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Spectrographs
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Literature:
Astronomical Optics, Daniel Schneider
Astronomical Observations, Gordon Walker
Stellar Photospheres, David Gray
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Spectral Resolution
d
1 2
Consider two monochromatic beams
They will just be resolved when they have a wavelength separation of d
Resolving power:
d = full width of half maximum of calibration lamp emission lines
R = d
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R = 15.000
R = 100.000
R = 500.000
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Spectral Resolution
The resolution depends on the science:
1. Active Galaxies, Quasars, high redshift (faint) objects:
R = 500 – 1000
2. Supernova explosions:
Expansion velocities of ~ 3000 km/s
d/ = v/c = 3000/3x105 = 0.01
R > 100
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R = 3.000
R = 30.000
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35.0000.160100000
60.0000.09130000
100.0000.05310000
140.0000.046000
200.0000.0283000
Rth (Ang)T (K)
3. Thermal Broadening of Spectral lines:
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3000001K
1000003G0
1200025F5
375080F0
2000150A0
R1Vsini (km/s)Sp. T.
4. Rotational Broadening:
1 2 pixel resolution, no other broadening
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5. Chemical Abundances:
Hot Stars: R = 30.000
Cool Stars: R = 60.000 – 100.000
Driven by the need to resolve spectral lines and blends, and to accurately set the continuum.
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6 Isotopic shifts:
Example:
Li7 : 6707.76
Li6 : 6707.92
R> 200.000
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7 Line shapes (pulsations, spots, convection):
R=100.000 –200.000
Driven by the need to detect subtle distortions in the spectral line profiles.
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Line shapes due to Convection
Hot rising cell
Cool sinking lane
•The integrated line profile is distorted.
• Amplitude of distortions ≈ 10s m/s
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R = 200.000
R > 500.000
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8 Stellar Radial Velocities:
RV(m/s) ~ R–3/2 ()–1/2 wavelength coverage
R (m/s)100 000 1 60 000 3 30 000 7 10 000 40 1 000 1200
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collimator
Spectrographs
slit
camera
detector
corrector
From telescope
Anamorphic magnification:
d1 = collimator diameter
d2 = mirror diameter
r = d1/d2
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slit
camera
detector
correctorFrom telescope
collimator
Without the grating a spectograph is just an imaging camera
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A spectrograph is just a camera which produces an image of the slit at the detector. The dispersing element produces images as a function of wavelength
without disperser
without disperser
with disperser
with disperser
slit
fiber
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Spectrographs are characterized by their angular dispersion
d
d
Dispersing element
ddA =
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f
dl
dd
dld = f
In collimated light
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S
dd
dld = S
In a convergent beam
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Plate Factor
P = ( f A)–1
= ( f )–1
dd
P = ( f A)–1
= (S )–1
dd
P is in Angstroms/mm
P x CCD pixel size = Ang/pixel
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w
h
f1
d1
A
D
f
d2
w´
h´
D = Diameter of telescope
d1 = Diameter of collimator
d2 = Diameter of camera
f = Focal length of telescope
f1 = Focal length of collimator
f2 = Focal length of collimator
A = Dispersing element
f2
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w
h
f1
d1
A
D
d2
f
w´
h´
f2
w = slit width
h = slit height
Entrance slit subtends an
angle and ´on the sky:= w/f
´= h/f
Entrance slit subtends an angle
and ´on the collimator:= w/f1
´= h/f1
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w´ = rw(f2/f1) = rDF2
h´ = h(f2/f1) = ´DF2
F2 = f2/d1r = anamorphic magnification due to dispersing element = d1/d2
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w´ = rw(f2/f1) = rDF2
This expression is important for matching slit to detector:2 = rDF2 for Nyquist sampling (2 pixel projection of slit).1 CCD pixel () typically 15 – 20 m
Example 1:
= 1 arcsec, D = 2m, = 15m => rF2 = 3.1
Example 2:
= 1 arcsec, D = 4m, = 15m => rF2 = 1.5
Example 3:
= 0.5 arcsec, D = 10m, = 15m => rF2 = 1.2
Example 4:
= 0.1 arcsec, D = 100m, = 15m => rF2 = 0.6
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5000 A
4000 An = –1
5000 A
4000 An = –2
4000 A
5000 An = 2
4000 A
5000 An = 1
Most of light is in n=0
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b
The Grating Equation
m = sin + sin b 1/ = grooves/mm
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dd =
m cos =
sin + sin cos
Angular Dispersion:
Linear Dispersion:
ddx
dd=
ddx
=1fcam
1
d/d
dx = fcam d
Angstroms/mm
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Resolving Power:
w´ = rw(f2/f1) = rDF2
dx = f2 dd
f2 dd
rDF2
R = /d = Ar
1
d1
D
=rA
D
d1
For a given telescope depends only on collimator diameter
Recall: F2 = f2/d1
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D(m) (arcsec) d1 (cm)
2 1 10
4 1 20
10 1 52
10 0.5 26
30 0.5 77
30 0.25 38
R = 100.000 A = 1.7 x 10–3
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What if adaptive optics can get us to the diffraction limit?
Slit width is set by the diffraction limit:
=
D
R = r
A D
d1
D=
Ar
d1
R d1
100000 0.6 cm
1000000 5.8 cm
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For Peak efficiency the F-ratio (Focal Length / Diameter) of the telescope/collimator should be the same
collimator
1/F 1/F1
F1 = F
F1 > F
1/f is often called the numerical aperture NA
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F1 < F
d/
1
But R ~ d1/
d1 smaller => must be smaller
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Normal gratings:
• ruling 600-1200 grooves/mm
• Used at low blaze angle (~10-20 degrees)
• orders m=1-3
Echelle gratings:
• ruling 32-80 grooves/mm
• Used at high blaze angle (~65 degrees)
• orders m=50-120
Both satisfy grating equation for = 5000 A
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Grating normal
Relation between blaze angle , grating normal, and angles of incidence and diffraction
Littrow configuration:
= 0, = =
m = 2 sin
A = 2 sin
R = 2d1 tan D
A increases for increasing blaze angle
R2 echelle, tan = 2, = 63.4○
R4 echelle tan = 4, = 76○
At blaze peak + = 2
mb = 2 sin cos
b = blaze wavelength
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3000
m=3
5000
m=2
4000 9000
m=1
6000 14000Schematic: orders separated in the vertical direction for clarity
1200 gr/mm grating
2
1
You want to observe 1 in order m=1, but light 2 at order m=2, where 1 ≠ 2 contaminates your spectra
Order blocking filters must be used
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4000
m=99
m=100
m=101 5000
5000 9000
9000 14000
Schematic: orders separated in the vertical direction for clarity
79 gr/mm grating
30002000
Need interference filters but why throw away light?
In reality:
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collimator
Spectrographs
slit
camera
detector
corrector
From telescope
Cross disperser
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y ∞ 2
y
m-2
m-1
m
m+2
m+3
Free Spectral Range m
Grating cross-dispersed echelle spectrographs
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Prism cross-dispersed echelle spectrographs
y ∞ –1
y
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Cross dispersion
y ∞ · –1 =
Increasing wavelength
grating
prism
grism
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Cross dispersing elements: Pros and Cons
Prisms:
Pros:
• Good order spacing in blue
• Well packed orders (good use of CCD area)
• Efficient
• Good for 2-4 m telescopes
Cons:
• Poor order spacing in red
• Order crowding
• Need lots of prisms for large telescopes
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Cross dispersing elements: Pros and Cons
Grating:
Pros:
• Good order spacing in red
• Only choice for high resolution spectrographs on large (8m) telescopes
Cons:
• Lower efficiency than prisms (60-80%)
• Inefficient packing of orders
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Cross dispersing elements: Pros and Cons
Grisms:
Pros:
• Good spacing of orders from red to blue
Cons:
• Low efficiency (40%)
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So you want to build a spectrograph: things to consider
• Chose R product– R is determined by the science you want to do– is determined by your site (i.e. seeing)
If you want high resolution you will need a narrow slit, at a bad site this results in light losses
Major consideration: Costs, the higher R, the more expensive
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• Chose and , choice depends on – Efficiency– Space constraints– „Picket Fence“ for Littrow configuration
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normal
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• White Pupil design? – Efficiency– Costs, you require an extra mirror
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Tricks to improve efficiency:White Pupil Spectrograph
echelle
Mirror 1
Mirror 2Cross disperser
slit
slit
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• Reflective or Refractive Camera? Is it fed with a fiber optic?
Camera pupil is image of telescope mirror. For reflective camera:
Image of Cassegrain hole of Telescope
camera
detector
slit
Camera hole
Iumination pattern
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• Reflective or Refractive Camera? Is it fed with a fiber optic?
Camera pupil is image of telescope mirror. For reflective camera:
Image of Cassegrain hole
camera
detector
A fiber scrambles the telescope pupil
Camera hole
ilIumination pattern
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Cross-cut of illumination pattern
For fiber fed spectrograph a refractive camera is the only intelligent option
fiber
e.g. HRS Spectrograph on HET:
Mirror camera: 60.000 USD
Lens camera (choice): 1.000.000 USD
Reason: many elements (due to color terms), anti reflection coatings, etc.
Lost light
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• Stability: Mechanical and Thermal?
HARPS
HARPS: 2.000.000 Euros
Conventional: 500.000 Euros
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Tricks to improve efficiency:Overfill the Echelle
d1
d1
R ~ d1/
w´ ~ /d1
For the same resolution you can increase the slit width and increase efficiency by 10-20%
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Tricks to improve efficiency:Immersed gratings
Increases resolution by factor of n
n
Allows the length of the illuminated grating to increase yet keeping d1, d2, small
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Tricks to improve efficiency:Image slicing
The slit or fiber is often smaller than the seeing disk:
Image slicers reformat a circular image into a line
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Fourier Transform Spectrometer
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Interferogram of a monchromatic source:
I() = B()cos(2n)
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Interferogram of a two frequency source:
I() = B1()cos(21) + B2(2)cos(22)
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Interferogram of a two frequency source:
I() = Bi(i)cos(2i) = B()cos(2)d–∞
+∞
Inteferogram is just the Fourier transform of the brightness versus frequency, i.e spectrum
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