Page 1 Lecture 14 Part 1: AO System Optimization Part 2: How to be a savvy user and consumer of AO Claire Max Astro 289, UC Santa Cruz Feburary 21, 2013.

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Lecture 14Lecture 14Part 1: AO System OptimizationPart 1: AO System Optimization

Part 2: How to be a savvy user Part 2: How to be a savvy user

and consumer of AOand consumer of AO

Claire MaxAstro 289, UC Santa Cruz

Feburary 21, 2013

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Optimization of AO systemsOptimization of AO systems

• If you are If you are designingdesigning a a newnew AO system: AO system:– How many actuators?

– What kind of deformable mirror?

– What type of wavefront sensor?

– How fast a sampling rate and control bandwidth (peak capacity)?

• If you are If you are usingusing an an existingexisting AO system: AO system:– How long should you integrate on the wavefront

sensor? How fast should the control loop run?

– Is it better to use a bright guide star far away, or a dimmer star close by?

– What wavelength should you use to observe?

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Issues for designer of Issues for designer of astronomical AO systemsastronomical AO systems

• Performance goals:Performance goals:– Sky coverage fraction, observing wavelength,

degree of image compensation needed for science program

• Parameters of the observatory:Parameters of the observatory:– Turbulence characteristics (mean and variability),

telescope and instrument optical errors, availability of laser guide stars

• AO parameters chosen in the design phase:AO parameters chosen in the design phase:– Number of actuators, wavefront sensor type and

sample rate, servo bandwidth, laser characteristics

• AO parameters adjusted by user: AO parameters adjusted by user: integration time on wavefront sensor, wavelength, guide star mag. & offset

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Example: Keck Observatory Example: Keck Observatory AO AO ““Blue BookBlue Book””

• Made scientific case for Keck adaptive optics system

• Laid out the technical tradeoffs

• Presented performance estimates for realistic conditions

• First draft of design requirements

The basis for obtaining funding commitment The basis for obtaining funding commitment from the user community and observatoryfrom the user community and observatory

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What is in the Keck AO Blue What is in the Keck AO Blue Book?Book?

• Chapter titles:Chapter titles:

1. Introduction

2. Scientific Rationale and Objectives

3. Characteristics of Sky, Atmosphere, and Telescope

4. Limitations and Expected Performance of Adaptive Optics at Keck

5. Facility Design Requirements

• Appendices: Technical details and overall Appendices: Technical details and overall error budgeterror budget

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Other telescope projects Other telescope projects have similar have similar ““BooksBooks””

• Keck Telescope (10 m): – Had a “Blue Book” for the telescope concept itself

• Thirty Meter Telescope: – Series of design documents: Detailed Science

Case, Science Based Requirements Document, Observatory Requirements Document, Operations Requirements Document, etc.

These documents are the kick-off point These documents are the kick-off point for work on the for work on the ““Preliminary DesignPreliminary Design””

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First, look at individual terms in First, look at individual terms in error budget one by oneerror budget one by one

• Error budget terms– Fitting error– WFS measurement error– Anisoplanatism– Temporal error

• Figures of merit– Strehl ratio– FWHM– Encircled energy– Strehl ratio

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Fitting error: dependence of Fitting error: dependence of Strehl on Strehl on λλ and DM degrees of and DM degrees of freedomfreedom

• Assume very bright natural guide star

• No meas’t error or anisoplan-atism or band-width error

Deformable mirror fitting error only

Strehl increases for smaller subapertures and shorter observing wavelengths

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Decreasing fitting error

• Assume very bright natural guide star

• No meas’t error or anisoplan-atism or band-width error

Deformable mirror fitting error only

Strehl increases for smaller Strehl increases for smaller subapertures and longer observing subapertures and longer observing wavelengthswavelengths

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Strehl increases for longer Strehl increases for longer λλ and better seeing (larger rand better seeing (larger r00))

Decreasing fitting error

• Assume very bright natural guide star

• No meas’t error or anisoplan-atism or band-width error

Deformable mirror fitting error only

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Wavefront sensor measurement Wavefront sensor measurement error: Strehl vs error: Strehl vs λλ and guide star and guide star magnitude magnitude

Assumes no DM fitting error or other error terms

But: SNR will decrease as you use more and more subapertures, because each one will gather less light

Strehl increases for brighter guide stars

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Strehl increases for brighter guide Strehl increases for brighter guide stars stars

Decreasing measurement error

Assumes no DM fitting error or other error terms

bright star

dim star

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Strehl vs Strehl vs λλ and guide star and guide star angular separation angular separation (anisoplanatism)(anisoplanatism)

