22PH503 - Astronomy and Astrophysics - Unit 1 - Astronomical Instruments

Astronomy and Astrophysics
Ms Dhivya R
Assistant Professor
Department of Physics
Sri Ramakrishna College of Arts and Science
Coimbatore - 641 006
Tamil Nadu, India
1

Unit I – Astronomical Instruments
 Light and its properties
 The Earth’s Atmosphere
 Electromagnetic Radiation
 Telescopes - Optical telescopes
 Types of Reflecting Telescopes
 The Refracting telescope
 Relative Advantages of Reflectors and
Refractors
 Radio telescopes
 The Hubble space telescope (HST)
2

Light and its properties
 Light is the important source of information for an astronomer.
 Light is an electromagnetic waves
 Wavelength λ
 E & H are always numerically equal.
 In a free space, the energy per unit volume in an electric field is
E2/8π and the energy per unit volume in a magnetic field is H2/8π.
 The total energy W per unit volume is W = E2/8π + H2/8π = E2/4π =
H2/4π
3

 The intensity of the EM radiation at any point in space is defined as
energy the flows per unit time per unit area perpendicular to the
direction of propagation.
 I= cE2/4π = cH2/4π
 The number of oscillations in unit time of E and H at a given point is
called frequency
λν = c
 The numerical value of ν is very high. It is 6 x 1014 for green light.
 Monochromatic
 Reflection – smooth surface – regular
 Reflection – rough surface – reflected irregularly - diffused
 Reflectivity - ratio of the energy associated with reflected radiation
to the incident radiation
 “Albedo” reflectivity from an astronomical body
4

 Refraction
 Snell’s law
 Diffraction
 Polarization
5

Ancient Astronomical
Instruments
 Astrolabe
 Quadrant
 The Armillary
Sphere
6

The Earth’s Atmosphere
7

Electromagnetic Radiation
8

Telescopes
A telescope is a
tool that
astronomers use to
see faraway
objects. Most
telescopes, and all
large telescopes,
work by using
curved mirrors to
gather and focus
light from the night
sky.
9
Telescopes
Optical
telescopes
Reflecting Refracting
Radio
Telescopes

Optical Telescopes
 Telescopes which are used for observation in the optical
wavelengths of the electromagnetic spectrum are known as
optical telescopes.
 Two types of optical telescope
 Reflecting telescopes (largest telescopes in the world)
 Refracting telescopes (general use and small in size)
 The fundamental principles is the same for both the
telescopes.
Magnifying power:
The ratio of the focal length F of the objective to the focal length f of the
eyepiece
q=F/f
Brightness of Image:
Depends upon the aperture and the focal length of the objective.
10

Types of Reflecting Telescopes
 There are 3 main types of reflecting telescopes.
 Newtonians,
 Cassegrains, and
 Coude Foci
 They all share many things in common like the use of
mirrors instead of lenses, but their designs vary.
 Reflecting telescopes are known for being powerful
because of their high apertures and being budget friendly
because of the materials they use and how easy they are
to manufacture
11

Newtonian Reflecting Telescopes
Characteristics of reflecting telescopes
 Reflecting telescopes were invented by Isaac Newton in the 17th
century. The big innovation they had over the telescopes at the time
is they used mirrors instead of lenses to reflect the light – hence the
name – instead of refracting it into a single point. This allowed for
much bigger telescopes that also fixed chromatic aberrations (wrong
colors and colored rings around the objects).
 One important note about reflectors is that they require more
maintenance than other types of telescopes. Because of their open
design, some dust will inevitably enter the tube. This means the
interior and the primary mirror need to be cleaned regularly.
 With regular use, the mirrors also tend to move a little, so once
every few months, reflectors need to be re-aligned. This process is
called collimation, and while it is simple once you get the hang of it,
it can seem intimidating for beginners. Collimating a telescope can
take anywhere from 5 minutes to 1 hour depending on the type of
reflector.
12

