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What’s the
best LGS upgrade
for LINC-NIRVANA?
Sebastian Egner
Wolfgang Gaessler, C. Arcidiacono,
Th. Bertram, B. Goldman, E. Masciadri,
J. Stoesz
Ringberg, 02 November 2007
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Overview
• Introduction:
What is LINC-NIRVANA?
• Motivation:
Why should we want Laser-Guide-Stars for
LINC-NIRVANA?
• Selection:
Which Laser-Guide Star would be the best for
LINC-NIRVANA?
• Implementation:
How can a LGS be implemented into LINC-NIRVANA?
• Conclusion:
What should we do?
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
What is LN?
LINC-NIRVANA:
• Fizeau (imaging) interferometer for the LBT:
combine the light from the two primary mirrors of the LBT to achieve
an effective resolution of a 23m telescope
• Science detector:
– 10x10’’ FoV of science detector
– J to K-band
– 0.005”/pixel (diff. limit in J: 0.011’’)
•
Fringe-tracker:
– measure and correct the OPD between the two arms to keep the fringes
– J to K-band
– 1x1.5’ FoV
•
MCAO system:
– 6’ diameter field of natural guide stars
– 20 NGS in each arm
– two deformable mirrors in each arm:
• Deformable secondary of the LBT (correcting ground-layer turbulence)
• Piezo DM inside the instrument (correct high-layer turbulence)
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
What is LN?
LINC-NIRVANA:
• Fizeau (imaging) interferometer for the LBT:
combine the light from the two primary mirrors of the LBT to achieve
an effective resolution of a 23m telescope
• Science detector:
– 10x10’’ FoV of science detector
– J to K-band
– 0.005”/pixel (diff. limit in J: 0.011’’)
•
Fringe-tracker:
– measure and correct the OPD between the two arms to keep the fringes
– J to K-band
– 1x1.5’ FoV
•
MCAO system:
– 6’ diameter field of natural guide stars
– 20 NGS in each arm
– two deformable mirrors in each arm:
• Deformable secondary of the LBT (correcting ground-layer turbulence)
• Piezo DM inside the instrument (correct high-layer turbulence)
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
What is LN?
LINC-NIRVANA:
• Fizeau (imaging) interferometer for the LBT:
combine the light from the two primary mirrors of the LBT to achieve
an effective resolution of a 23m telescope
• Science detector:
– 10x10’’ FoV of science detector
– J to K-band
– 0.005”/pixel (diff. limit in J: 0.011’’)
•
Fringe-tracker:
– measure and correct the OPD between the two arms to keep the fringes
– J to K-band
– 1x1.5’ FoV
•
MCAO system:
– 6’ diameter field of natural guide stars
– 20 NGS in each arm
– two deformable mirrors in each arm:
• Deformable secondary of the LBT (correcting ground-layer turbulence)
• Piezo DM inside the instrument (correct high-layer turbulence)
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
What is LN?
LINC-NIRVANA:
• Fizeau (imaging) interferometer for the LBT:
combine the light from the two primary mirrors of the LBT to achieve
an effective resolution of a 23m telescope
• Science detector:
– 10x10’’ FoV of science detector
– J to K-band
– 0.005”/pixel (diff. limit in J: 0.011’’)
•
Fringe-tracker:
– measure and correct the OPD between the two arms to keep the fringes
– J to K-band
– 1x1.5’ FoV
•
MCAO system:
– 6’ diameter field of natural guide stars
– 20 NGS in each arm
– two deformable mirrors in each arm:
• Deformable secondary of the LBT (correcting ground-layer turbulence)
• Piezo DM inside the instrument (correct high-layer turbulence)
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
What is LN?
LINC-NIRVANA:
• Fizeau (imaging) interferometer for the LBT:
combine the light from the two primary mirrors of the LBT to achieve
an effective resolution of a 23m telescope
• Science detector:
– 10x10’’ FoV of science detector
– J to K-band
– 0.005”/pixel (diff. limit in J: 0.011’’)
•
Fringe-tracker:
– measure and correct the OPD between the two arms to keep the fringes
– J to K-band
– 1x1.5’ FoV
•
MCAO system:
– 6’ diameter field of natural guide stars
– 20 NGS in each arm
– two deformable mirrors in each arm:
• Deformable secondary of the LBT (correcting ground-layer turbulence)
• Piezo DM inside the instrument (correct high-layer turbulence)
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
What is LN?
