Comet Ephemerides with Geometry and Visibility Info
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Transcript of Comet Ephemerides with Geometry and Visibility Info
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Comet Ephemerides with Geometry and Visibility
Info
by Steve Albers
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Ephemeris Output● DEC, RA, Distance from Sun and Earth
● Magnitude, Tail Length, Tail orientation
● Phase Angle, Elongation
● Rise/Set Times, Time/Altitude for best
viewing
● Visibility compared to naked eye, binocular
thresholds
○ expressed as “effective” or corrected
magnitude
● Output computed daily or more frequently
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Visibility Computation
● Effective Magnitude Adjusted from Actual
Magnitude
○ compare to naked eye 6.0 threshold
● Effective Magnitude Adjustments
○ extinction
○ sky brightness (moonlight, twilight, daylight
at best observation time)
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Sky Brightness Computation
● Nighttime
○ moderate light pollution assumed,
increasing near horizon
○ increased by scattering/glare from the moon
● Twilight
○ empirical relationship of limiting magnitude
and comet/sun altitude difference
● Daytime
○ scattering/glare based on elongation from
sun, comet altitude and solar altitude
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Internet Demo?
http://laps.noaa.gov/cgi/albers.homepage.cgi
(optional)
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Visibility Examples● PANSTAARS reaching (barely) naked eye
magnitude limit in March
● Ikeya-Seki visible naked eye in Japan within
hours of perihelion, otherwise head was likely
invisible naked-eye (despite impressive tail)
● ISON orbit similar to Ikeya-Seki, though will be
dimmer and may never reach naked-eye
brightness
● Hale-Bopp longest most visible comet in last
50 years
● McNaught (2007) visible naked eye from
Longmont area in bright twilight, and in
binoculars during the daylight
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Sky Brightness Computation
● It’s fine to calculate sky brightness and
limiting magnitude at individual points in the
sky for an ephemeris, however…
● Wouldn’t it be nice to show an image of sky
brightness over the entire sky??
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Simulated All-Sky Images Compared with the LAS All-Sky
Camera
by Steve Albers, Vern Raben, and the NOAA LAPS Group
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Simulation Ingredients
● 3-D Gridded Cloud Analyses (or
forecasts)
○ Cloud liquid, ice, rain, snow,hail
○ NOAA’s LAPS model (developed at
ESRL)
● Locations of Sun, Moon, Planets, Stars
● Specification of Aerosols (haze)
● Specification of Light Pollution
● Specify Vantage Point
○ Latitude, Longitude, Elevation
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LAPS Cloud
analysis
METARMETAR
METAR
OAR/ESRL/GSD/Forecast Applications Branch 10
First Guess
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Visualization Technique
● Illumination of clouds, air, and terrain are pre-
computed
● Sky brightness based on sun, moon, planets,
stars
● Ray Tracing from Vantage Point to each sky
location
● Scattering by Intervening Clouds, Aerosols, Air
● Terrain shown where its along the line of sight
● Physically and Empirically based for best
efficiency
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Image Navigation● Overall correction based on optical axis centering,
spherical rotation, and radial lens “distortion”
• Need to rotate around Lambda Draconis?• Except that near horizon offsets are just in azimuth
(zenith rotation)
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Cloud Illumination Example
Cloud Illumination (and scattering)
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Nighttime Clouds (and stars)
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Background Sky Brightness
● Source can be sun or moon
● Rayleigh Scattering by Air Molecules (blue sky)
○ Minimum brightness 90 degrees from light
source
○ Blue-Green sky color near horizon far from
sun
● Mie Scattering by Aerosols (haze)
○ Brighter near the light source (aureole)
● Added sky brightness from planets, stars, light
pollution, airglow
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Daylight Clear Sky
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Nighttime Comparison
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Clear Air Illumination● Cloud shadows in clear air can show
crepuscular rays
● Brightness and color changes shown during
twilight
○ 3-D orientation of Earth’s shadow considered
○ Secondary scattering needed to reduce
contrast in Earth’s shadow that appears
opposite the sun
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Twilight Comparisons
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Twilight Comparisons
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Terrain Illumination
● Topography data allows showing mountains
near the horizon
● Terrain Albedo (e.g. a dark forest)
● Adjusted by cloud shadows
● Show snow cover (future enhancement)
● Terrain can be obscured by intervening clouds,
haze, or clear air (very long distances)
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Main Ray-Tracing Step● Trace from viewer into sky at ~1x1 degree grid
● Ray path travels through clear air, aerosols,
clouds, and may hit terrain
● First estimate is clear sky value (background
sky)
● Scattered by clouds (can show up either bright
or dark)
○ depends on optical depth of cloud and
elongation from sun, as well as pre-computed
cloud illumination
● Cloud/Aerosol scattering can obscure distant
terrain
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More on Cloud/Precip Scattering● Mie scattering phase function means thin clouds
are brighter near the sun (silver lining), cloud
corona
● Thick clouds are the opposite, being lit up better
when opposite the sun
● Rayleigh scattering by clear air can redden
distant clouds
● Future enhancement would be to add rainbows &
halos
○ (with clouds/precip at specific elongation
angles)
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Final Display
● Cylindrical grid (panoramic view) can be
calculated at either 1x1 or 0.5x0.5 degree
spacing
● Currently just shows at and above the horizon
○ future enhancement to show below the
horizon
● Convert to polar grid (shown here)
○ good for overhead views, and for camera
comparison
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Cylindrical Panoramic View (½ degree resolution)
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Example Animation #1
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Example Animation #2
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Internet Demo of All-sky Web page?
(optional)
http://laps.noaa.gov/allsky/allsky.cgi
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What’s next?
The sky is the limit!