Astronomy Basics

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Astronomy Basics

• Where is it?
• Angular positions in the sky
• How far is it?
• How to see it?
• Telescopes

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Positions on the Celestial Sphere

• How to locate (and track) objects from a
  spinning, orbiting platform in space.

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Positions on the Celestial SphereThe
Altitude-Azimuth Coordinate System

• Coordinate system based on observers local
  horizon
• Zenith - point directly above the observer
• North - direction to north celestial pole NCP
  projected onto the plane tangent to the earth at
  the observers location
• h altitude - angle measured from the horizon to
  the object along a great circle that passes the
  object and the zenith
• z zenith distance - is the angle measured from
  the zenith to the object zh90?
• A azimuth - is the angle measured along the
  horizon eastward from north to the great circle
  used for the measure of the altitude

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Changes in the Sky

• Coordinates continuously changing in alt-az
  system for all celestial objects (except
  geo-stationary satellites)
• Earths rotation
• Earths orbit about Sun
• Proper motion of objects
• The moon
• Planets
• Asteroids
• Comets
• Satellites.

Milky Way From Frisco Peak. Paul Ricketts ?
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Motion of Stars about NCP
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Earths Rotation

• Earths rotation is responsible for the rapid
  motion of objects through the sky

Mud springs point 2 hour exposure of NCP. Paul
Ricketts?
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Tilt of Earths Axis

• Position of sun, moon and planets on celestial
  sphere significantly influenced by the tilt of
  Earths axis.
• Stars far enough away that seasonal variation of
  position on celestial sphere not significantly
  influenced by the tilt of Earths axis
• On timescale of thousands of years, however,
  position of even stars move on celestial sphere
  due to precession!!!!

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Earths Orbit about the Sun

• Due to the Earths motion about the Sun
• Line of sight to the sun sweeps through the
  constellations. The sun apparently moves through
  the constellations of the zodiac along a path
  known as the Ecliptic
• The constellations that are visible each night at
  the same time changes with the season
• A given star will rise approximately 4 minutes
  earlier each day

The Ecliptic.The path of the sun through the year
in equatorial coordinates.
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Equatorial Coordinate System

• Coordinate system that results in nearly constant
  values for the positions of distant celestial
  objects.
• Based on latitude-longitude coordinate system for
  the Earth.
• Declination - coordinate on celestial sphere
  analogous to latitude and is measured in degrees
  north or south of the celestial equator
• Right Ascension - coordinate on celestial sphere
  analogous to longitude and is measured eastward
  along the celestial equator from the vernal
  equinox ? to its intersection with the objects
  hour circle

Hour circle
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Positions on the Celestial SphereThe Equatorial
Coordinate System

• Hour Angle - The angle between a celestial
  objects hour circle and the observers meridian,
  measured in the direction of the objects motion
  around the celestial sphere.
• Local Sidereal Time(LST) - the amount of time
  that has elapsed since the vernal equinox has
  last traversed the meridian.
• Right Ascension is typically measured in units of
  hours, minutes and seconds. 24 hours of RA would
  be equivalent to 360?.
• Can tell your LST by using the known RA of an
  object on observers meridian

Hour circle
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What is a day?
The period (sidereal) of earths revolution about
the sun is 365.26 solar days. The earth moves
about 1? around its orbit in 24 hours.

• Solar day
• Is defined as an average interval of 24 hours
  between meridian crossings of the Sun.
• The earth actually rotates about its axis by
  nearly 361? in one solar day.
• Sidereal day
• Time between consecutive meridian crossings of a
  given star. The earth rotates exactly 360? w.r.t
  the background stars in one sidereal day 23h
  56m 4s

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Annalemma

• Mean (average) solar day is 24 hours
• Equation of time

Position of sun at noon
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Local Sidereal Time

• LST 100.46 0.985647 d long 15UT
• d is the days from J2000, including the
  fraction of
• a day
• UT is the universal time in decimal hours
• long is your longitude in decimal degrees,
  East positive.
• Add or subtract multiples of 360 to bring LST in
  range 0 to 360
• degrees.

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Precession of the Equinoxes

• Precession is a slow wobble of the Earths
  rotation axis due to our planets nonspherical
  shape and its gravitational interaction with the
  Sun, Moon, etc
• Precession period is 25,770 years, currently NCP
  is within 1? of Polaris. In 13,000 years it will
  be about 47? away from Polaris near Vega!!!
• A westward motion of the Vernal equinox of about
  50 per year.

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Celestial Coordinates Links

• http//spiff.rit.edu/classes/phys445/lectures/rade
  c/radec.html
• http//home.att.net/srschmitt/script_celestial2ho
  rizon.html
• http//www.coyotegulch.com/articles/StellarCartogr
  aphy/na0002.html
• http//tycho.usno.navy.mil/sidereal.html
• http//www.jgiesen.de/elevaz/basics/astro/stposeng
  l.htm
• http//idlastro.gsfc.nasa.gov/
• http//idlastro.gsfc.nasa.gov/ftp/pro/astro/hor2eq
  .pro
• http//idlastro.gsfc.nasa.gov/ftp/pro/astro/eq2hor
  .pro

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Distance and Brightness

• Stellar Parallax
• The Magnitude Scale

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Stellar Parallax

• Trigonometric Parallax Determine distance from
  triangulation
• Parallax Angle One-half the maximum angular
  displacement due to the motion of Earth about the
  Sun (excluding proper motion)
• With p measured in radians

