The Art of Seeing Management

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


    Ron Wodaski

    Advanced Imaging Conference 2005

  • OverviewOverview

    Defining Terms

    Evolving Perceptions

    The Nature of Seeing

    Observatory and Site Improvements

  • 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

  • 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.

  • 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

  • Seeing CausesSeeing CausesWavefront ErrorsWavefront Errors

    Phase distortion (arrival time)



    phase distortion + diffraction effects

    also called amplitude distortion, scintillation

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

  • Wavefront error demonstration

  • 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

  • Evolving PerceptionsEvolving Perceptions

    The History of Seeing

    70s: Uh-oh

    80s: Seeing the problem

    90s: Quantifying the problem

    Today: Solving the problem

  • 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

  • 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

  • 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

  • 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)

  • 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

  • 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)

  • 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 doesnt 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

  • NCEP Reanalysis data provided by the NOAA-CIRES ESRL/PSD Climate Diagnostics branch, Boulder, Colorado, USA, from their Web site at:

    Wind speed at 200 millibarspredicts high-altitude seeing:

    (Contribution of near seeing effects omitted.)

    32291912Wind, 200mb


  • 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)

  • 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


    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

  • 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

  • Controlling Near SeeingControlling Near Seeing

    Site improvements

    Observatory improvements

    Telescope improvements

  • Site Improvements: 1Site Improvements: 1

    Elevate telescope above ground

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



    Heat sources (e.g., concrete walkways)

    Locate observatory on highest available ground

  • 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

  • Site Improvements: 3Site Improvements: 3

    Determine primary wind direction(s)

    Use wind rose built from RAWS data

    Place observatory for best wind flow

    Orient observatory for least wind resistance

    Wind rose

    RAWS data

  • 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)

  • AEOS Telescope Thermal Conditioning

  • 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

  • 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

  • Wind Loading: 1Wind Loading: 1

  • Wind Loading: 2Wind Loading: 2

  • 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