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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Fundamentals of Echelle Spectroscopy
Moletai Summer School August 2012
Eric Stempels, Uppsala University
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Overview
● A different introduction to optics?
● Dispersive elements
● Making a spectrograph
● Improving a spectrograph
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Astronomical observables
Color : spectroscopy (photometry)
Position : astrometry
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Spectroscopy
Detectors cannot measure the wavelength of light!
with filters = photometry(simple, efficient, but very low resolution)
with dispersive element = spectroscopy(technically more demanding, but gives high resolution)
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Simple refraction
● Speed of light in medium is c/n● Refraction is actually light taking the quickest path
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Simple imager
Collimator Camera
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Simple spectrograph
Slit
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Fourier perspective of optics
G =∫ F x , t G x e2 i x sin /dx
Switch variables :
which is a Fourier transformation and holds for all G(x)
G =F 0∫G x e2 i x dx
=x sin/
First, for a single, wide slit :
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Fourier perspective of optics
With a lens we can determine G() :
G()
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Transmission grating
High dispersion can be obtained by :
● High order number (n)● Small distance between grooves (d)
sinsin ==nd
d sin d sin
d
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Transmission grating efficiency
At high orders, efficiency becomesan issue
Directing light towards higher ordersnot a solution, because it introducesphase shifts
Solution : blazed reflection grating
d sin d sin
d
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Blazed grating
d
sinsin==nd
Best performance if :(no shadowing)
=
Central blaze directed tohigher orders
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Echelle
d=
Easy to work in veryhigh orders sinsin==n
d
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Resolution of an echelle
and thus :This can be used to calculatethe resolution that can beachieved for a given echelleand slit width
sinsin ==nd
= d cosn
d d
=d cosn
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Spectral resolution
● Lowresolution spectroscopy
R
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Grating and echelle : order overlap
For n > 1
Order sorting filter or crossdisperser
Now we can make a very efficient highresolution spectrograph
1 23
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Simple crossdispersed spectrograph
Gives 2dimensional spectrum
Very efficient use of small CCDs
Disadvantage limited slit length
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Telescope + Spectrograph
Ratio of diameters determines magnification
Cannot have small PSF (=high resolution) and have small beam
Very expensive to have a highresolution spectrograph on a large telescope
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Example possible resolution for FIES
FIES has an echelle with 31.6 grooves/mm anda blaze angle of 63.5 degrees.
The FIES beam has a diameter of 15 cmThe NOT mirror has a diameter of 250 cm
The fiber diameter is 1 arcsecond on the sky
= d cosn
sinsin==nd
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Further improvements to the spectrograph
● White pupil
● Temperature stabilization
● Fibers
● Image slicer
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Whitepupil design
Collimator Grating Crossdisperser
Camera
Camera
CrossdisperserGratingCollimator
But, with one more optical element :
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Whitepupil design
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Temperature Stabilization
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Temperature Stabilization
Very high precision and stability...
...for a very high price
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Use optical fibers
Fiber
Allows mechanical stabilization
Spectrograph
Telescope
Note that fiber does notchange magnification
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Improve 'stability' with simultaneous ThAr
If the interorder space is large, one can add a secondslit/fiber. For example :
● Simultaneous calibration lamp light● Sky background● Multiobject fiberfed spectroscopy
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
BowenWalraven image slicer
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Image slicer
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Resolution is not everything
Spectral resolution is not the only parameter thatdetermines how good a spectrograph is
Other important factors are :
● Telescope● Size of the 'slit' on the sky● Short and longterm stability of the instrument● Efficiency (throughput)
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Throughput of FIES
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
Optical layout of UVES
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Moletai Summer School August 2012Fundamentals of echelle spectroscopy
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