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You Can't You Can't Image What Image What You Can't See You Can't See The Art of Seeing The Art of Seeing Management Management Ron Wodaski Advanced Imaging Conference 2005

The Art of Seeing Management

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Page 1: The Art of Seeing Management

You Can't You Can't Image What Image What You Can't SeeYou Can't SeeThe Art of Seeing The Art of Seeing

ManagementManagement

Ron Wodaski

Advanced Imaging Conference 2005

Page 2: The Art of Seeing Management

OverviewOverview

• Defining Terms

• Evolving Perceptions

• The Nature of Seeing

• Observatory and Site Improvements

Page 3: The Art of Seeing Management

• Seeing components:– Image motion

– Scintillation (speckle patterns)

• The causal chain that creates seeing:– Atmospheric turbulence

– Mixing of layers withdifferent temperatures

– Variable refractive indices

– Shake it all about…

Seeing DefinedSeeing Defined

Freeze Frame

Page 4: The Art of Seeing Management

Mixing It UpMixing It Up

• Vertical mixing is always bad – it creates seeing problems by bringing air masses of different temperature into contact

• Horizontal movement of air transports seeing problems, moving them in front of your optics like a bad movie.

Page 5: The Art of Seeing Management

Wind Is Your FriendWind Is Your Friend

• Observatories were built to block wind

• But: Wind breaks up convection, improving seeing

• A laminar 30mph wind provides awesome seeing (but only if your telescope can handle the wind loading!)

New style

Old style

Page 6: The Art of Seeing Management

Seeing CausesSeeing CausesWavefront ErrorsWavefront Errors

• Phase distortion (arrival time)

• Tilt

• Breakup

–phase distortion + diffraction effects

–also called amplitude distortion, scintillation

• The sum of the angular position errors fatten up the stars.

Page 7: The Art of Seeing Management

Wavefront error demonstration…

Page 8: The Art of Seeing Management

Fixing the SeeingFixing the Seeing

• Adaptive Optics– Uses some form of wavefront sensing to measure the wavefront error

– Bend the mirror to match the wavefront

• Image reconstruction– Speckle interferometry

– Lucky imaging

– Aperture synthesis (interferometry using multiple instruments)

• Make the air stand still

Page 9: The Art of Seeing Management

Evolving PerceptionsEvolving Perceptions

The History of Seeing

• 70s: Uh-oh

• 80s: Seeing the problem

• 90s: Quantifying the problem

• Today: Solving the problem

Page 10: The Art of Seeing Management

70s: What did we do wrong?70s: What did we do wrong?

• Big telescopes ruled the Earth

• Seeing problems were worse than expected

–Expected optical quality not achieved

–Expected resolution not achieved

• Initially, minimal research conducted to identify source of problems

Page 11: The Art of Seeing Management

80s: Seeing the Problem80s: Seeing the Problem

• Empirical discoveries about the sources of the problem

–Convection effects inside the dome

–Weak mixing a problem (long persistence of thermal effects)

–Conjecture about roles of:

• Dome/outside boundary

•Mirror/air boundary

Page 12: The Art of Seeing Management

90s: Quantifying the Problem90s: Quantifying the Problem

• Greatest fluctuations occur near heat sources

• Greatest velocity of flow occurs away from heat sources (can create time-dependent seeing problems if it intersects with another heat source, by moving the problem into the optical path)

• Varying emissivity can create temperature differences without a heat source

Page 13: The Art of Seeing Management

Heat/Turbulence SourcesHeat/Turbulence Sources• Convective airflow from mirror/air temperature difference. Roughly 0.25 to 0.5” worsening of seeing per degree Celsius of mirror/air delta T (0.38 arcsec/°C6/5)

• Convective airflow from dome interior to exterior (0.1 arcsec/°C6/5)

• Convective flow from instrument waste heat• Airflow via doors from heat sources• Turbulence across the slit• Heat retained/released by structural elements• Convective airflow from floor, dome, and walls• Disturbance of stationary temperature boundaries (inversions)

Page 14: The Art of Seeing Management

Today: Solving the ProblemToday: Solving the Problem• Ventilation equalizes temperature

– Mix air between spaces of different temperature before observing

– Move massive amounts of air in/out of observatory

• Remove waste heat– Water cooling– Ducting and fans

• Match emissivities– Aluminum ideal (tape, plates, dome, etc.

• Fast-moving air breaks up convection– Perfect for mirror seeing– Can be done while imaging– Wind is your friend, if your setup can handle the loading

Page 15: The Art of Seeing Management

The Nature of SeeingThe Nature of Seeing

• High Altitude Seeing (minor)–Affected by latitude

–Predicted using wind velocity at tropopause (V200mb)

• Near Seeing (major)–Varies with elevation above sea level

–Greatest effects near ground

–Many sources controllable

• Dome/Instrument Seeing (critical)

Page 16: The Art of Seeing Management

Latitude and SeeingLatitude and Seeing• “Jet Stream” seeing a minor componement of total seeing

• Proportional to the wind velocity at pressure of 200mb

• Velocity is seasonal• The jet stream doesn’t increase turbulence; it makes turbulence move past you faster (shorter coherence time)

Typical best summer velocities:Lat 10: 8 m s-1

Lat 20: 10 m s-1

Lat 30: 15 m s-1

Lat 40: 18 m s-1

Typical worst winter velocities:Lat 10: 12 m s-1

Lat 20: 25 m s-1

Lat 30: 33 m s-1

Lat 40: 38 m s-1

Page 17: The Art of Seeing Management

NCEP Reanalysis data provided by the NOAA-CIRES ESRL/PSD Climate Diagnostics branch, Boulder, Colorado, USA, from their Web site at: http://www.cdc.noaa.gov/cdc/data.ncep.reanalysis.derived.html

Wind speed at 200 millibarspredicts high-altitude seeing:

(Contribution of near seeing effects omitted.)

