Design team reports: Wavefront sensors

V. Velur, J. Bell, A. Moore, C. Neyman V. Velur, J. Bell, A. Moore, C. Neyman Design Meeting (Team meeting #10)Design Meeting (Team meeting #10)

Nov. 5Nov. 5thth, 2007, 2007

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Agenda

• WBS definition• Inputs (as per WSPS)• More input from science team, consensus from Systems

team.• Preliminary WFS design parameters and assumptions.• Justification for certain parameter choices• Technical Challenges• Deliverables (as per revised scope)• Time allocation• Update.

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WBS definition (Initially allocated: 240 hours. Estimate: 260 hrs., Approved on 10/12/07)

• Develop a design concept for each of the required NGAO wavefront sensors: • 3.2.3.5.1 High Order LGS Wavefront Sensors: Given the functional and performance

requirements, develop a design concept for the laser guide star high order wavefront sensors. Take into consideration the possible need for both open and closed loop wavefront sensing.

 • 3.2.3.5.2 High Order NGS Wavefront Sensor: Given the functional and performance

requirements, develop a design concept for the natural guide star high order wavefront sensor(s). Take into consideration the possible need for both open and closed loop wavefront sensing. Include consideration of ADC packaging (ADC design is covered in WBS 3.2.3.8).

 • 3.2.3.5.3 Low Order NGS Wavefront Sensors: Given the functional and performance

requirements, develop a design concept for the low order natural guide star wavefront sensors for the purpose of determining tip/tilt and other low order modes in laser guide star observing mode. Take into consideration the possible need for both open and closed loop wavefront sensing. Include consideration of ADC packaging (ADC design is covered in WBS 3.2.3.8).

 • 3.2.3.5.4 Calibration Wavefront Sensor: Given the functional and performance requirements,

develop a design concept for the calibration wavefront sensor which will use natural guide star light as a truth wavefront. This sensor will be periodically used to reset the references of the high order wavefront sensors in laser guide star mode. Include consideration of ADC packaging (ADC design is covered in WBS 3.2.3.8).

Inputs:• Optical design of the Cascaded relay.

• FRD– Type of each WFS (SH/ PYR)– What order/ # of sub-apertures.– The position each sensor in the Optical Relay.– The FoR for each sensor– Positioning accuracy– Choice of detector(s) for each WFS (pixel size).– Pixel geometry specifics like guard bands, pixel geometry and spot size (for LGS with

appropriate elongation) for each WFS– Centroiding accuracy, dynamic range, and linearity specifications from FRD for each WFS– The Field Stop/ Spatial filter specification– TT sensor specification (FoV, dynamic range, etc.)

• SRD (specifically input of the type of sources on which tip-tilt sensor needs to work and performance margin for binaries, elongated/ asymmetric sources.

• NGAO System Architecture Definition (KAON 499)

• Mechanical drawing(s) w/ space constraints and packaging issues clearly stated for the of Cascaded relay.

• Specification on pick-offs for the WFSs (including the ones shared by the TT(FA) sensors inside the d-NIRI) and rotation if necessary. (input must come from 3.2.3.11)

• Wavefront sensor error budget spreadsheet.

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Need consensus from Science Team & Systems Engg. Team and on:

• Requirements on TT sensing – (Extended objects? Binaries?) – what size, what brightness, what

separation? What performance is required?

• Wavefront sensors– Location of different wavefront sensors

– Type of WFS

– Sensing band

• Extended object guiding (for TWFS and NGS WFS)? - If so, what are the requirements?

• A document was sent out to address Science team members’ concerns – no input/ comments were obtained.

• Some reasonable assumptions have been made as of this time.

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WFS design input parameters

Assumptions:

1TT(FA) shall use extra pixels when guiding on

extended objects.

