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LGS WFS Design Status & Issues
Dekany, Delacroix, & Velur
Caltech Optical Observatories
LGS WFS concept at SF review
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LGS WFS parameters
WFS type (#
of sensors)
Location
Sensing
wavelength
(nm)
Input
platesc
ale
(um/")
# of subapertures
Detector
platescale(
"/pixel)
Filters
Pupil
sampling
Field of
regard
(arcsec)
Comments
LGS
HOWFS
(9)
after
WF
relay
589 nm
727
16x16,
32x32,
64x64
1.45
no
filter
64x64,
32x32,
16x16
174
Theta-phi
OSM
Location
Type
Range of travel
Min. step size
(est.)
LGS channel
(individual unit
focus)
delta Z
~10 mm
10 um
LGS lenslet changer
deltaX,
Y
delta X = 20 mm, delta
Y = 2 mm
1 um
LGS channel relay
and camera focus
for pupil sampling
change
delta Z
5 mm
0.1 um
Pick off (theta
mech)
Pick off (phi mech.)
LGS WFS unit overall
travel
Rotation of the WFS
(sans the central
WFS)
delta
theta
delta
phi
0.2 (UP)/ 115"
(LA)
0.2" (UP)/ 300"
(LA)
360 deg.
360 deg.
delta Z
130 mm
1 mm
delta
(thetaphi)
360 deg.
100"
WFS type
Length in Z
[mm]
Breadth X
[mm]
Width Y
[mm]
# of
sensors
Pick off mech.
Cooling
LGS WFS
850
150
150
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LGS theta-phi
3 stage
Peltier
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SF review LGS WFS conceptual design
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Post SF review changes to LGS WFS architecture
•
•
•
•
•
•
4 fixed + 3 (PnS) LGS sensors (as compared to 1 fixed + 8 mobile sensors)
Field of regard of PnS sensors is 120 arcsec dia. and the fixed sensors are
positioned to accept LGS light at 10 arcsec. radius (as compared to 10-150
arcsec continuous FoR (dia.) for sensors in the pentagon formation and 10174” arcsec continuous FoR (dia.) for PnS sensors)
Only 2 pupil sampling scales – 64x64, 32x32 (no 16x16).
LGS TT mirror to be implemented on the downlink – not appraised as part
of WFS costs, neither have the motion controls for these mirrors defined as
part of LGS WFS (this was a recent change).
LGS WFS’s are not refrigerated and are fed through a window (or a pair of
windows) – sensors are not required to operate -17C but need to work at -5
C.
Lesser motion control
Total laser power has been reduced and the sodium return numbers have been scaled
accordingly for performance in Mauna Kea.
5
Current optical design (Lick)
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LGS WFS design concept (needs refinement)
Camera Rotation axis
Reflective
tetrahedron
Fixed
frame
4 Fixed Cameras
Camera Translation
LGS WFS TT stages are not implemented yet (we
realized the need to accommodate TT sensor last
week).
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FLAMINGOS-2 OIWFS
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LGS WFS issues
•
The most extreme RMS wavefront error in the LGS path is 1.5-1.7micron, and the
largest spot size is about 250 miliarcsec - these static aberrations on the LGS caused
a shift of the centroids of 5% of a sub-aperture (at most) [Kupke]
Laser Guide Star Size Calculation
Finite Object Size
We have started working
with Lick to understand
what WF quality is required
at the LGS input (Gemini
and TMT may have overspecified their sensor).
Intrinsic guide star diameter
0.00
arcsec
Inherent aberrations in the uplink beam:
0.90
arcsec
Beam movement contribution to uplink
0.33
arcsec
Residual seeing contribution to uplink
0.60
arcsec
Diameter of point source laser at Na layer:
1.08
arcsec
Natural seeing FWHM at GS wavelength
0.58
arcsec
Subaperture Tip/Tilt corrected FWHM
0.47
arcsec
Contribution due to seeing
0.47
arcsec
0.00
m
1.27
arcsec
Avg. Elongation
0.85
arcsec
Contribution due to elongation
0.85
arcsec
0.25
arcsec
RMS blurring due to atmospheric dispersion
0.000
arcsec
Total size of detected return beam:
1.47
arcsec
Charge Diffusion
0.25
pixels
Contribution due to Charge Diffusion
0.40
arcsec
0.66
arcsec
1.66
arcsec
Uplink formation of the beacon(s)
Perfect Uplink AO?
NO
Seeing
Elongation
Distance from LLT to telescope axis:
Use Max. Elongation?
NO
System Aberrations
Aberrations in AO thru to WFS
Atmospheric Dispersion
ADC in HOWFS?
NO
Sensing Approach
Pyramid WFS?
NO
Charge Diffusion
Subaperture Diffraction
Lambda/d (for sensing)
Spot size used for centroiding
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LGS WFS design concept
Pick off
arms search
area
Pick off arms
effective 120”
search area
Tetrahedron to direct
light to fixed WFS’s.
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Issues & to-dos:
• Need to update all relevant requirements and nail down as many #s
and parameters as possible.
• Refine the LGS sensor concept to work as 4 fixed + 3 PnS
(conceptual work has already started).
• Redesign pick off arm with pupil access and TTM.
• Now that a 256x256 pixel CCD is being talked about in meetings –
should we make it our baseline? If so, what is the pixel size?
• We need to model 2 more windows in zemax as the sensor resides
outside the refrigerated chamber.
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LGS spots with sensor in the current optical design
The AO design team
and the WFS team are
trying to hash out
details to be able to
build a integrated
optical model with
version control.
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Other WFS related changes
• No narrow field TWFS.
• No ADC on NGS WFS.
• No ADC in the TWFS.
• In general, the system has undergone many changes, any design
work without nailing down requirements might mean re-work.
• Next 2 weeks will mostly be spent in updating the requirements
database.
• Alex is already obtained a Contour account and is getting familiar
with the software, the requirements and the PBS.
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