Post on 04-Jul-2020
The GMT Consortium Large Earth Finder
Sagi Ben-AmiSmithsonian Astrophysical Observatory
The Giant Magellan Telescope• The GMT is one of the three next generation optical
telescope.• Segmented Gregorian design (six off axis and one on axis 8.4m
mirrors) with an effective aperture of 25.4m diameter.• F/8.2 with a FoV of 24’. Plate Scale of 1’’ mm-1.• Site: Cerro Las Campanas in Chile’s Atacama desert.
The GMT Consortium Large Earth Finder• The GMT Consortium Large Earth Finder (G-Clef) is a general
purpose visible echelle spectrograph that offers precision radial velocity capabilities.
• G-Clef was chosen as the 1st light instrument for the GMT.
G-CLEF: Science Cases• Weighing planets in the
TESS Catalogue.
• O2 in the transmission spectra of exoplanets:A-Band absorption features
between 7600-7700Å .
G-CLEF: Science Cases• Near-field cosmology:
Characterization of metal poor stars – the fossils of structure formation at the earliest phase after the big bang.
• Flash Spectroscopy of SNe:Direct measurements of CSM composition, unique properties of the progenitor.
Science Requirements
So how do we achieve all of these ?• A fiber-fed high dispersion spectrograph thermally stabilized deployed at a gravity
invariant location. • Pass band: 3500-9500Å
• Different resolution is achieved by feeding the spectrograph with fibers of different core diameters.
dl = Pw '
Optical Path: Fiber Run
(Pupil Slicer)
Front-end F/# converters.
Vacuum Enclosure Interface
(Scrambler)
Spectrograph F/# converters.
(Exposure Meter)
0 . 4 9
0 . 1 4
0 . 1 2
0 . 1 0
0 . 1 0
0 . 1 2
0 . 1 4
0 . 0 7
0 . 3 0
0 . 3 4
0 . 3 8
0 . 3 8
0 . 2 0 0 . 2 0
0 . 7 6
0 . 3 2 5
0 . 6 5 0
0 . 6 5
6 . 0 0 6 . 0 0 6 . 0 0
B U F F E RC L A D D I N G
CORE
H O R I Z O N T A L A X I S N O T T O S C A L EB U F F E R
C L A D D IN G
CORECORE
C L A D D I N G
B U F F E R
B U F F E R
C L A D D IN G
CORE
F I B E R L A Y O U T
N S - P R V M o d e P R V M o d e M R M o d e H T M o d e
1 0 0 : 1
1 O F 1
T h e d a t a i n t h i s d o c u m e n t i n c o r p o r a t e p r o p r i e t a r y r i g h t s o f t h e S m i t h s o n i a n A s t r o p h y s i c a l O b s e r v a t o r y . A n y p a r t y a c c e p t i n g t h i s d o c u m e n t d o e s s o i n c o n f i d e n c e a n d a g r e e s t h a t i t s h a l l n o t b e d u p l i c a t e d i n w h o l e o r i n p a r t , n o r d i s c l o s e d t o o t h e r s , w i t h o u t t h e c o n s e n t o f t h e S m i t h s o n i a n A s t r o p h y s i c a l O b s e r v a t o r y .
C O M P O N E N TW E I G H T ( k g ) :
D E S C R I P T I O N
D W GN O .
S C A L E
S H E E T
C H E C K E DB Y :
G R P L E A DA P P R O V A L :
± 0 . X X = ± 0 . 2 5 ± 0 . X = ± 0 . 5
N O D E C I M A L = ± 1A N G L E S = ± 1 º
S M I T H S O N I A N A S T R O P H Y S I C A L
O B S E R V A T O R Y1 0 0 A c o r n P a r k D r i v e
C a m b r i d g e , M A 0 2 1 4 0
R E V I S I O N
S H E E T S I Z E
T H I R D A N G L E P R O J E C T I O N
T O L E R A N C E S U N L E S SO T H E R W IS E N O T E D
P B S N O .
