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ModernObservational/Instrumentation

TechniquesAstronomy 500

Andy Sheinis, Sterling 5520,2-0492sheinis@astro.wisc.eduMW 2:30, 6515 SterlingOffice Hours: Tu 11-12

Telescopes

• What parameters define telescopes?– Spectral range– Area– Throughput– FOV– Image Quality– Plate scale/Magnification– Pointing/tracking

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Telescopes• Examples of telescopes?

– Refracting• Galilean (1610)• Keplerian (1611, 1834)• Astronomical• Terrestrial

– Reflecting (4x harder to make!)• Newtonian• Gregorian (1663)• Cassegrain (1668)• Ritchey-Cretien (1672)

– Catadioptric• Schmidt (1931)• Maksutov (1944)

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A Omega

0.070.00310SALT73.9476.5LSST22.6873.6PanStarrs3.280.28.1Subaru3.2413.6CFHT6.093.92.5SDSS

M^2 sq.degrees

squaredegrees

MA omegaFOVdiametername

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Refractors/ ChromaticAberration

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Newtonian

Cassegrain

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Spherical Primary

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• For a classical cassegrain focus orprime focus with a parabolic primaryyou need a corrector.

• The Richey-Chretien design has ahyperbolic primary and secondarydesigned to balance out coma andspherical in the focal plane.

Optics Revisited

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Where Are We Going?• Geometric Optics

– Reflection– Refraction

• The Thin Lens– Multiple Surfaces– Matrix Optics

• Principle Planes• Effective Thin Lens

– Stops• Field• Aperture

– Aberrations

Ending with a word aboutray tracing and opticaldesign.

Snell’s Law

Index = n

Index = n’

θ’’θ P

Q

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Focal Length Defined

FF’A’

Object at Infinity

Rss

2

'

11=+

Rs

2

'

1=

fs

1

'

1=Definition Application

fss

1

'

11=+

C

ABCD Matrix Concepts

• Ray Description– Position– Angle

• Basic Operations– Translation– Refraction

• Two-Dimensions– Extensible to Three

Ray Vector

Matrix Operation

System Matrix

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Ray Definition

x1

α1

Translation Matrix

• Slope Constant• Height Changes

x1

α1=α2

z

x2

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Refraction Matrix (1)

• Height Constant• Slope Changes

x1

α2

α1

(θ Ref. to Normal)

Refraction Matrix (2)Previous Result

Recall Optical Power

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Cascading Matrices (1)Generic Matrix:

Determinant (You can show thatthis is true for cascaded matrices)

V1 R1 T12 R2 V’2

Light Travels Left to Right, butBuild Matrix from Right to Left

The Simple Lens (Matrix Way)

z12

Front Vertex,V Back Vertex, V’

Index = n Index = nL Index = n’

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Building The Simple LensMatrix

Simple Lens Matrix

z12

V V’

n nL n’

The Thin Lens AgainSimple Lens Matrix

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Thin Lens in Air Again

Thick Lens Compared to Thin

z12

V V’

n nL n’

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Imaging Cameras

• Imagers can be put at almost any focus,but most commonly they are put atprime focus or at cassegrain.

• The scale of a focus is given by S=206265/(D x f#) (arcsec/mm)Examples:1. 3m @f/5 (prime) 13.8 arcsec/mm (0.33”/24µpixel)2. 1m @f/3 (prime) 68.7 arcsec/mm (1.56”/24µpixel)3. 1m @f/17 (cass) 12.1 arcsec/mm (0.29”/24µpixel)4. 10m @f/1.5 (prime) 11.5 arcsec/mm (0.27”/24µpixel)5. 10m @f/15 (cass) 1.15 arcsec/mm (0.03”/24µpixel)

• Classical cassegrain (parabolic primary + convex hyperbolicin front of prime focus) has significant coma.

!

C =3"

16 f 2 for 3m prime focus, 1'' @2.2'

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Direct Cameradesign/considerations

LN2 can DewarCCDdewar windowPreamp

baffles Shielded cable to controllershutterFilter wheel

Field corrector/ADC

Primary mirror

Shutters

• The standard for many years has been multi-leaf irisshutters. As detectors got bigger and bigger, the finiteopening time and non-uniform illumination patternstarted to cause problems.

• 2k x 2k 24µ CCD is 2.8 inches along a diagonal.• Typical iris shutter - 50 milliseconds to open. Center

of a 1s exposure is exposed 10% longer than thecorners.

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Shutter vignetting patternproduced by dividing a 1second exposure by a 30second exposure.

Double-slide system

• The solution for mosaic imagers andlarge-format CCD has been to go to a35mm camera style double-slidesystem.

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Filter Wheel

• Where do you put the filter? There is atrade off between filter size and howwell focused dust and filterimperfections are.

Drift Scanning

• An interesting option for imaging is to park thetelescope (or drive it at a non-sidereal rate) and letthe sky drift by.

• Clock out the CCD at the rate the sky goes by andthe accumulating charge ``follows’’ the star imagealong the CCD.

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Drift Scanning

• End up with a long strip image of the sky witha `height’ = the CCD width and a length setby how long you let the drift run (or by howbig your disk storage is).

• The sky goes by at 15 arcseconds/second atthe celestial equator and slower than this by afactor of 1/cos(δ) as you move to the poles.

• So, at the equator, PFCam, with 2048 x 0.3”pixels you get an integration time per objectof about 40 seconds.

Drift Scanning• What is the point?

– Superb flat-fielding (measure objects on many pixels andaverage out QE variations)

– Very efficient (don’t have CCD readout, telescope setting)• Problem:

– Only at the equator do objects move in straight lines, as youmove toward the poles, the motion of stars is in an arccentered on the poles.

• Sloan digital survey is a good example• Zaritsky Great Circle Camera is another

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Direct Imaging

• Filter systems– Photometry

• Point sources– Aperture– PSF fitting

• Extended sources (surface photometry)• Star-galaxy separation

Filter Systems

• There are a bunch of filter systems– Broad-band (~1000Å wide)

– Narrow-band (~10Å wide)– Some were developed to address

particular astrophysical problems, someare less sensible.

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3100Å is the UVatmospheric cutoff

1.1µ siliconbandgap

Filter Choice: Example

• Suppose you want to measure the effectivetemperature of the main-sequence turnoff in aglobular cluster.

color relative time to reach δTeff=100 B-V 4.2 V-R 11.5 B-I 1.0 B-R 1,7

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Narrow-band Filters

• Almost always interference filters andthe bandpass is affected bytemperature and beam speed:

ΔCWL = 1Å/5˚C ΔCWL = 17Å; f/13 f/2.8