LGS WFS Design Status & Issues

Dekany, Delacroix, & Velur

Caltech Optical Observatories

LGS WFS concept at SF review

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LGS WFS parameters

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LGS HOWFS

(9)

after WF

relay589 nm 727

16x16, 32x32, 64x64

1.45no

filter

64x64, 32x32, 16x16

174Theta-phi

OSM

Location Type Range of travelMin. step size

(est.)LGS channel

(individual unit focus)

delta Z ~10 mm 10 um

LGS lenslet changerdeltaX,

Ydelta X = 20 mm, delta

Y = 2 mm1 um

LGS channel relay and camera focus for pupil sampling

change

delta Z 5 mm 0.1 um

Pick off (theta mech)delta theta

360 deg.0.2 (UP)/ 115"

(LA)

Pick off (phi mech.)delta phi

360 deg.0.2" (UP)/ 300"

(LA)LGS WFS unit overall

traveldelta Z 130 mm 1 mm

Rotation of the WFS (sans the central

WFS)

delta (theta-

phi)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 9 LGS theta-phi 3 stage Peltier

WFS type (# of sensors)

LocationSensing

wavelength (nm)

Input platesc

ale (um/")

# of sub-apertures

Detector platescale("

/pixel)Filters

Pupil sampling

Field of regard

(arcsec)Comments

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 10-174” 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.

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Current optical design (Lick)

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LGS WFS design concept (needs refinement)

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Fixed frame

Camera Rotation axis

Camera Translation4 Fixed Cameras

Reflective tetrahedron

LGS WFS TT stages are not implemented yet (we realized the need to accommodate TT sensor last week).

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]

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Laser Guide Star Size Calculation

Finite Object Size

Intrinsic guide star diameter 0.00 arcsec

Uplink formation of the beacon(s)

Perfect Uplink AO? NO

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

Seeing

Natural seeing FWHM at GS wavelength 0.58 arcsec

Subaperture Tip/Tilt corrected FWHM 0.47 arcsec

Contribution due to seeing 0.47 arcsec

Elongation

Distance from LLT to telescope axis: 0.00 m

Use Max. Elongation? NO 1.27 arcsec

Avg. Elongation 0.85 arcsec

Contribution due to elongation 0.85 arcsec

System Aberrations

Aberrations in AO thru to WFS 0.25 arcsec

Atmospheric Dispersion

ADC in HOWFS? NO

RMS blurring due to atmospheric dispersion 0.000 arcsec

Total size of detected return beam: 1.47 arcsec

Sensing Approach

Pyramid WFS? NO

Charge Diffusion

Charge Diffusion 0.25 pixels

Contribution due to Charge Diffusion 0.40 arcsec

Subaperture Diffraction

Lambda/d (for sensing) 0.66 arcsec

Spot size used for centroiding 1.66 arcsec

We have started working with Lick to understand what WF quality is required at the LGS input (Gemini and TMT may have over-specified their sensor).

LGS WFS design concept

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Pick off arms search

area

Pick off arms effective 120”

search area

Tetrahedron to direct light to fixed WFS’s.

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

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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.

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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