Characteristics of Newtonians
 No front mirror
 The primary mirror is at the back of the tube
 A secondary mirror bounces the light to the eyepiece
 Relatively short focal length
 Large apertures
 Easy and cheap to manufacture
 Lower contrast
 Free or almost free of color errors
13

14

Cassegrains Reflecting
Telescopes
15

Coude Foci Reflecting Telescopes
16

Types of Reflecting Telescopes
17

Refracting Telescopes
18

Refracting Telescopes
19

Reflectors vs Refractors
20

Relative advantages of Reflectors
and Refractors
 Lenses Vs Mirrors
 Lenses – Optically
Homogeneous
 Mirrors – Mechanically
Homogenous (Easier to
Achieve)
 Lenses – Chromatic
Aberration – Affect
Photographic quality
 Mirrors – Perfectly
Achromatic – Quality Pictures
are achieved
 Lenses – Smaller in Size -
Image definition – Wide Field
and Good
 Mirrors – Large in size -
decreased defining power
21
For the same size of aperture
 Refractors – Costly
 Reflectors – Less Costly
Usage
 Refractors – visual
observation and
Measurement – Objective
Prism Spectrograph
 Reflectors – faint object
observation –
photography,
spectroscopy,
photoelectric photometry

Radio Telescopes
 Radio Waves – Shorter RW – Absorbed ; Long
RW about 50 m –completely reflected back.
 RW of wavelength ranging few CM to few M are
only available for study.
 We use radio telescopes to study naturally
occurring radio light from stars, galaxies, black
holes, and other astronomical objects.
 We can also use them to transmit and reflect
radio light off of planetary bodies in our solar
system.
 These specially-designed telescopes observe the
longest wavelengths of light, ranging from 1
millimeter to over 10 meters long.
 They work similar to optical telescopes except
that the RT works on only one frequency at a
time.
 Resoving power of RT very low compared to OT
22

Radio Telescopes
23

Radio Telescopes
 Radio telescopes collect weak radio
light waves, bring it to a focus,
amplify it and make it available for
analysis.
 We use radio telescopes to study
naturally occurring radio light from
stars, galaxies, black holes, and
other astronomical objects.
 We can also use them to transmit
and reflect radio light off of
planetary bodies in our solar
system.
 These specially-designed
telescopes observe the longest
wavelengths of light, ranging from 1
millimeter to over 10 meters long.
24

Radio Telescopes in India
25

Hubble Space Telescope
26

SOLAR PANELS
Named after the
famous American astronomer
Edwin Hubble, the Hubble
telescope is powered by six
nickel-hydrogen batteries,
which provide power to the
spacecraft during orbit while it
flies through the Earth’s
shadow. The scientific
instrumentation and onboard
computers draw approximately
2800 watts, which are charged
by two 2.45 x 7.56m solar
panels.
27

COMMUNICATIONS
ANTENNAS
On-board the Hubble
telescope are two identical S-
Band transmitters. In 1998 one
of the transmitters failed,
however the secondary
transmitter has kept up with the
extra load by rotating the
telescope to maintain
communication. The radio
waves converted from the solid-
state recording system are sent
to a NASA communication
satellite, which in-turn relays the
information to Earth.
28

APERTURE DOOR
The adaptive optics on the
Hubble telescope allow it to
conduct high-resolution
optical visualization over a
wide field of view. The
aperture door itself can be
closed if needed, to
prevent light from external
sources from entering the
telescope
29

30
LENGTH
13.2m
DIAMETER
2.4m

1. The Wide Field Planetary Camera (WFPC) – survey selected
area of Faint Objects
2. The Faint Object Spectrograph (FOS) – perform low resolution
spectroscopy
3. The High-Resolution Spectrograph (HRS) – perform very high-
resolution spectroscopy
4. High Speed Photometer (HSP) – to study brightness
fluctuations
5. Faint Object Camera (FOC) – Study Very Faint Object
31

22PH503 - Astronomy and Astrophysics - Unit 1 - Astronomical Instruments

22PH503 - Astronomy and Astrophysics - Unit 1 - Astronomical Instruments

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