Strehl increases for smaller angular offsets and longer observing

wavelengths

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Strehl increases for smaller Strehl increases for smaller angular offsets and longer angular offsets and longer observing wavelengthsobserving wavelengths

20 arcsec

10 arcsec

4 arcsec

0 arcsec

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Seeing limited

(TIP-TILT)

PSF with bright guide star: more PSF with bright guide star: more degrees of freedom ⇒ more energy degrees of freedom ⇒ more energy in corein core

Point Spread Function very bright star, λ = 2.2 μm, D / r0 = 8.5

1

0.1

0.01

0.001

0.0001

0 0.1 0.2 0.3 0.4 0.5

Radius (arcsec)

Pea

k in

tens

ity r

elat

ive

to d

iffra

ctio

n lim

it

uncorrected

218 DOF

50 DOF

24 DOF

12 DOF

2 DOF

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2.2 μ

1.65 μ

1.25 μ

0.88 μ

0.7 μ

uncorrected

What matters for spectroscopy What matters for spectroscopy is is ““Encircled EnergyEncircled Energy””

Fraction of light encircled within diameter of xx arc sec Fraction of light encircled within diameter of xx arc sec

Enc

ircle

d E

ner

gy F

ract

ion

Diffraction limited

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Overall system optimizationOverall system optimization

• Concept of error budget

– Independent contributions to wavefront error from many sources

• Minimize overall error with respect to a parameter such as integration time or subaperture size

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Error model: mean square wavefront Error model: mean square wavefront error is sum of squares of component error is sum of squares of component errorserrors

• Mean square error in wavefront phase

Meas’t Timelag Fitting Isoplan. Tip-tilt

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Signal to Noise Ratio for a fast Signal to Noise Ratio for a fast CCD detectorCCD detector

• Flux is the average photon flux (detected photons/sec)

• Tint is the integration time of the measurement,

• Sky background is due to OH lines and thermal emission

• Dark current is detector noise per sec even in absence of light (usually due to thermal effects)

• Read noise is due to the on-chip amplifier that reads out the charge after each exposure

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Short readout times needed for Short readout times needed for wavefront sensor ⇒ read noise is wavefront sensor ⇒ read noise is usually dominantusually dominant

• Read-noise dominated: read noise >> all other noise sources

• In this case SNR is

where Tint is the integration time, npix is the number of pixels in a subaperture, R is the read noise/px/frame

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Now, back to calculating Now, back to calculating measurement error for Shack-measurement error for Shack-Hartmann sensorHartmann sensor

• Assume the WFS is read-noise limited. Then

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Error model: mean square wavefront Error model: mean square wavefront error is sum of squares of component error is sum of squares of component errorserrors

Tcontrol is the closed-loop control timescale, typically ~ 10 times the integration time Tint (control loop gain isn’t unity, so must sample many times in order to converge)

Flux in a subaperture will increase with subap. area d2

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Integration time trades temporal Integration time trades temporal error against measurement errorerror against measurement error

From Hardy, Fig. 9.23

Measurement errorr0= 0.1 m

Temporal error1 / 0 = 39 Hz

Optimum integration time

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First exercise in optimization: First exercise in optimization: Choose optimum integration timeChoose optimum integration time• Minimize the sum of read-noise and temporal errors by finding optimal integration time

• Sanity check: optimum Tint larger for long τ0, larger read noise R, and lower photon Flux

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Similarly, subaperture size d Similarly, subaperture size d trades fitting error against trades fitting error against measurement errormeasurement error

• Smaller d: better fitting error, worse measurement error

Hardy, Figure 9.25

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Solve for optimum subaperture Solve for optimum subaperture size dsize d

dopt is larger if r0, read noise, and npix are larger, and if Tint and I are smaller

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Keck 2 AO error Keck 2 AO error budget examplebudget example

(bright TT star)(bright TT star)

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Summary: What can you optimize Summary: What can you optimize when?when?

• Once telescope is built on a particular site, you don’t have control over 0, θ0 , r0

• But when you build build your AO system, you CAN optimize choice of subaperture size d , maximum AO system speed, range of observing wavelengths, sky coverage, etc.

• Even when you are observing with an existingexisting AO system, you can optimize:

– wavelength of observations (changes fitting error)

– integration time of wavefront sensor Tint

– tip-tilt bandwidth

– brightness and angular offset of guide star

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How to be a savvy user and How to be a savvy user and consumer of AO systems? Topicsconsumer of AO systems? Topics

• What kinds of astronomy are helped by AO?