LINC-NIRVANA:
GWS field
• Fizeau (imaging) interferometer for the LBT:
combine the light from the two primary mirrors of the LBT to achieve
an effective resolution of a 23m telescope
• Science detector:
•
Fringe-tracker:
2
arcmin
– 10x10’’ FoV of science detector
– J to K-band
– 0.005”/pixel (diff. limit in J: 0.011’’)
– measure and correct the OPD between the two arms to keep the fringes
– J to K-band
MHWS field
– 1x1.5’ FoV
•
MCAO system:
– 6’ diameter field of natural guide stars
– 20 NGS in each arm
– two deformable mirrors in each arm:
6 arcmin
• Deformable secondary of the LBT (correcting ground-layer turbulence)
• Piezo DM inside the instrument (correct high-layer turbulence)
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Why LGS for LN?
Why thinking about LGS for LINC-NIRVANA?
• currently “Phase-A study” for LGS at the LBT
(more details in the talks by S. Rabien and L. Busoni)
• Assumption here:
– LINC-NIRVANA will be working!
– We will get a LGS system at the LBT!
– But not yet sure what kind of LGS:
maybe we can push in our favor?
• Answers to questions like:
– Is an upgrade for LINC-NIRVANA from multiple natural guide
stars to LGS useful?
– What is the possible increase in sky-coverage / performance /
throughput?
– How easily can LINC-NIRVANA be upgraded for a LGS WFS?
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Why LGS for LN?
Increase in sky-coverage?
• not directly:
sky coverage for LINC-NIRVANA limited by OPD guide star,
not by MCAO system:
galactic plane
MCAO: 95% / 45% (galactic plane, galactic poles)
OPD: 50% / 6% (C. Arcidiacono)
• BUT:
OPD guide star can be fainter if Strehl ratio is higher:
higher S/N ratio for fringe-tracking
• increase the Strehl-ratio from 10% to 25%
for 0.5’ off-axis OPD guide star:
galactic pole
 ~1 mag fainter OPD guide star,
 ~40% increase in sky coverage
at the galactic poles (Th. Bertam, C. Arcidiacono)
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Why LGS for LN?
Increase in performance / throughput?
for LINC-NIRVANA this can be achieved by:
– reduce OPD error AND / OR
– increase Strehl ratio of science object
• Reduce OPD error:
– same argument as for sky-coverage:
increase of the Strehl ratio for the off-axis OPD guide star
– better S/N ratio on the OPD sensor
 better determine the OPD of the atmosphere
– for increase of Strehl ratio from 10% to 25%
 2 times less OPD error,
 extract more high-spatial frequency information
(23m telescope information) from the images
• Increase Strehl ratio for science object:
– content of high-spatial frequency information ~ linear with Strehl-ratio
• overall performance is product of individual high-spatial
frequency information
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Why LGS for LN?
Increase in performance / throughput?
for LINC-NIRVANA this can be achieved by:
– reduce OPD error AND / OR
– increase Strehl ratio of science object
• Reduce OPD error:
– same argument as for sky-coverage:
increase of the Strehl ratio for the off-axis OPD guide star
– better S/N ratio on the OPD sensor
 better determine the OPD of the atmosphere
– for increase of Strehl ratio from 10% to 25%
 2 times less OPD error,
 extract more high-spatial frequency information
(23m telescope information) from the images
• Increase Strehl ratio for science object:
– content of high-spatial frequency information ~ linear with Strehl-ratio
• overall performance is product of individual high-spatial
frequency information
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Why LGS for LN?
Increase in performance / throughput?
for LINC-NIRVANA this can be achieved by:
– reduce OPD error AND / OR
– increase Strehl ratio of science object
• Reduce OPD error:
– same argument as for sky-coverage:
increase of the Strehl ratio for the off-axis OPD guide star
– better S/N ratio on the OPD sensor
 better determine the OPD of the atmosphere
– for increase of Strehl ratio from 10% to 25%
 2 times less OPD error,
it makes sense to
higher
 extract
moresky-coverage
high-spatial frequency information
(23m telescope information) from the images
look deeper into LGS
possibly higher performance 
• Increase Strehl ratio for science object:
for
higher
throughput
– content
of high-spatial
frequency information
~ linear with Strehl-ratio
LINC-NIRVANA
• overall performance is product of individual high-spatial
frequency information
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Why LGS for LN?