32291912Wind, 200mb

0.850.750.550.45Seeing

Page 18: The Art of Seeing Management

Altitude and ElevationAltitude and Elevation• Altitude:

– Height above sea level

• Elevation:– Height above local ground level

• Surface layer:– The air close to the ground disturbed by interaction with ground, trees, buildings, etc.

• Boundary layer:– Height at which pressure is 200 millibars

• Free Air:– Between surface and boundary layers. Typically contributes from 0.5” to 0.1” to total seeing (altitudes from 1000 to 10000 meters)

Page 19: The Art of Seeing Management

Altitude and SeeingAltitude and Seeing• The facts:

– Density changes cause refraction

– At higher altitudes, air is less dense– Optical path errors are proportional to the density of the

medium

• The results:– Higher altitudes provide better seeing even under otherwise

adverse conditions.

– For an otherwise well-chosen site, altitude alone is a very good predictor of seeing quality.

• Note: Higher latitudes have lower air pressure, with similar results (approx. 25% at the poles)

• Local conditions typical of high-alitude sites may interfere:– Anabatic (upslope) winds– Katabatic (downslope) winds

Page 20: The Art of Seeing Management

Surface LayerSurface Layer

• Surface layer dominates seeing– Most disturbed layer

– Close to telescope

• Surface layer a factor below 10 meters

• Tall telescope enclosures provide better return than building on top of mountains

• A change from 4m to 10m elevation is same as placing telescope at 4200m!*

* Altitude, Elevation, and Seeing. Rene Racine, Publications of the Astronomical Society of the Pacific, Vol 117, No 830 p407

Page 21: The Art of Seeing Management

Controlling Near SeeingControlling Near Seeing

• Site improvements

• Observatory improvements

• Telescope improvements

Page 22: The Art of Seeing Management

Site Improvements: 1Site Improvements: 1

• Elevate telescope above ground

• Remove, or locate away from, sources of ground-effect seeing:

–Trees

–Buildings

–Heat sources (e.g., concrete walkways)

• Locate observatory on highest available ground

Page 23: The Art of Seeing Management

Site Improvements: 2Site Improvements: 2

• Explore nature of seasonal air flows around the observatory

– Cool air runs down hill

– Hill crowns, ridges, and mountain crests generate cool downflows at night

– Look for downflow turbulence sources (narrowing of valley; obstructions)

– Estimate (or measure) depth of downflowat/near your observatory

• Know the weather at all times

Page 24: The Art of Seeing Management

Site Improvements: 3Site Improvements: 3

• Determine primary wind direction(s)

– Use wind rose built from RAWS datahttp://www.wrcc.dri.edu/wraws/

– Place observatory for best wind flow

– Orient observatory for least wind resistance

Wind rose

RAWS data

Page 25: The Art of Seeing Management

Observatory ImprovementsObservatory Improvements• Identify heat sources so you can evaluate success of improvements

• Pre-observing dome flushing

• Active horizontal flushing at or slightly above mirror height while observing

• Dedicated fans and/or ducting for dealing with waste heat from primary heat sources

• Adequate distance between dome slit and telescope– Slit turbulence abatement

– Avoid wind excitation of tube, truss, or secondary assembly (resonance modes)

Page 26: The Art of Seeing Management

AEOS Telescope – Thermal Conditioning

Page 27: The Art of Seeing Management

Telescope Improvements: 1Telescope Improvements: 1

• Mirror seeing problems are the primary seeing issue

– Problem caused by convection resulting from the difference in temperature between mirror and air

– Air over the mirror surface will break up convection cells

• Simple fan(s)

• Air knife

• Ideal: Radial toward center with suction at center

Page 28: The Art of Seeing Management

Telescope Improvements: 2Telescope Improvements: 2

• Wind loading often an issue

–Excitation of truss secondary creates seeing-like problems

• Pistoning (changes focus)

•Wobble (makes image move)

• Causes of wind loading:

–Slit turbulence

–More direct wind than mount/truss/tube can handle

Page 29: The Art of Seeing Management

Wind Loading: 1Wind Loading: 1

Page 30: The Art of Seeing Management

Wind Loading: 2Wind Loading: 2

Page 31: The Art of Seeing Management

Image Processing TipsImage Processing Tips• Deconvolution reduces star sizes and improves detail– Can make things worse if S/N not good enough

• Curves on stars will work on any image to reduce blurry/fat stars– Star selection with Color Range– Enlarge/feather selection

• Expand by 4-8 pixels (image scale)• Feather by half expansion

– Make layer via copy– Make additional layers via cut if stars are on backgrounds of different brightness

See Zone System for Astro Imaging for details