The Art of Seeing Management
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Transcript of 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
ManagementManagement
Ron Wodaski
Advanced Imaging Conference 2005
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OverviewOverview
• Defining Terms
• Evolving Perceptions
• The Nature of Seeing
• Observatory and Site Improvements
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• 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
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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.
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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
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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.
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Wavefront error demonstration…
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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
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Evolving PerceptionsEvolving Perceptions
The History of Seeing
• 70s: Uh-oh
• 80s: Seeing the problem
• 90s: Quantifying the problem
• Today: Solving the problem
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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
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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
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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
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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)
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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
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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)
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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
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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
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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)
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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
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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
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Controlling Near SeeingControlling Near Seeing
• Site improvements
• Observatory improvements
• Telescope improvements
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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
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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
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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
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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)
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AEOS Telescope – Thermal Conditioning
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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
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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
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Wind Loading: 1Wind Loading: 1
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Wind Loading: 2Wind Loading: 2
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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