• For users of astronomical AO:

– How to plan your observations– What questions to ask when you get to the

telescope– Observing procedures

• For critical readers of AO papers in journals:– How to assess the reality of AO results reported

in the literature– Which data should you take seriously?– What are “danger signs” that should make you

doubtful?

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What kinds of observations will be What kinds of observations will be helped by AO? (1)helped by AO? (1)

• See details that were not previously present

– Qualitative: can make new morphological statements

– Quantitative: need to know Point Spread Function; need to understand PSF error bars

• Detect fainter objects/features

– Works for point sources

– But: IR AO systems inject more thermal background, because of many mirror surfaces. Also throughput to detector goes down.

– In astronomy, faint extended objects can actually be harder to see with AO. Limiting factor is sky background, and AO doesn’t improve this for extended objects.

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What kinds of observations will be What kinds of observations will be helped by AO? (2)helped by AO? (2)

• AO increases image contrast:

– Increased Strehl ratio ⇒ sharper edges, brighter features (if they are close to diffraction limit)

– Detecting faint things close to bright things:

» companions to bright stars

» host galaxies of quasars

» stellar and protoplanetary disks

– Caution: Contrast improvement may not be helped by AO for extended features, unless they have structure at λ / D

• AO permits more precise astrometry

– Can measure position of a point source more accurately if a) it is smaller, and b) it is brighter

– But need other stars in the field to create a reference frame

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See new details and structureSee new details and structure

• Structure is dramatically clearer

• But can be hard to measure quantitative brightness of features

– AO PSF “spills” light from bright features into fainter areas

Neptune, Keck, no AO Neptune, Keck, AO

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Spilling of light, Neptune bright Spilling of light, Neptune bright cloudsclouds

• Light from bright compact cloud region “spills” over limb, and into nearby dark areas

• How do you tell what the “real” intensity is, in a dark region?

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Will I detect fainter objects with Will I detect fainter objects with AO? (1)AO? (1)

• Assume a point source under Assume a point source under sky-background-limited sky-background-limited conditionsconditions. Total flux from object is Fobj (ergs/cm2 or

watts/m2).

• Generally choose size of pixel such that two pixels sample a typical point-source diameter. So within the area of the PSF, npix ~4

• The area of the PSF on the sky is ~ (2λ/D)2 for AO, but is ~ (2λ/r0)2

without AO

• So if all else is the same, the sky background Bsky within the PSF of a point source is (D/r0)2 larger

for the no-AO case

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Will I detect fainter objects with Will I detect fainter objects with AO? (2)AO? (2)

• Lick AO ( λ = 1.65 microns ): S = 0.4D = 3 m r0 = 0.6 m T ’ao / T’noao = 0.5

• At 1.65 microns, the sky background per arc sec is the same with and without AO, so

• If Strehl is < 0.3, AO doesn’t give sensitivity advantage even for point sources

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Galactic Center: NGS to LGS AOGalactic Center: NGS to LGS AO

Best NGS

Credit: Andrea Ghez’s group at UCLA

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Galactic Center: NGS to LGS AOGalactic Center: NGS to LGS AO

LGS

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Structure in extended objectsStructure in extended objects

• If goal is to see diffraction-limited structure, we need to achieve good signal to noise in each pixel

• Overall transmission T’AO ∝ D2

• AO pixels are usually diffraction-limited, so npix=aobj/apix= aobj/ (λ/D)2

• SNR with AO indep. of telescope diameter!

• The larger the object (aobj) the lower the SNR per pixel

• Can increase SNR by binning pixels

– If object is diffuse (aobj >> λ/D)

don’t need diffraction limited pixel sampling anyway

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AO yields higher contrast, for AO yields higher contrast, for small featuressmall features

• T. Rimmele

• AO for imaging surface of Sun

• Higher contrast on bright granules, dark regions in between where B field is emerging from sub-surface

AO off AO off

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Example of higher contrast: vision Example of higher contrast: vision science images of human retinascience images of human retina

• Austin Roorda and David Williams

Without AO With AO: resolve individual cones

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AO yields better contrast for faint AO yields better contrast for faint objects next to bright objectsobjects next to bright objects

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AO can permit more accurate AO can permit more accurate astrometry (precise position astrometry (precise position measurement)measurement)

• For a point source, accuracy of centroid measurement increases with intensity, decreases with image size

• AO helps both of these:

• But: need stars with known positions in field of view. AO field of view tends to be small

Binary stars areperfect for relativeastrometry

No AO

AO

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Questions? Discussion?Questions? Discussion?