W O R K F L O WS T A T E :
D R A F T E DB Y :
D E S I G N E DB Y :
C H I E F E N GA P P R O V A L :
R E L E A S E D A T E :
C O M P O N E N T M A T E R I A L :
B
S A O
Spectrograph Optical Design
Grating: A mosaic of three 300x400mm R4 operating at a quasi-Littrow configuration.
Mangin Fold Mirror to correct cylindrical field curvature
 =ldl
=dArf
d1
D
G-Clef Red Camera• The re-designed red camera is a 7-element
camera with one aspheric surface on the back surface of the 1st lens (Glass substrate).
• Power is mainly due to CaF2 positive lenses, with i-line negative and meniscus lenses to control chromatic aberrations.
Passband 5400-9520Å (Orders 65-113)
Focal Length 450mm
Beam Diameter 250mm
FoV 7.7°
Testable in collimated light (air-space).
Yes
0
5
10
15
20
0
5
10
15
20
0
5
10
15
20
5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 100000
5
10
15
20
Wavelength [Ang]
Box Edge Size [mm]
Run HE081715AGs − Ensquared Energy
Fiber5 67% AHsFiber5 80% AHs
Fiber6 67% AHsFiber6 80% AHs
Fiber7 67% AHsFiber7 80% AHs
Fiber8 67% AHsFiber8 80% AHs
0
5
10
15
20
0
5
10
15
20
0
5
10
15
20
5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 100000
5
10
15
20
Wavelength [Ang]
Box Edge Size [mm]
Run 0HE081715AGs − Ensquared Energy
Fiber1 67% AHsFiber1 80% AHs
Fiber2 67% AHsFiber2 80% AHs
Fiber3 67% AHsFiber3 80% AHs
Fiber4 67% AHsFiber4 80% AHs
Red Camera: PRV Ensquared Energy• Center-to-center distance between fibers in pseudo-slit
increased to 170μm.• 80% Ensquared energy below 18μm (Nyquist for STA 9μm
pixels) across the entire echellogram.
X-dispersers: VPH Gratings• VPH Grating: A modulation in the index of refraction induced
by holographic exposure of dichromatic gelatin. • Higher efficiency than common ruled gratings.
Blue X-disperser:A VPH-Prism.
3000 3500 4000 4500 5000 55004
5
6
7
8
9
10
11
12
13
Wavelength [Ang]
Angle (deg)
VPH Gelatin AoI and AoE
VPH Ghosts Mitigation: Tilted Fringes• Narcissistic Ghosts: Scattering from Gelatin-Glass interface
after reflection from the detector .• Littrow ghost: Recombination of cross dispersed orders by the
VPH .
m= 1; m̀= 0
Dm= 0
sinb ' =Dmls cosg
+ sina
Tilted Fringes
• By introducing a tilt to the imprinted fringes, we move the operation wavelength away from the Littrow configuration.
• The tilt should be large enough so that the ghosts are moved away from the detector.
sinbB = n2 sin arcsinsinan2
æèç
öø÷- 2f
é
ëê
ù
ûú
Db = a - bB
Narcissistic Ghost• The increased VPH-camera angle deflects ghost beams to
higher angles.
Reflected Beams
Narcissistic Beams
Detector
Littrow Ghost• Tilted fringes ensure that we no longer operate in Littrow
configuration, and so the recombined rays miss the detector after recombination.
Detector
Red Arm Echellogram• Entire Echellogram fits well onto the detector (92.4×92.2mm),
with at least 1mm for alignment in each direction.
−21.5 −21 −20.5 −20 −19.5 −19
40
40.5
41
41.5
42
42.5
43
[mm]
[mm]
Blue Arm Echellogram• Entire Echellogram fits well onto the detector (92.4×92.2mm), with at
least 0.5mm for alignment in each direction.