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Spectroscopy
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Spectral analysis is probably the most important method
for learning about the physics of the astronomical sources
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Simplest method to get spectral information is using filters
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In this case the size of the spectral elemet we can resolve is
the width of the filter
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More detailed information is obtained if the light is sent
through a dispersive element
Newton‘s experiment
Dispersive elements
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The core optical element of an astronomical spectrograph
is its dispersive element
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With a dispersive element, the angle at which the light
leaves it, is wavelength dependent.
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There are three kinds of dispersive elements:
 Prisms
 Grating
 Grisms
Prism 1
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When light moves from one medium to a denser medium, it is slowed
down and as a result either bent (refracted) or reflected.
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The refractive index of a material is a measure of the light speed in
that material. It depends on the wavelength.
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The white light entering the prism is a mixture of different
wavelengths, each of which gets bent slightly differently. Blue light is
slowed more down than red light and will therefore be bent more
than red light.
Prism 2
Grating 1
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The diffraction grating is the primary dispersing element in
most astronomical spectrometers
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It has significantly larger limiting resolution than a prism
of comparable size
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A grating is also versatile in the spectral formats it can
provide
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And it can be quite efficient over a reasonable spectral
range, though usually not as efficient as a prism
Grating 2
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A diffraction grating is a
reflecting or transparent element,
whose optical properties of an
external (or internal) surface are
periodically modulated.
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Most commonly the diffraction
gratings are realized as fine
parallel and equally spaced ruled
grooves on a material surface.
Grating equation 1
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The diffraction of light in a
plane reflectance grating is
governed by
sin + sin = m/d
 is the angle of incidence
m is the angle of diffraction
m denotes the order number
of the diffracted beam
d is the groove spacing of the
grating
Grating equation 2
Échelle grating 1
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For high resolution astronomical work échelle is the preferred
choise over a grating used in low order
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The reasons for this are:
 Larger luminosity
 Two dimensional format that permits broad spectral
coverage
Échelle grating 2
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A diffraction grating in which
the grooves are quite widely
spaced and have a zigzag or
step-like cross-sectional
profile.
The light to be dispersed is
made to fall on the grating at
right angles to the faces of
the grooves. This has the
effect of producing a series
of overlapping spectra with a
high degree of resolution.
A second, low-dispersion
grating, or a prism, arranged
perpendicular to the échelle,
serves to separate out the
overlapping spectra.
Échelle spectrum
Grism
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A grism is a transmission
grating replicated onto a prism
that has its angle chosen in such
a way that the desired order of
the grating passes through the
grism undeviated.
The grating groove spacing and
the prism angle for a given
prism material define the
location of the spectrum on the
detector, and the grating groove
angle and refractive index of
the grating define the grating
blaze function.
Schematic grism and the
optical light path through it
= prism angle
=groove angle
=deflection angle np=refractive index
d=grating groove spacing
Resolving power
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Resolving power (R) tells how small details we can resolve
in the spectrum
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It is defined as 
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So for example
 R=1000 at 6500 Å
gives =6.5 Å
or 300km/s
 R=10 000 at 6500 Å gives =0.65 Å
or 30km/s
 R=100 000 at 6500 Å gives =0.065 Å or
3km/s
Which resolving power to use for
your observations?
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Always „the larger the better“ is not the answer
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High resolution needs a lot of photons, so to get any signal
one needs a bright source and/or a large telescope
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Also, in some cases there is no need for high resoltion. If
the process you want to study produces velocities of
1000km/s, there is not much point studying it with
resolution of 1 km/s
Spectroscopy at NOT
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NOT has 4 instruments with spectroscopic capabilities:
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Optical multi purposes spectrograph, ALFOSC
Low to medium resoltion IR spectrograph, NOTCam
2 high resolution échelle spectrographs, FIES & SOFIN
ALFOSC
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15 possible grisms
Wavelengths covered between
3200-11000 Å
Resolving powers between 190
and 10 000
Long slits and 6.6 " -8 .6 " &
15" slits for échelle
spectroscopy and
spectropolarimetry
Also multi object spectroscopy
(MOS)
NOTCam, low resolution
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One grism with an echelle grating
Intermediate resolution (2-pixel R=2500, with dispersion 2.5-4.1
Angstrom/pixel) in J (5th and 6th order), H (4th order) and K (3rd
order) when used with the WFC
The grism is used in combination with the JHK filter set.
Slit width of 128 microns (1 arcsec)
NOTCam, medium resolution
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Also the High-Resolution
Camera (HRC) can be used to
obtain spectra.