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PARSEC/Light Year

• 1 radian 57.2957795? 206264.806
• Using p in units of arcsec we have
• Astronomical Unit of distance
• PARSEC Parallax Second pc
• 1pc 2.06264806 x 105 AU
• The distance to a star whose parallax angle p1
  is 1pc. 1pc is the distance at which 1 AU
  subtends an angle of 1
• Light year 1 ly 9.460730472 x 1015 m
• 1 pc 3.2615638 ly

• Nearest star proxima centauri has a parallax
  angle of 0.77
• Not measured until 1838 by Friedrich Wilhelm
  Bessel
• Hipparcos satellite measurement accuracy
  approaches 0.001 for over 118,000 stars. This
  corresponds to a a distance of only 1000 pc (only
  1/8 of way to center of our galaxy)
• The planned Space Interferometry Mission will be
  able to determine parallax angles as small as 4
  microarcsec 0.000004) leading to distance
  measurements of objects up to 250 kpc.

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The Magnitude Scale

• Apparent Magnitude How bright an object appears.
  Hipparchus invented a scale to describe how
  bright a star appeared in the sky. He gave the
  dimmest stars a magnitude 6 and the brightest
  magnitude 1. Wonderful smaller number means
  bigger brightness!!!
• The human eye responds to brightness
  logarithmically. Turns out that a difference of 5
  magnitudes on Hipparchus scale corresponds to a
  factor of 100 in brightness. Therefore a 1
  magnitude difference corresponds to a brightness
  ratio of 1001/52.512.
• Nowadays can measure apparent brightness to an
  accuracy of 0.01 magnitudes and differences to
  0.002 magnitudes
• Hipparchus scale extended to m-26.83 for the
  Sun to approximately m30 for the faintest object
  detectable

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Flux, Luminosity and the Inverse Square Law

• Radiant flux F is the total amount of light
  energy of all wavelengths that crosses a unit
  area oriented perpendicular to the direction of
  the lights travel per unit timeJoules/sWatt
• Depends on the Intrinsic Luminosity (energy
  emitted per second) as well as the distance to
  the object
• Inverse Square Law

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Absolute Magnitude and Distance Modulus

• Absolute Magnitude, M Defined to be the apparent
  magnitude a star would have if it were located at
  a distance of 10pc.
• Ratio of fluxes for objects of apparent
  magnitudes m1 and m2 .
• Taking logarithm of each side

• Distance Modulus The connection between a stars
  apparent magnitude, m , and absolute magnitude,
  M, and its distance, d, may be found by using
  the inverse square law and the equation that
  relates two magnitudes.
• Where F10 is the flux that would be received if
  the star were at a distance of 10 pc and d is the
  stars distance measured in pc. Solving for d
  gives
• The quantity m-M is a measure of the distance to
  a star and is called the stars distance modulus

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A Brief talk about Telescopes

• Types of Telescopes
• Refractor
• Reflector
• Newtonian
• Schmidt-Cassegrain
• .
• What Does a Telescope Do?
• Light Collection
• Image Formation
• Pointing
• Go-To Telescopes

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Types
Refractor Reflector Catadioptric
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Light Collection
The aperture of the optical instrument allows
light coming from a source to be collected for
image formation. The larger the aperture the more
light is collected, therby allowing dimmer
objects to be seen.
Meade LX200 14 diameter
Human Eye 1/4 diameter
99,314 mm2
126 mm2
Our 14 Meade has 788 times larger area than your
eye
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Light Collection
Magnitude limit of 14 scope

• Assume that the unaided eye can see down to 6th
  magnitude.
• The amount of light collected increases with
  light collection area
• 14 scope hase 788 times the area of your eye
• Magnitude Definition
• Each 5 magnitudes ? 100 times the light
• Each magnitude ? 2.51 times the
  light

With 14 scope should be able to see down to
magnitude 13.24
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Image Formation
Lenses or Mirrors Focus light in such a way that
the light rays emanating from one point on the
object is focused to one point in the focal plane
thereby forming an image of the object
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Eyepieces and Magnification

• Need eyepiece to examine image
• Magnification

Primary Focal Length -----------------------------
Eyepiece Focal Length
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Resolution

• The ability to make out detail of an object
• Separate binary stars
• See features on extended objects
• Diffraction limit
• Maximum useful magnification

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Focal Ratio

• Focal Length/Aperture
• Speed/Brightness of optics
• f/8 requires 4x exposure time of f/4
• Field of View
• Smaller is faster and wider

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Telescope Pointing

• Mount types
• Altitude-Azimuth
• Equatorial Fork
• German Equatorial
• Dobsonian

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Altitude-Azimuth Mount
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Equatorial Fork Mount
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German Equatorial
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Dobsonian Mount
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Go-To Telescopes

• Alignment of Equatorial Mount Scopes
• Basic Setup
• Computer
• Location and Time
• Use of Catalogs
• Use of Coordintes
• Telescope
• Finderscope alignment
• Basic Usage
• Finding and Centering Objects
• Focusing

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Warnings !!!!!

• Know the basic operation before turning on scope
• Be prepared to switch off/ stop scope from
  slewing
• Watch cables..