Download Spectrometer The instrument used in the space telescope

Spectrometer
The instrument used for the
astronomers
MinGyu Kim
2010. 04. 28
Primary applications of spectroscopy
•
•
•
•
SED
Spectral classification
Radial velocity determination
Spectral line formation
-abundance analysis-from equivalent widths
-line profile analysis(thermal, collisions, rotation)
-zeeman effect(magnetic fields)
-stellar age(Li abundance, etc.)
The basic spectrograph
•
•
•
•
Slit : isolates portion of sky that is imaged in a single wavelength
Collimater : makes beam parallel
Dispersing element : diseperses light as a function of wavelength
Camera : forms image of object(star or slit) on detector
Dispersion
• The important design characteristics of a
spectrograph are the dispersion(dθ/dλ),
which defines how widely the various
wavelengths are spread out, and the
resolution, which describes the minimum
difference in wavelength that can be
determined.
• dx=dθ·fcam →dx/dλ=dθ/dλ·fcam
• Here, dx/dλ is properly defined as linear dispersion,
astronomers define linear reciprocal dispersion
dλ/dx[A/mm].
• 50~200A/mm : low dispersion(spectral classification)
• 10~50A/mm : medium dispersion(radial velocities)
• <10A/mm are high dispersion(line profiles)
If blurred image wider
than the slit is coming
from the universe as
below, the resolution
is already determined
from that.
•
•
•
•
Resolution
Resolving power is the ability of the components of an imaging device to measure the angular
separation of the points in an object.
The imaging system’s resolution can be limited either by aberration or by diffraction causing blurring
of the image.
The minimum angle that can be resolved is δθ=1.22·λ/d radians(angular resolution).
The angular resolution may be converted into a spatial resolution, ᅀl=1.22fλ/D
Types of Spectroscopes
• Spectroscope
-utilized for visual observation of spectra
• Spectrograph
-photographic recording of spectra
• Spectrometer
-obtaining digital recording
Classifications in detail
Classify with resolution
Low Resolution spectrograph
Medium Resolution spectrograph
High Resolution spectrograph
Classify with the device dispersing light
Prism
Grating
Echelle
Grism
Prism-based
• Using principle of refraction, a prism
shaped like a triangular.
• This happens because a ray of light is
refracted as it passes from air to glass,
and this refraction is slightly different,
depending on the specific wavelength of
light.
Disadvantage
• Incoming light is not linearly dispersed,
so that the distances between the
various constituent wavelengths are not
equal.
Diffraction grating-based
• A diffraction grating is an
optical component with a
periodic structure, which
splits and diffracts light into
several beams travelling in
different directions(spaced
usually from 200 to 1,000 per
mm).
• The directions of these
beams depend on the
spacing of the grating and
the wavelength of the light
so that the grating acts as
the dispersive element.
• gratings are classified as either reflection
or transmission gratings.
-Reflection gratings
-Transmission gratings :the transmitted
beam is diffracted into multiple orders.
• Dispersion increases
with order m, and the
red light is dispersed
through a greater
angle than blue. In the
higher orders, the end
of the blue overlap the
red. It may be
necessary to insert a
colored glass filter to
isolate the order of
interest.
• The useful spectral
range(over which
orders will not overlap)
is approximately λredλblue=λcentral/m
Disadvantage
• Successive, fainter spectra are referred to
as a first-order spectrum, second-order
spectrum, and so on. This effect, of
course, may give rise to significant light
losses.
• Incoming light is not linearly dispersed,
so that the distances between the
various constituent wavelengths are not
equal.
Echelle
• Desires both high spectral dispersion and broad
wavelength coverage.
• Light passed throw the slit go into the cross
dispersers and disperse the light as above figure.
Grism
• Here a grating has been bonded to the
surface of a prism.
• Incidence angle and the transmittance
angle is approximately same so, it disperse
the light just put it along the ray.
• The deviation of the beam by the prism is
compensated by the deviation of the
central wavelength in the grating resulting
in a spectrum centered on the system
optical axis.
• prism and grating
are non linear
dispersion(each
are stretch out to
one direction),
however, if you
use grism, there
is no deviation so
either direction is
evenly dispersed.
Classification with resolution
High resolution spectrograph(HRS)
• It can discern fine features in the spectra
from astronomical sources by spreading
the spectrum out more than the other
spectrographs.
