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Astro 500

A500/L-6

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Find values for WIYN & SALT instr.:

A500/L-6

• Detector gain, read-noise, system efficiency• WIYNØ WHIRCØ Bench SpectrographØ ODIØ NEID• SALTØ SALTCAMØ RSSØ HRS

Assignment: vWork as a teamvTabulate informationvAssess availability, ease of accessvBring to class to present

What did you find?

4A500/L-6

Telescopes & Optics

• Defining the telescope & observatory• Mounts• Foci• Optical designs• Geometric optics• Aberrations

Outline

Conceptually separate

Critical for understanding telescope and instrument capabilities and preliminary instrument design

Essential for detailed instrument design and for evaluating data.

5A500/L-6

Telescopes as Facilities

• The Observatory: far more than telescope

ØOpto-mechanical and thermal controlØAcquisition & guidingØTelemetry and sensingØ Instrumentation and instrument interfaces (ports)ØSoftware for telescope and instrument controlØTechnical support and maintenanceØData storage and transferØSoftware pipelines for data reduction and analysisØEnvironment for observer and operatorØPersonnel management, technical and scientific leadership

6A500/L-6

Telescopes: broader relevance

• Telescopes are examples of optical systems serving as cameras, i.e., producing images from sources at infinity (collimated light).

• Spectrographs and re-imaging systems essentially use variants of these systems as collimators and cameras.

7A500/L-6

Telescopes

• What parameters define telescopes?ØCollecting Area (A)ØFOV (W)Ø Image QualityØCostØThroughputØSpectral rangeØPlate scale/MagnificationØPointing/tracking

highly coupled

We will focus on optical/IR telescopes;Most of this parameter space is relevant for light collection at other wavelengths.

Telescopes as cameras

Telescopes

8A500/L-6

Examples of Telescopes

ØRefractingo Galilean (1609)o Keplerian (1611)

ØReflecting (4x harder to make!)o Newtonian (1672)o Gregorian (1673)o Cassegrain (1668)o Ritchey-Cretien (1927)

ØCatadioptrico Schmidt (1931)o Maksutov (1944)

ØMulti-mirroro MMT (1977)o Keck (1992)o HET (1996), SALT (2005)o all ELT’s (2014+)

Trends to larger collecting area, but more compact in mounting and focal-ratio (speed).

Why?

9A500/L-6Credit: R. Racine

Newton

HerschelParsons

Yerkes

Hale

Galileo

Keck 1&2VLT1-4

ELT’s

SALT

10A500/L-6

Telescope Mounts

Ø Equatorial*o German (Yerkes)o English Yoke or English Crosso Fork (Shane 3m)o Horseshoe (CTIO & KPNO 4m)

Ø Alt-Az (Altitude-Azimuth)o ARC, WIYN, ARC, SOARo Keck, VLT, Subaru, LBT, MMT

MegallanØ Transit

o Arecibo (radio)o LMT - liquid mirror telescope

Ø Transit-Azimutho HET, SALT

Easiest for tracking

Best for flexure, but pointing limitations

Best compromise

Most compact but requires computer control; has field rotation

Cheapest but limited sky coverage (40%)

Cheap and up to 70% sky coverage

First: 1977

3.5-4.5m

6.5-10m

*https://en.wikipedia.org/wiki/Equatorial_mount

11A500/L-6

Telescope FociØPrimeØNewtonianØCassegrain

ØFolded CassegrainØNasmythØCoude

What’s the difference?

Where’s prime focus?

12A500/L-6

Types of Telescopes

Category Primary Secondary Corrector Name Principal Aberrations

Singlet lens Spherical refractor spherical + chromatic

Singlet mirror Paraboloid mirror

Newtonian coma + astigmatism

Doublet mirrors Paraboloid Hyperbaloid Cassegrain comaHyperbaloid Hyperbaloid Ritchey-Chertien (RC) astigmatism (twice

Cassegrain field)Parabaloid Ellipsoid Gregorian field curvatureEllipsoid Spherical Dall-KirkhamEllipsoid Ellipsoid Aplonatic Gregorian