Wavelength ranges in the JHK
bands: 1.26-1.34 micron (Pabeta), 1.57-1.67 micron, 2.072.20 micron (Br-gamma).
Respective dispersions are 0.87,
1.08, 1.39 A/pix. With the
dedicated 0.5 arcsec slit, the
resolutions are R=5700, 5700,
4900 respectively.
FIES 1
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Cross-dispersed high-resolution echelle spectrograph,
R=60 000
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Spectral range 4000-8300 Å is covered without gaps in a
single setting
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Mounted in a well insulated building separate from and
adjacent to the NOT dome.
FIES 2
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The optical fibre connecting the telescope with the spectrograph is
permanently mounted with a movable 45-degree mirror
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Being a permanently mounted instrument, FIES can be used at any
time, also for short periods of time while other instruments are
mounted.
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Equipped with a 100 micron fibre, corresponding to 1.3 arcseconds on
the sky at a spectral resolution of R =40 000.
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We expect a new and more extensive fiber unit to be installed, which
contains a new 100 micron fiber (but with exit slit) offering a
resolution of R = 60 000
SOFIN 1
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Échelle spectrograph
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Wavelength coverage 3500-11000Å
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Three different spectral resolutions
R = 30 000, 80 000, and 170 000
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One of the few very high resolution
spectrographs in the northern
hemisphere.
SOFIN 2
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The resolution is altered by
changing one of the three
different optical cameras
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The higher the spectral
resolution the smaller the part
of the spectral range which can
be covered by the ccd. The
change of the spectral setting is
done by turning the échelle
grating and the cross-dispersion
prism.
Polarised light
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The electric field vector may be
arbitrarily divided into two
perpendicular components x and y
For a simple harmonic wave, the
two components have exactly the
same frequency.
Components have two other
defining characteristics that can
differ:
 Amplitude
 Phase
The shape traced out in a fixed
plane by the electric vector as such a
plane wave passes over it is a
description of the polarisation state
Plots show electric field vector
(blue), its x and y components
(red/green) and the path traced by
the tip of the vector in the plane
(purple)
Polarisation states
Linear
Circular
Elliptical
Why is it usefull?
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Mechanisms for creating polarisation:
 Scattering by small dust particles or free electrons
 Various absorption and emission processes in magnetic
fields
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Polarisation provides information about:
 The geometry of the light emitting source
 And the radiation process involved
How to observe polarisation
Polarimeter
Telescope
Spectrometer
Detector
Calibration
opics
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Polarimeters consist of optical elements such as retarders
and polarisers that change the polarisation state of the
incoming light in a controlled way
The detector only measures intensities
The various intensity measurements are then combined to
get the polarisation state f the incoming light
Wave plates 1
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A wave plate (or retarder)
works by shifting the phase of
the light wave between two
perpendicular polarization
components. A typical wave
plate is simply a birefringent
crystal with a carefully chosen
thickness.
The wave plate is characterized
by the amount of relative phase
that it imparts on the two
components
Wave plates 2
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Quater wave plate creates a quarter wavelength phase shift and can
change linearly polarized light to circular and vice versa.
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Half wave plate retards one polarisation by a half wavelength, or 180
degrees. This type of wave plate rotates the polarization direction of
linear polarized light.
Polarisers 1
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Polariser is an optical element that produces (at least partially)
polarised light from unpolarised input beam
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Many types of polarisers, for example
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Polaroid
Christal polarisers
Polaroid
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First polaroid by Edwin Land in 1928 used herapathite
chrystals, which were spread as a thin layer between
supporting sheets
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The chrystals have a magnetic dipole moment, so that if
the suspension is placed in a very strong magnetic field
they become oriented to form a uniform dichroic layer
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Nowadays other materials are used, e.g., polyvinyl alcohol
and polyvinylene
Chrystal polarisers
A calcite crystal laid upon a paper with some
letters showing the double refraction
Wollaston prism
Polarimetry with ALFOSC
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ALFOSC has a polarimetry unit FAPOL which can be
equiped either with half wave plate (linear polarisation) or
quater wave plate (cicular + linear polarisation)
A calcite plate is needed in the aperture wheel to provide a
simultaneous measurement of the ordinary and the
extraordinary components.
The unvigneted field is limited by the size of the calcites to
140" in diameter.
Both photo- and spectropolarimetry can be obtained
Turpol
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Simultaneous five-colour
polarimeter (UVBRI)
Double image chopping
polarisation levels
0.01%
detectable
Rotatable quater-wave plate:
simultaneous cicular and linear
polarissation
Best efficiency for only linear
obtained with a half-wave
retarder
Calcite plate polarising beam
splitter yields direct
elimination of sky back-grund
polarization