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How to plan observations ahead How to plan observations ahead of timeof time

• First requirement: understand what Strehl ratio you will need for your science project to succeed

• Estimate exposure time needed to achieve good SNR– Some AO systems have exposure time calculators– Or check with folks who have observed in the past

• Refer to web pages to see what brightness guide star, at what distance, at what zenith angle, you will need

• Check AO system web page for maximum offset between science target and guide star

• Search star catalogues to find guide stars

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Star catalogs for guide star searchStar catalogs for guide star search(After B. MacIntosh)(After B. MacIntosh)

Incomplete near bright stars, funky close to big galaxies

Colors 20USNO B1.0

Unreliable (but good near bright stars), locally searchable in IRAF

None 15HST Guide Star

Very accurateColors11-12

Tycho 2

Very accurate, catalogue available as IDL file

Colors 9Hipparcos

Old catalogue, bright starsGood for quick PSFs

* Types

9SAO/PPM

NotesSpectral info?

Mag LimitCatalog

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Finding a guide star: ToolsFinding a guide star: Tools

• VizieR http://vizier.u-strasbg.fr/viz-bin/VizieR

– Has the ability to do constrained searches – limited in position, magnitude, etc. – from a list of input targets

– Results can be read into IDL or spreadsheet for sorting and processing

• Aladin (one of Vizier’s capabilities)

– http://aladin.u-strasbg.fr/java/nph-aladin.pl

– Can overplot a Digital Sky Survey image of your target with all the stars it can find from the USNO B1 catalogue. Very useful for finding guide stars.

– Gives B and R magnitudes of all USNO stars.

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• Aladin and USNO B1 catalog: virtues and pitfalls

• Great user interface, many surveys

• But gets confused near galaxies, nebulosity

• Check out potential guide stars by eye!

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Some observatories have their own online guide star tools

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Other questions to address prior Other questions to address prior to observing with AO systemto observing with AO system

• PSF calibrations

– What is my PSF star calibration strategy?

– What is the impact of anisoplanatism?

• Observing time

– Calculate exposure times needed for good SNR

– Have I accurately estimated AO’s overhead (wasted time)?

• Calibration and flat-fielding issues

– How will I calibrate the sky fluxes (offset skies, dithering, other?)

– How will I calibrate detector response variations?

– How will I calibrate photometry (brightness measurement)?

» Usually observe photometric standard stars

» How often? In what sequence?

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““PSF starsPSF stars””

• Before, after, and sometimes intermingled with observing science target, observe “PSF star”

• Constraints:– If science target is offset in angle from guide star,

can find PSF star pair with similar relative offset– Should be at ~ same zenith angle as science target

(but typically an hour or two earlier or later)– PSF star should produce same number of wavefront

sensor counts as guide star for your science target.

• In practice it’s hard to meet all these conditions

• With LGS, I typically end up using the tip-tilt star as PSF

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Sometimes you find creative Sometimes you find creative endeavors on the web (!)endeavors on the web (!)

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Laser guide star observing Laser guide star observing requires more preparationrequires more preparation

• US observatories have to submit target list to US Space Command (satellite avoidance) in advance– Not good form to destroy the detector on a

billion dollar satellite

• Specific formats required

• Check web pages for instructions

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Questions to ask when you get to Questions to ask when you get to the telescopethe telescope

• If possible, come a day early and watch the previous night’s observers use the AO system

• Ask for a “lesson” in how to control the AO system from the instrument interface

• Typically the AO system is calibrated each afternoon– Observatory staff will use an internal light

source to measure non-common-path errors every day (before observing)

– Instructive to watch this process if you’ve never seen it before

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Why we must calibrate for non-Why we must calibrate for non-common-path errorscommon-path errors

Schematic of Lick AO system (one generation ago)To near-IR camera

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Overview of the calibration Overview of the calibration process (usually done by process (usually done by observatory staff)observatory staff)

• Close dome, lights out, flatten the deformable mirror

• Turn on internal light source (e.g. optical fiber with diode or laser light)– Record centroid positions on wavefront

sensor– Record image of internal reference on

camera• Adjust deformable mirror shape until image of

internal reference has highest Strehl ratio• Record new positions of centroids on

wavefront sensor. These will be the “reference centroids” to which AO loop will control.