(AG)best images but large obstruction

Multiplets Spherical Aspheric lens or achromate doublet

Schmidt v. wide field

Spherical Hyperbaloid Aspheric lens Schmidt-Cassegrain “

Spherical Spherical Spherical meniscus lens

Maksutov “

Spherical 4-mirror asphere HET, SALT Low cost

13A500/L-6

Singlets

14A500/L-6

Doublets*

*with a confusing mixture of labels for telescope types and foci

15A500/L-6

Multiplets: Catadioptrics

Spherical Primary

16A500/L-6

• Trade-offs primarily between cost, desired field of view (at given image-quality) and collecting area.

• Image-quality and field-of-view concern aberrations.• 6 principal aberrations:

Ø ChromaticØ SphericalØ ComaØ AstigmatismØ DistortionØ Field curvature

Why different designs?

Lenses only

The Big 3

Not strictly aberrationsOff-

axis

Rad

ial

17A500/L-6

Optics• Geometric Optics

ØReflectionØRefractionØThin lensØSpherical opticsØConics

ØStopsØPupilsØChief raysØMarginal rayØSystem design

ØTelescope summary

• AberrationsChromaticSphericalComaAstigmatismDistortionField curvature

Whe

re w

e’re

hea

ded

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Reading

• Walker - ok place to start• Schroeder - ferocious, but very rigorous and

remarkably general and complete• McLean - fairly complete, but missing basicsØ Recommend:

1. Start with Walker to get an overview2. Pick up McLean and a basic optics book (e.g.,

Meyer-Arendt, “Introduction to Classical and Modern Optics”) and work out quoted results; return to Walker to reconcile results and nomenclature.

3. Then hit Schroeder’s text running to go to expert level.

A500/L-6

19A500/L-6

Some Everyday Concepts

• Specular and Diffuse Reflection• Refraction

Specular Diffuse

Refraction

20A500/L-6

The Thin Lens

f

f

Object

Image

h’

h

s

s’

€

1f

=1s

+1s'

m =−s's

=h'h

= magnification

f− ratio = f /D, D = clear aperture

€

1f≡ P = optical power

chief ray

focal ray

parallel ray &marginal ray

21A500/L-6

The Thin Lens

f

f€

1f

=1s

+1s'

m =−s's

=h'h

= magnification

f− ratio = f /D, D = clear aperture

Object at ∞

D

22A500/L-6

Spherical OpticsFocal Length Defined

f

Object at Infinity

€

1f

=1s

+1s'

=2R

Rs2

'1=

fs1

'1=

DefinitionC

R

s s’

f = R/2 is referred to as the paraxial focal-length

23A500/L-6

Snell’s Law

Index = n

Index = n’

q’q P

Q

n and n’ depend on l

€

1V

= (nF − nC ) /(nD −1)

= dispersive power

F,D,C: solar-spectrum Fraunhofer lines (486, 589, 656 nm).V vs n is called a glass table.

B

C

Fermat’s principle: least time

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An Aside: Implications of Snell’s Law

qa

z

d

Impact of wedges and and plane parallel plates on an optical beam

n n n

q

q’

€

α ≈ (n −1)θ

€

z ≈ (n −1)tn

€

d ≈ t sinθ 1− cosθncosθ '

%

& '

(

) *

t

If beam is converging, astigmatism introduced. Can be eliminated with wedge.

A500/L-6

25A500/L-6

The Thick Lens

f1Object

h

s

€

n2

f2

=n2 − n1

R= refractive power

f2 /n2 = reduced focal lengthR = radius of curvature

chief ray

focal ray

parallel ray &marginal ray

n1

f2

n2

Image

h’

s’

26A500/L-6

Optical Power, P

Two surfaces• separation d• index n

€

P ≡ 1f

= P1 + P2 −dnP1P2

R1

R2

n

Two-surface spherical lens in air or vacuum• R1, R2 radii of curvature• R>0: center of curvature behind lens

f

€

P =1f

= (n −1) 1R1

−1R2

+d(n −1)nR1R2

#

$ %

&

' (

units : dioptor (m−1)

Lensmaker’s formula: