Comet Ephemerides with Geometry and Visibility Info
description
Transcript of Comet Ephemerides with Geometry and Visibility Info
Comet Ephemerides with Geometry and Visibility
Info
by Steve Albers
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
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)
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
Internet Demo?
http://laps.noaa.gov/cgi/albers.homepage.cgi
(optional)
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
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??
Simulated All-Sky Images Compared with the LAS All-Sky
Camera
by Steve Albers, Vern Raben, and the NOAA LAPS Group
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
LAPS Cloud
analysis
METARMETAR
METAR
OAR/ESRL/GSD/Forecast Applications Branch 10
First Guess
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
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)
Cloud Illumination Example
Cloud Illumination (and scattering)
Nighttime Clouds (and stars)
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
Daylight Clear Sky
Nighttime Comparison
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
Twilight Comparisons
Twilight Comparisons
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)
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
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)
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
Cylindrical Panoramic View (½ degree resolution)
Example Animation #1
Example Animation #2
Internet Demo of All-sky Web page?
(optional)
http://laps.noaa.gov/allsky/allsky.cgi
What’s next?
The sky is the limit!