Low resolution spectrograph(LRS)
• To see the faint sources and background
of the universe.
Medium resolution
spectrograph(MRS)
• MRS allows for higher resolution
spectroscopy than the LRS and better
sensitivity than the HRS and, therefore,
bridges a gap between these two
instruments.
Applications for astronomers
A small basic spectroscope
• A simple grating-based
slit spectroscope.
• Such an instrument on
a moderate telescope
is capable of providing
reasonable spectra of
the brighter stars,
sufficient for spectral
classification, etc.
Conventional Cassegrain
spectroscope
Dichroic mirror ,depending on material
made of, reflects some light and
transmit other light.
• A grating-based slit
spectroscope attached
to the Cassegrain focus
of a large telescope.
• It uses a dichroic
mirror to split the
incoming light into a
red and a blue beam,
each of which is then
separately directed into
a spectroscope
optimized for those
wavelengths.
Transmission grating spectroscopes
• Designed to obtain the spectra of up to 100 objects
simultaneously at low dispersions and down to about
magnitude 23.
• The spectroscope uses lenses throughout in order to avoid
the obstructions caused by the secondary mirrors in
reflecting systems.
• The entrance aperture is a mask with up to 100 precisely
positioned slits to match the positions of objects in the FOV.
• Because of the width of the entrance field, the optics of this
spectroscope need to have their aberrations corrected over
a much wider FOV than those for normal spectroscopes
and this is the reason uses transmission optics.
COUDE spectroscopes
• This instrument uses
reflection gratings.
• Three separate imaging
elements are
incorporated into the
one mounting and can
be selected by changing
the angle of the grating.
• Much better dispersion
and spectral resolution
possible compared with
Cassegrain spectroscope.
Grism-based spectroscope
• It is designed for low
dispersion spectroscopy
uses when a high efficiency
instrument is required for
the study of the spectra of
very faint objects.
Multi-Object spectroscopes
• The TGS can obtain the spectra of up to
100 objects simultaneously, however, a
mask containing slits positioned to
match the objects to be studied and this
has to be made up for every observation.
• So, a more flexible system is to use
optical fibers to direct the light from the
primary mirror to the instrument.
• It can observe up to
30objects
simultaneously over
a 1deg field and the
fiber optics are
moved to the
calculated positions
of those objects by
computer controlled
positioning arms.
• Three fibers are
carried by each arm.
• One intercepts the
light from the object
to be observed(2.6’’).
• The 2nd the light from
the nearby sky to
provide background
subtraction(2.6’’).
• The 3rd provides a
separate direct image
for positioning
purposes(36’’ by 36’’).
Echelle grating spectroscope
• An echelle grating can give
high dispersions and
spectral resolutions, but in
order to achieve these it
usually has to be used at
high spectral orders and so
must be allied to a cross
disperser.
• Spectroscope use an
echelle grating as the
primary disperser and then
a low dispersion spherical
grating which acts both as
the cross disperser and as
the imaging element.
Infrared spectroscopes
• The main
difference between
such an IR and
those of visible is
the need to reduce
background noise.
• This require
cooling system.
• It is encased in a
vacuum chamber
and cooled down
below 80K.
Spacecraft-borne spectroscopes
• Any spectroscope
operating at
wavelengths shorter
than about 350nm, in
the longer IR or
microwave regions, has
to be lifted above the
Earth’s atmosphere.
• Two detectors : one is for shorter wavelength and
the other is for longer.
• Two alternative imaging concave mirrors
• Two cross dispersing concave gratings
Fabry-Perot spectroscopes
• A filter isolates one of the
transmitted orders, and the
beam is re-imaged.
• Fabry-perot etalons are
most likely to be found in
an observatory in use as
narrow band filters.
• In operation, the spacing of
the etalon is varied to scan
its transmitted wavelength
over the free spectral range
of the order being used.
Etalons : various, with gaps ranging from 15 to 500um, velocity resolutions
ranging from 7 to 150km/s, spectral resolutions ranging from 500 to 60,000.
Dispersion is caused by the interference between the multiple reflections of light
between the two reflecting surfaces. Constructive interference occurs if the
transmitted beams are in phase, and this corresponds to a high-transmission
peak of the etalon