Astr 3130 Objectives
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Understand sky motions. Target accessibility vs. time of year.
Coordinate systems → Pointing a telescope/scheduling a program
Follow a photon from outside the atmosphere to a scientific paper.
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Atmospheric transmission, turbulence, refraction
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Image formation by a telescope. Limitations to image quality –
diffraction, seeing, aberration.
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Detectors and capturing photons, conversion of photon signals to
numbers, image representation
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Observing techniques
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Image math and calibration
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Photometric and astrometric extraction of point sources
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Introduction to basic astronomy research tools
Quantitative analysis of results and associated statistics.
Write up lab results and develop professional skills (proposal writing)
Astr 3130 Grade Breakdown
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Observing Notebook – 15%
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Midterm – 15%
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Lab Prep / Day Assignments – 20%
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Labs – 40%
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Final Assignment – 10%
The ASTR 3130 Iceberg
Astr 3130 Time Investments
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Class – Tuesdays and Thursdays 11:00-12:15
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Lab sections – Tuesday or Wednesday 9:00-11:00
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Or as otherwise arranged
Alternative times driven by weather or extended field trips
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e.g. Fan Mountain visit/observation
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“Day” problems
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Lab-prep quizzes
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Observation, data reduction, and report writing
Astr 3130 Time Investments
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Class – Tuesdays and Thursdays 11:00-12:15
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Lab sections – Tuesday or Wednesday 9:00-11:00
–
●
Or as otherwise arranged
Alternative times driven by weather or extended field trips
–
e.g. Fan Mountain visit/observation
●
“Day” problems
●
Lab-prep quizzes
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Observation, data reduction, and report writing
Teamwork vs. Individual Work
Astr3130 – o - Meter
Hardest
course you'll
ever love
(Majewski odd years)
Why are they
doing this to
me????
(this semester?)
Astr 3130 Tools
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Clear Sky Chart
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SAOimage ds9
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XEphem
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Aperture Photometry Tool
The Celestial Sphere
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From our perspective on Earth the stars appear embedded on a
distant 2-dimensional surface – the Celestial Sphere.
The Celestial Sphere
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Although we know better, it is helpful to use this construct to think
about how we see the night sky from Earth.
The Sphere turns, the Earth stays fixed.
Spheres
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Some terminology
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Great circle vs. small circle
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For a rotating sphere there are
two well defined “pivots” - the
poles and a fundamental plane
perpendicular to the polar axis
through the center of the
sphere.
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Note that the circumference of
a small circle is cos(δ) smaller
than the great circle.
The fundamental plane defines
a great circle – the equator.
Meridians are great circles that
intersect the equator at 90
degrees
δ
Coordinates on Spheres
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Two angles define a location on a spherical surface.
A prime meridian establishes the zeropoint of longitude
Longitudes increase positively in the direction opposite the planet's spin.
Local Perspective: Altitude, Azimuth, and Zenith
Altitude
Local (Altitude/Azimuth) coordinates
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Azimuth measured positively toward the East from North
Altitude measured up from the Horizon plane
A Personal Perspective: Horizon and Zenith
Horizon
A Personal Perspective: Horizon and Zenith
Reference Points on the Celestial Sphere
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Extend the Earth's poles and equator onto the sky and you have
defined the celestial poles and celestial equator.
The Celestial Poles
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The “North Celestial Pole” lies overhead for an observer at the
North Pole and on the horizon for an observer on the Equator
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The altitude of the pole equals your latitude.
The Celestial Poles
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The “North Celestial Pole” lies overhead for an observer at the
North Pole and on the horizon for an observer on the Equator
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The altitude of the pole equals your latitude.
To Pole
John Dobson 1915 - 2014
The Celestial Poles
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The “North Celestial Pole” lies overhead for an observer at the
North Pole and on the horizon for an observer on the Equator
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The altitude of the pole equals your latitude.
Why is This Important?
As an observer you need to develop a comfortable “feel” for the accessible sky.
Where are your sources? Which are setting???
Which are too “low” for reasonable observation?
Will the Sun or Moon interfere?
For ground based observation the Earth is your spacecraft. It enforces hard limits on
your observations.
In space (e.g. Hubble or Spitzer) there will be Sun and/or Earth avoidance constraints.
Angle c is the altitude
Angle b is the zenith angle, z
1
Airmass =
= sec( z )
cos( z )
For a plane parallel atmosphere
(not what is pictured at left, but a
good approximation for the small
zenith angles most astronomers
care about).
