for data reduction and analysis

• Environment for observer and operator

• Personnel management, technical and scientific leadership

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Telescope Parameters

• Collecting area is most important parameter– collected light scales as aperture diameter squared (A=r2)

• Length is a practical parameter that impacts mass and dome size

• Delivered image quality (DIQ)– function of optical design aberrations – function of atmospheric properties at observing site

• f/ratio determines plate scale and field of view

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Thin Lens Equation

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Refracting/Reflecting Telescopes

Refracting Telescope: Lens focuses light onto the focal plane

Reflecting Telescope:

Concave Mirror focuses light onto the focal

plane

Almost all modern telescopes are reflecting telescopes.

Focal length

Focal length

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Disadvantages of Refracting Telescopes

• Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect). Can be

corrected, but not eliminated by second lens out of different material.

Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges

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The Powers of a Telescope:Size Does Matter

1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared:

A = (D/2)2

D

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Telescope Size and SNR

• In source shot noise limited case, SNR goes as telescope diameter

teleteleshot

ii

DtFASS

S

N

S

N

S

2

• For faint sources, i.e., read noise limited cased, SNR goes as telescope diameter squared

2noise read

2 teleteleread

ii

DtFAN

S

N

S

N

S

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Reflecting Telescopes

• Most modern telescopes use mirrors, they are “reflecting telescopes”

• Chromatic Aberrations eliminated

• Fabrication techniques continue to improve

• Mirrors may be supported from behind

• Mirrors may be light-weighted

Mirrors may be made much larger than refractive lenses

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Basic Designs of Optical Reflecting Telescopes

1. Prime focus: light focused by primary mirror alone

2. Newtonian: use flat, diagonal secondary mirror to deflect light out side of tube

3. Cassegrain: use convex secondary mirror to reflect light back through hole in primary

4. Nasmyth (or Coudé) focus (coudé French for “bend” or “elbow”): uses a tertiary mirror to redirect light to external instruments (e.g., a spectrograph)

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Prime Focus

fSensor

Mirror diameter must be large to ensure thatobstruction does not cover a significant fraction of

the incoming light.

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Newtonian Reflector

Sensor

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Cassegrain Telescope

Sensor

Secondary Convex Mirror

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Feature of Cassegrain Telescope

• Long Focal Length in Short Tube

Location of Equivalent Thin Lens

f

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Coudé or Nasmyth Telescope

Sensor

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Plate Scale

focal length

x

F

Fx

/206265s/mm)(arcsecond platescale

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Field of View

• Two telescopes with same diameter, different F#, and same detector have different “Fields of View”:

Small F# Large F#

large small

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Optical Reflecting Telescopes

• Concave parabolic primary mirror to collect light from source– modern mirrors for

large telescopes are thin, lightweight & deformable, to optimize image quality

3.5 meter WIYN

telescope mirror, Kitt

Peak, Arizona

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Thin and Light (Weight) Mirrors

• Light weight Easier to point– “light-duty” mechanical systems cheaper

• Thin Glass Less “Thermal Mass”– Reaches Equilibrium (“cools down” to ambient temperature) quicker

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Hale 200" TelescopePalomar Mountain, CA

http://www.astro.caltech.edu/observatories/palomar/overview.html

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200" mirror (5 meters)for Hale Telescope

• Monolith (one piece)

• Several feet thick

• 10 months to cool

• 7.5 years to grind

• Mirror weighs 20 tons

• Telescope weighs 400 tons

• “Equatorial” Mount – follows sky with one motion

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Keck telescopes, Mauna Kea, HI

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400" mirror (10 meters) for Keck Telescope

• 36 segments

• 3" thick

• Each segment weighs 400 kg (880 pounds)– Total weight of mirror is 14,400 kg (< 15 tons)

• Telescope weighs 270 tons

• “Alt-azimuth” mount (left-right, up-down motion)– follows sky with two motions + rotation

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Optical Reflecting Telescopes

Schematic of 10-meter

Keck telescope

(segmented mirror)

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History and Future of Telescope Size

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Optical Telescopes: Resolution

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Optical Telescopes: Collecting Area

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Optical Telescopes: LSST

person!

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Optical Telescopes: LSST

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Optical Telescopes: Giant Magellan Telescope

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Optical Telescopes: Thirty Meter Telescope

person!

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Thirty Meter Telescope vs. Palomar

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Optical Telescopes: E-ELT (now 39m?)

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Optical/IR Telescopes: JWST

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Optical/IR Telescopes: JWST