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AO tuning for your guide starAO tuning for your guide star

• Wavefront sensor camera frame-rate and AO control loop gain optimized for your guide star

– For fainter guide stars, want slower frame rate

– Typically need 100-200 counts per subaperture per wavefront sensor frame, for good AO performance

– For fainter stars, use lower control loop gain (lower bandwidth)

• AO operator will take a sky background measurement for the wavefront sensor

– Subtracted from each wavefront sensor frame

• Based on number of wavefront sensor counts, AO operator will run a program to optimize the AO system performance (trade frame rate against counts on wavefront sensor)

• Then offset from PSF star’s guide star to the PSF star itself, turn on AO system, take images or spectra

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Re-tuning the AO system during Re-tuning the AO system during the nightthe night

• When does operator re-tune AO system?– At each new telescope pointing– When background changes (clouds, moon)– When flexure changes (after slew, long

integrations)– Whenever observer requests an updated

tune-up

• I usually keep an eye on the number of wavefront sensor counts per subaperture– When it drops considerably below its original

value, ask for a re-tuning

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Other observing procedures are Other observing procedures are same as for any infra-red same as for any infra-red observationsobservations

• Take sky backgrounds– Necessary in IR: science target can be

dimmer than the sky background!– Can nod to sky so that your science target

is entirely off the detector, or– If your science target is small enough, can

get sky bkgnd just from dithering target on detector

• Observe photometric standard stars several times during the night (if needed)

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Other observing procedures are Other observing procedures are same as for any infra-red same as for any infra-red observationsobservations

• Dithering and nodding:

• IR array non-uniformity requires sky measurement and subtraction

• To obtain a sky subtraction, usually need a multiposition dither (1-2-3-4 etc.)– If your science target is

big, good to get a separate sky too

5 (sky)

1 2

3 4

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Questions? Discussion?Questions? Discussion?

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How to assess the reality of AO How to assess the reality of AO results reported in the literatureresults reported in the literature

• Which data should you take seriously?

• What are “danger signs” that should make you doubtful?

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Taking data seriously: Three big Taking data seriously: Three big issuesissues

1. Strehl ratio and variability

2. Effect of using a non-point-source as a guide star or tip-tilt star

3. Signal to noise ratio

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Taking data seriously: Three big Taking data seriously: Three big issuesissues

1) Strehl:– Don’t trust low-

Strehl results

– How low is low? My rule of thumb: “low” is S < 10%

– Problems: unstable photometry, variable PSFs

Credit: J. Christou et al.

Higher Strehl ratios are more stable

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Big issues, continuedBig issues, continued

2) Finite-size object used as “guide star”– Frequently produces artifacts on point

spread function– Sometimes “double-star” PSF– Look for independent measurement of PSF if

possible

– Also there can be issues with using finite-sized object as tip-tilt star

» Most important example: using bright nucleus of a galaxy as the tip-tilt reference

» The more point-like it is, the better» No firm rules here about what to do – try it!

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Big issues, continuedBig issues, continued

3) Signal to noise ratio of AO image or spectrum– Rules of thumb (Hardy):

» SNR needed to recognize an object in a noisy background: SNR > 5

» SNR needed for spectroscopy is much larger: people use numbers like 20, 50, 100 per resolution element (depends on the application)

Be sure to look carefully at section of published paper where SNR is discussed.– If it isn’t discussed, try to estimate it

yourself.

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““Journal of Irreproducible ResultsJournal of Irreproducible Results””

• Danger signs:

• Low Strehl ratios (e.g. 5% - 15%)

• Use of an extended source as a “natural guide star” – Can give PSFs that are double, or that have

several lumps

• Use of a “guide star” that IS a point source, but that is embedded in a fuzzy region– Also can give odd PSFs

• Look for repeatable independent measurements of PSF

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Radio galaxy 3C294 seen with UH Radio galaxy 3C294 seen with UH AO systemAO system

Diffraction spike from guide star (a double star?)

Stockton et al.UH AO SystemCFHT Telescope

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3C294 images from Hubble, Keck 3C294 images from Hubble, Keck AOAO

Hubble (0.7 micron)

Keck AO (1.6 microns):Bright point-like core, plus fuzz on right

From Wim de Vries. LLNL

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Example of dangers from Example of dangers from extended guide star: Frosty Leo extended guide star: Frosty Leo nebulanebula

• UH AO system

• Closed AO loop on one of the big blue blobs

• Concluded central star is double

• Not confirmed by subsequent observations

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ESOESO’’s VLT MACAO Observations of s VLT MACAO Observations of Frosty LeoFrosty Leo

• Is it a binary star?

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ConclusionsConclusions

• AO systems can yield flakey results if:– Guide star is extended, or too faint– Strehl is too low or too variable

• Need good signal to noise (but that is no different from “regular” observations)

• Need thoughtful preparation before an observing run

• But…. RESULT CAN BE WORTH THE TROUBLE!