Increase in performance / throughput?
for LINC-NIRVANA this can be achieved by:
– reduce OPD error AND / OR
– increase Strehl ratio of science object
• Reduce OPD error:
– same argument as for sky-coverage:
increase of the Strehl ratio for the off-axis OPD guide star
– better S/N ratio on the OPD sensor
 better determine the OPD of the atmosphere
– for increase of Strehl ratio from 10% to 25%
 2 times less OPD error,
it makes sense to
higher
 extract
moresky-coverage
high-spatial frequency information
(23m telescope information) from the images
look deeper into LGS
possibly higher performance 
• Increase Strehl ratio for science object:
for
Strehl ratio
0.7
0.2
0.03
1.0
higher
throughput
– content
of high-spatial
frequency information
~ linear with Strehl-ratio 0.0
LINC-NIRVANA
• overall performance is product of individual high-spatial
frequency information
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Which LGS for LN?
4 options for LGS:
Single Rayleigh
Multiple Rayleigh
(6 to 30km altitude)
(6 to 30km altitude)
Single Sodium
Multiple Sodium
(90 km altitude)
(90 km altitude)
we would like to have:
• large field (1.5’ diameter) for OPD guide star & science object:
– sky-coverage can only be increased, when Strehl ratio for
off-axis OPD guide star AND on-axis science object is high
– performance increase requires good correction of OPD guide star
AND science object
• good on-axis correction for good science images
– increase in throughput and performance
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Which LGS for LN?
1st step:
have a look at the atmospheric turbulence above Mt. Graham:
• SCIDAR measurements for 16 nights (more observations this week)
– measure the strength of the turbulence as a function of the altitude
above the mountain (CN2 profile)
• Integral turbulence profile:
– total amount of turbulence below a given altitude
– we find: concentration of the turbulence near the ground
75% below 2 km (conventional G-SCIDAR)
50% of turbulence below 170m (newly developed HVR-GS method)
– is multiple Rayleigh already enough?
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Which LGS for LN?
1st step:
have a look at the atmospheric turbulence above Mt. Graham:
• SCIDAR measurements for 16 nights (more observations this week)
– measure the strength of the turbulence as a function of the altitude
above the mountain (CN2 profile)
• Integral turbulence profile:
– total amount of turbulence below a given altitude
– we find: concentration of the turbulence near the ground
75% below 2 km (conventional G-SCIDAR)
50% of turbulence below 170m (newly developed HVR-GS method)
75% below 2 km
– is multiple Rayleigh already enough?
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Which LGS for LN?
Height above the telescope
20 km
1st step:
have a look at the atmospheric turbulence above Mt. Graham:
• SCIDAR measurements for 16 nights (more observations this week)
– measure the strength of the turbulence as a function of the altitude
above the mountain (CN2 profile)
• Integral turbulence profile:
– total amount of turbulence below a given altitude
– we find: concentration of the turbulence near the ground
75% below 2 km (conventional G-SCIDAR)
50% of turbulence below 170m (newly developed HVR-GS method)
75% below 2 km
– is multiple Rayleigh already enough?
0 km
number of the night
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Which LGS for LN?
More quantitative estimate of the correction efficiency of various
configurations:
• Assumption:
perfect Adaptive Optics system (incl. tomography)
• Consider only cone-effect
• Calculate for each turbulent layer the correction efficiency:
upper limit for the fraction of the turbulence which can be seen and corrected by AO system
•
•
Multiply the measured vertical turbulence profiles with this correction efficiency profile
Study case:
– LGS System:
• 3 laser guide stars, triangular arrangement
• 6, 30 & 90 km altitude
• 0 – 12’ FoV diameter
– Atmosphere:
• turbulence profile as measured at Mt. Graham
• assume 0.8’’ seeing in V-band
• 30% Strehl ratio
 requires correction of between 85% (K) and 93% (J) of the
turbulence
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Which LGS for LN?
More quantitative estimate of the correction efficiency of various
configurations:
• Assumption:
perfect Adaptive Optics system (incl. tomography)
• Consider only cone-effect
• Calculate for each turbulent layer the correction efficiency:
upper limit for the fraction of the turbulence which can be seen and corrected by AO system
•
•
Multiply the measured vertical turbulence profiles with this correction efficiency profile
Study case:
– LGS System:
• 3 laser guide stars, triangular arrangement
• 6, 30 & 90 km altitude
• 0 – 12’ FoV diameter
– Atmosphere:
• turbulence profile as measured at Mt. Graham
• assume 0.8’’ seeing in V-band
• 30% Strehl ratio
 requires correction of between 85% (K) and 93% (J) of the
turbulence
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Which LGS for LN?
