Astronomical Techniques - 1

Please pick up a
copy of the lecture
notes from the
front

About Me
Dr Ben Maughan
Degree: Cardiff
Ph.D: Birmingham
Chandra Fellow:
Harvard-Smithsonian
Centre for
Astrophysics

Research:
X-ray properties of
galaxy clusters
Cosmology

Room 4.19
ben.maughan@bristol.ac.uk

Resources
Lecture Notes
You need to make additions and extra notes as
appropriate
Extra annotations in red

Course website

http://www.star.bris.ac.uk/bjm/lectures/astech
all material also on blackboard
copies of lecture slides and handouts
lecture videos (can have early access by request)

Resources
Books see handout, but I refer most to
Astronomy, Principles and Practice, Roy & Clarke (IoP)
Observational Astronomy, Birney et al. (Cambridge)

Podcasts
Skeptics guide to the Universe
More or less...

Blogs...
Bad Astronomy
XKCD what if
In the Dark...

Bristol Astronomical
Society
UoB Physics Colloquia
Science Cafe
Skeptics in the Pub

http://www.star.bris.ac.uk/bjm/interesting

Lectures

Week

Mon 10am

Tue 10am

Wed 12pm

Thu 12pm

Thu 2pm

13

Wodehouse

Frank

Mott

Practical
Tyndall

14

Wodehouse

Frank

Mott

Practical
Tyndall

Course Overview
Coordinate & Time systems
Telescopes
Detectors at different wavelengths

Coordinates I - Objectives
Understand use of spherical coordinates
Explain & use:
Alt-azimuth/horizon system
Equatorial system

A) Coordinates I
Wide range of techniques used to study
astronomical objects
First must consider how describe position of objects
Need to know where to point telescope at given time

The Celestial Sphere

From our point of view, astronomical objects (stars,
galaxies etc) appear to lie on spherical surface
Distances not important for describing position
Define coordinate grid on this sphere using circles
Given by intersections of planes with spherical surface

Spherical Coordinates
Great circles are intersections of sphere with
planes containing centre of sphere
c.f. longitude on Earth

Spherical Coordinates
Small circles are intersections of sphere with
planes NOT containing centre of sphere
c.f. latitude on Earth

Spherical Coordinates
Choose a great circle as the equator of sphere
define grid of small circles parallel to equator and great
circles perpendicular to equator
poles are points 90 from all points on equator

Spherical Coordinates
The position of any point on sphere then described
by 2 angles
angle around equator and angle up/down from equator

Need origin on equator to measure angle from

Coordinate system defined by equator and origin

A1) Alt-Azimuth System

Observer O at latitude
Most obvious system:
use horizon as equatorial
plane
use zenith/nadir (up/down)
as poles

Need origin from which to

measure angles around
equator

Alt-Azimuth System
Celestial sphere appears to
rotate about projection of
geographic North pole
North Celestial Pole

Define North by dropping

perpendicular from North
Celestial Pole to horizon
Use as origin for angles

Alt-Azimuth System
Consider observer's view of the sky
half of celestial sphere on plane of observer's horizon
mark on cardinal points N, E, S, W and Zenith Z and North
Celestial Pole P

Arc PZS is called the observer's meridian

Part of the great circle through P and Z

Alt-Azimuth System
Coordinates of a star Q defined by drawing great
circle through Z and Q cuts horizon at X
Position of Q then given by:

Alt-Azimuth System
Coordinates of a star Q defined by drawing great
circle through Z and Q cuts horizon at X
Position of Q then given by:
altitude (or elevation) a the arc QX

Alt-Azimuth System
Coordinates of a star Q defined by drawing great
circle through Z and Q cuts horizon at X
Position of Q then given by:
altitude (or elevation) a the arc QX
azimuth, A the arc NX eastward from N

Alt-Azimuth System
The arc QZ is called the zenith distance of point Q
What is the altitude of P (North Celestial Pole)?

Alt-Azimuth System

Alt-Azimuth System
The arc QZ is called the zenith distance of point Q
What is the altitude of P (North Celestial Pole)?
equal to latitude of the observer

 What are a & A of star Q?

 What are a & A of star Q?

N.B. These lines are not really parallel meet at zenith

 What are a & A of star Q?

Q'

Alt-Azimuth System
Apparent rotation of celestial sphere about P causes
a & A to change continuously
Local System coordinates of an object different
for different observers at same time
same observer at different times

Example
Sketch the view of the sky of an observer at La
Palma observatory, latitude =28N. Mark on the
zenith, north celestial pole, cardinal points, and the
position of a star with azimuth A=300 and altitude
a=30

Example
Sketch the view of the sky of an observer at La
Palma observatory, latitude =28N. Mark on the
zenith, north celestial pole, cardinal points, and the
position of a star with azimuth A=300 and altitude
a=30

O
Observer's
horizon

Example
Sketch the view of the sky of an observer at La
Palma observatory, latitude =28N. Mark on the
zenith, north celestial pole, cardinal points, and the
position of a star with azimuth A=300 and altitude
a=30
Z
P
O
Observer's
horizon

Example
Sketch the view of the sky of an observer at La
Palma observatory, latitude =28N. Mark on the
zenith, north celestial pole, cardinal points, and the
position of a star with azimuth A=300 and altitude
a=30
Z
Angle PN = = 28

P
E
N

O
W

Observer's
horizon

Example
Sketch the view of the sky of an observer at La
Palma observatory, latitude =28N. Mark on the
zenith, north celestial pole, cardinal points, and the
position of a star with azimuth A=300 and altitude
a=30
Z
Angle PN = = 28

P
E
N
a = angle XQ = 30
A = angle NESWX
A = 300

Q
X

O
W

Observer's
horizon

A2) Equatorial System

The simplest system in which coordinates do not
change with time is the Equatorial System

Equatorial System
Define equator of coordinate system as the projection
of Earth's equator onto celestial sphere
Celestial Equator
Cuts horizon at E & W
Cuts observer's meridian at T

Poles are North and South Celestial Poles

Equatorial System

Equatorial System
Coordinates of star Q defined by great circle through
P and Q cutting celestial equator at X

Equatorial System
Coordinates of star Q defined by great circle through
P and Q cutting celestial equator at X
Declination is arc QX, range +90N to -90S

Equatorial System
Coordinates of star Q defined by great circle through
P and Q cutting celestial equator at X
Declination is arc QX, range +90N to -90S
Right Ascension, RA arc X measured east from First
Point of Aries, in hours, mins, secs with 360 = 24
hours

For direction east,

imagine you are
travelling on
outside of sphere

Equatorial System
As Earth rotates, star Q moves along small circle
RA and of Q remain constant

Star crosses observer's meridian at T'

Star's transit
Time between transits is 1 sidereal day (sidereal = with
respect to the stars)

Equatorial System
Hour Angle, HA: arc TX, measured west from
observer's meridian in hours, mins, secs

Equatorial System
While RA & are constant, HA changes continuously
HA of acts as clock, specific to particular observer
Observer's Local Sidereal Time, LST

For any object Q,

LST = RA(Q) + HA(Q)

Equatorial System
For any object Q,
LST = RA(Q) + HA(Q)
So, given RA and of object, and LST of our
observatory from sidereal clock
Determine HA and find object on sky

Equatorial System

Coordinates I - Summary
Astronomical coordinate systems describe positions
with 2 angles
Systems defined by equator and poles
Alt-azimuth: equator = horizon, poles = zenith, nadir
a & A vary with time and location

Equatorial: equator = celestial equator, poles = NCP,

SCP
RA & do not vary

Hour Angle of gives observer's Local Sidereal Time