Observational Astrophysics in the visible light

Shai Kaspi

Technion - March 2015

Atmospheric transmission

Celestial Sphere

Angular Size or Distance

Measured in degrees of arc

Angular Size and Distance

Earth Coordinate System• Earth rotation axis between north and south poles and the equator

which slice into half right between the poles enable to define the:

• Latitude - is defined as an angle in degrees, North or South of the equator, with the vertex of the angle at the center of the Earth.

• Longitude is an angle East or West of the Greenwich Meridian in England, which is arbitrarily defined to be 0º longitude.

Altitude - Azimuth Coordinate System • Local coordinate system.

• Zenith - The overhead point.

• Nadir - is the underfoot point.

• Azimuth is measured on a parallel to the horizon, in degrees East from North.

• Altitude is measured in degrees above the horizon, so that the zenith is at 90º altitude, while the horizon is 0º altitude.

The Equatorial (Celestial) Coordinate System • celestial equator is the projection of Earth's

equator onto the sky.• North and South celestial poles - are the points

on the sky directly above Earth's North and South poles.

• Declination (Dec) - the North-South celestial coordinate measured from the celestial equator. Measured in degrees.

• Right Ascension (RA) - the East-West celestial coordinate measured from an arbitrary zero-point set at the March equinox, which is the Sun's position on the sky on the date that night and day are of equal length. RA is measured in hours, minutes, seconds and increases from West to East.

• Hour Angle - The angular distance, measured westward along the celestial equator, between the celestial meridian of the observer and the hour circle passing through a celestial body.

Vega is at 18h 36m 56s and 38º 47' 01"

Motion on the sky• The coordinate of an object on

the sky changes due to the precession of the earth axis at a cycle of 26,000 years.

The shift is 1 degree in 72 years.

• RA, Dec, and Epoch

• Self motion (Proper motion, movement of planets, asteroids etc.

To compute the angular distance between two stars:

Star 1: ra1 and d1 in degrees

Star 2: ra2 and d2 in degress

cos(A) = sin(d1)sin(d2) +

+cos(d1)cos(d2)cos(ra1-ra2)

Galactic Coordinate System (GCS)• celestial coordinate system which is

centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane. Similar to geographic coordinates, positions in the galactic coordinate system have latitudes (e) and longitudes (l). The northern galactic pole is located in the constellation of Coma Berenices while the southern pole lies in Sculptor.

One mirror telescope Segmented telescope