6 km
our requirement
Sebastian Egner
90 km
30 km
Laser guide stars for LINC-NIRVANA
our requirement
Ringberg, 02 November 2007
Which LGS for LN?
Sodium LGS: what is the optimal separation?
single LGS
3 LGS, 1’
3 LGS, 2’
3 LGS, 3’
3 LGS, 4’
3 LGS, 6’
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Which LGS for LN?
Configuration summary:
• (multiple) Rayleigh:
cannot correct the turbulence at high altitudes:
 correction is not good enough
• Single Sodium:
– acceptable correction efficiency
– but small field size

only useful in mixed-mode (NGS MCAO + LGS AO)
for Strehl-boost on-axis or for OPD guide star
• Multiple Sodium:
– good correction over a large field
– compromise between FoV and Strehl-ratio on-axis

significant increase in overall performance (OPD correction,
Strehl-ratio, sky-coverage) achievable over 1.5’ field
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Modify LN for LGS?
How can sodium LGS wavefront sensing be
implemented into LINC-NIRVANA?
Which WFS for the LGS?
•
MH-WFS (FoV 2’):
– close to the optical aixs of the science detector
– use for the correction of the high order modes (isoplanatic angle)
and thus as the WFS for the LGS
– opto/mechanical modifications?
•
GWS (FoV 6’):
–
–
–
–
use 12 NGS for tip/tilt correction
isokinetic angle (tip/tilt correction) is larger than isoplanatic angle
average the tip/tilt signal from all NGS around the optical axis
can use very faint guide stars (optical co-addition)
 no limitation in sky-coverage
– no opto-mechanical modifications required
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Modify LN for LGS?
MH-WFS: can it also sense LGS?
Optical image quality?
•
Sodium LGS:
– optical image quality remains good
(even better than for NGS because of single wavelength…)
– pupil images remain good, very little distortion (important for pyramid)
– small defocus: 307 mm,
can be easily achieved by adding a trombone into the beam to the MH-WFS
 Sodium LGS can be relatively easily implemented into LINC-NIRVANA,
open question: do pyramids work with LGS?
•
Rayleigh LGS:
– large defocus and vignetting:
• 30 km (920 mm defocus, 25% vignetted)
• 6 km (4.5m, 90% vignetted)
 Rayleigh LGS require complete re-design of WFS system
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Modify LN for LGS?
MH-WFS: can it also sense LGS?
Optical image quality?
•
Sodium LGS:
– optical image quality remains good
(even better than for NGS because of single wavelength…)
– pupil images remain good, very little distortion (important for pyramid)
– small defocus: 307 mm,
can be easily achieved by adding a trombone into the beam to the MH-WFS
 Sodium LGS can be relatively easily implemented into LINC-NIRVANA,
open question: do pyramids work with LGS?
• Rayleigh LGS:
poly-chromatic
WFS-PSF for NGS
– large defocus and vignetting:
mono-chromatic WFS-PSF for
sodium LGS
• 30 km (920 mm defocus, 25% vignetted)
• 6 km (4.5m, 90% vignetted)
 Rayleigh LGS require complete re-design of WFS system
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007
Conclusion
LGS is useful for LINC-NIRVANA:
•
•
for sky-coverage:
for performance:
•
for throughput:
increase in off-axis Strehl-ratio (multiple sodium LGS)
increase on-axis and / or off-axis Stehl-ratio
(single or multiple sodium LGS)
increase on-axis and / or off-axis Strehl ratio
(single or multiple sodium LGS)
Impact on
LGS Strategy
Performance
Sky-Coverage
Instrument
Single Rayleigh
6 – 30 km
Low: high-altitude
turbulence
Low: no increase in
Strehl-ratio cfg. NGS
MCAO
Huge: re-design WFS
unit
Multiple
Rayleigh
2’ / 6 – 30 km
Medium for 30 km:
cone-effect
Low: little increase in
off-axis Strehl-ratio
Huge: re-design WFS
unit
Single Sodium
90 km
Medium: cone-effect,
Strehl-boost on-/offaxis when combining
with NGS MCAO
Medium: Strehl-boost
off-axis for OPD guide
star when combining
with NGS MCAO
Low: only refocus WFS
unit, good image quality
Medium: for NGS/LGS
combination
Multiple Sodium
2’ / 90 km
High: good correction
over 2’ FoV
High: better Strehlratio offaxis
Low: only refocus WFS
unit, good image quality
Sebastian Egner
Laser guide stars for LINC-NIRVANA
Ringberg, 02 November 2007