The Celestial Poles
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The “North Celestial Pole” lies overhead for an observer at the
North Pole and on the horizon for an observer on the Equator
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The altitude of the pole equals your latitude.
The Celestial Poles
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The rotating Earth makes it look like the Celestial Sphere is
spinning about the celestial poles.
http://www.atscope.com.au/BRO/warpedsky.html
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In the Southern Hemisphere
there is no good pole star (at
present).
Note that there are some
stars (near the pole) that
never set below the horizon
- “Circumpolar Stars”
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For an observer at the
North or South pole
every star is circumpolar.
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At the Equator there are
no circumpolar stars
Given the altitude of the
pole, circumpolar stars have
declinations between 90 and
90-lat degrees.
Polaris
The Celestial Equator in the Sky
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The Celestial Equator is the locus of all points lying 90 degrees
from the celestial pole.
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It is a great circle around the celestial sphere.
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Since the celestial sphere “turns” around the poles. The celestial
equator is a fixed reference line in the sky (rotating over itself).
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The celestial equator runs from the horizon due east, up in the sky
(90-lat) degrees and back down to the horizon due west.
Stars “above” the celestial equator have positive declination (at least as
seen from Charlottesville).
The Meridian
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Every line of celestial longitude is a meridian of longitude, but we
recognize the line of longitude, or simply the great circle line,
running overhead as “THE” meridian.
Locating Stars on the Celestial Sphere
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Just like geographical coordinates on
the Earth each star has a celestial
address.
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This address is impermanent because
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Stars move steadily as they randomly
drift in the Galaxy.
The coordinate system (tied to the Earth)
shifts as the Earth precesses like a top.
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Precession is slow (26,000
years/cycle) but even over a decade
its effects are significant.
Coordinates are the analog of latitude and longitude, called
Declination and Right Ascension respectively.
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Declination is straightfoward and is simply the angular distance a
star lies above or below the celestial equator measured in degrees.
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The north celestial pole is at a declination of +90 degrees
The declination of the bright star Vega is +38:47:01.9 (at least in the
year 2000 it was – more on that later), so +dd:mm:ss.s in general.
Celestial Motion at Different Declination
Right Ascension
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Right Ascension (longitude) is trickier
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If you point your finger at a particular Declination the declination value
remains unchanged, but Right Ascension ticks away as the sky (actually
the Earth) rotates.
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Right Ascension is thus naturally measured in units of time – hh:mm:ss.s
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One hour of right ascension is 15 degrees of celestial longitude (not 15
angular degrees, except at the equator)
The sky rotates by at 15 arcseconds per second at the Equator
Since lines of RA converge toward the pole – 1 minute of RA spans a
different angle depending on Declination – a factor of cos(Dec) comes into
play.
Right Ascension/Longitude needs an arbitrary zeropoint (Greenwich
on Earth, the “First Point of Aries” on the sky).
This reference point is the intersection celestial equator and ecliptic at
of the location of the Sun at the Spring Equinox.
The Sun and the Celestial Sphere
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As the Earth orbits the Sun we seen the Sun in different locations
against the backdrop of stars.
The Earth reaches the same location in its orbit on the same
calendar date each year.
The Sun and the Celestial Sphere
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Said another way, the Sun finds itself fixed at a different location
on the celestial sphere each day (more or less) – as a result on
that day it behaves like any other given star, following a path
dictated by the rotation of the Earth.
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The Sun does not lie on the
celestial equator but
follows a path inclined by
23 ½ degrees.
The path crosses the
celestial equator at 2 points
(the vernal and autumnal
equinox) marking the
instant of the beginning of
Spring and Fall
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The intersection point
depends on the absolute
direction of the Earth's
rotation axis. Since this
axis precesses the
reference point drifts
around the sky.
The Precession of the Equinox
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The Sun does not lie on the
celestial equator but
follows a path inclined by
23 ½ degrees.
The path crosses the
celestial equator at 2 points
(the vernal and autumnal
equinox) marking the
instant of the beginning of
Spring and Fall
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The intersection point
depends on the absolute
direction of the Earth's
rotation axis. Since this
axis precesses the
reference point drifts
around the sky.
Celestial Motion at Different Declination
Celestial Motion at Different Declination
Convergence of Longitude at the Pole
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On Earth one degree of latitude (equivalent of declination) is
111.3 km at any latitude.
One degree of longitude is 111.3km * cos(latitude)
A minute of Right Ascension is 15 minutes of arc at the equator, but a smaller angle at
higher latitudes.
Equatorial Telescope Mounting