232x32 NGS case: Planetary sources between 7-10

mag.3 16x16 and 32x32 for LGS is for a de-scope option

4Truth sensor works on a 21st mag. Star with 21 maSec (1D) jitter with 0.6" spot at 1 Hz (EBS

analysis)

WFS type LocationSensing

wavelength (nm)

Input PS (um/")# of sub-apertures

Detector PS("/pixel)

Filters Comments

NGS HOWFSnear NF sci.

inst.400-900 2254 32x32, 64x64 1.5 no filter Steering mirrors for OSM

LGS HOWFS after WF relay 589 nm 72716x16, 32x32,

64x641.45 no filter

Has to track Na layer, tilted focal plane, field dependent

aberrations. Steering mirrors for OSM

TT inside d-NIRI 1.16 - 1.33 727 1x1 0.030J, H and J

& Hwill use d-NIRI OSM

TTFA inside d-NIRI 1.16 - 1.33 727 2x2 0.030J, H and J

& Hwill use d-NIRI OSM

Truth sensornear NF sci.

inst.400-900 2254 5x5 1.6 no filter

do we need two (one at d-NIRI ?) OSM can be steering mirrors for the NF one, and d-NIRI default OSM for truth sensor at the d-

NIRI channel

The TTFA #s are TBC

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Justification for certain parameter choices

• Spot size at WFS governs the plate scale at detector. The spot sizes in the previous chart are based on error budget. The EBS calculation takes the following effects into account:

• No uplink correction• Finite spot size due to aberrations in the uplink beam (for LGS WFS)• Residual seeing in uplink beam (for LGS WFS)• Natural seeing at GS wavelength on downlink (for all WFS)• Elongation due to location of LLT (averaged for LGS).• Extended object guiding for TWFS and NGS-WFS.

• A NGS 15” off-axis that is 21 mv or brighter is assumed for the TWFS

• We will use f/46.5 for calculating plate scale at NF relay focal plane.• NGS WFS needs to guide on slightly extended objects so its FoV is

larger than the LGS WFS.• All WFS are SHWFS.

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Technical Challenges• OSM details have to be figured out for each WFS• It is a hard problem to package 9 LGS WFSs with:

– 5 beacons that lie on a circle with variable radius (focal spots radius varying from 7 mm-146mm)

– 3 roving beacons that go anywhere.– Individual translation stages to account for LGS focal plane with variable tilt.– Combination of doublet and one focusing lens to keep the pupil at the lenslet for a Na-layer

object distance that varies from 90Km-180Km.Should we do this at all (refer to Brian’s note).– The WFS has to move to account for the Na-layer distance varying with zenith angle.– Motion control: 1 lens 1D(T), each LGS WFS 1D(T), whole LGSWFS package 1D(T), radial in-

out for each (but central) WFS 1D (T). OSM (field steering mirrors? 2x2D (tilt)), mechanism to pick off roving beacons! [(T)- translation]

– Shearing spherical plates to create correct coma (do these just need to shear or rotate and sheer?)

• Switching lenslets/ relay optics to allow for multiple pupil sampling scales• The question of on-axis (bright) TT star for guiding?• IR sensors:

– FoV vs. sky (DO we need to guide on extended sources?– OSM, optical design, ME design and packaging strongly dependent on d-NIRI progress.– TT WFS channels shall have individual MEMS DMs (1/2”) to sharper TT stars.

• LGS WFSs will have a 1/2” pupil mirror in the design that can be replaced by a MEMS DM at a later stage.

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Deliverables (as per revised scope)

• Feed into relevant sections of FRD version 2.0 (in particular update TT sensor requirements and performance based on the type of source).

• LGS pick off mechanism concepts.• Conceptual designs and first order optical design for the LGS WFSs,

TT(FA) sensors.• First order Mechanical packaging. • Preliminary mechanical design and 3D model (at least a cartoon

showing the envelopes occupied by the WFSs).• Acceptance and completeness of concepts and conceptual design with

information on what needs to be done during the preliminary design phase.

• Update the terms in the error budget spreadsheet based on conceptual design.

• Documentation for all the above.

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Estimated of time required to complete task

1.First order design: •LGS WFS: 40 hrs.•NGS HOWFS: 16 hrs.•TT and TTFA sensor: 40 hrs.•TWFS/ Calibration WFS: 16 hrs.•Total: 112 hrs.

2.First order Mechanical packaging•LGS WFS: 8 hrs.•NGS HOWFS: 4 hrs.•TT and TTFA sensor: 16 hrs. (need to talk to the IWG)•TWFS/ Calibration WFS: 4 hrs.•Total: 32 hrs.

•Preliminary mechanical design and 3D model i.e. at least a cartoon showing the envelopes occupied by the WFSs. •LGS WFS: 16 hrs.•NGS HOWFS: 12 hrs.•TT and TTFA sensor: 24 hrs. (Assuming we get enough input from the IWG)•TWFS/ Calibration WFS: 16 hrs.•(The hours above also include the time required to integrate the design into the Cascaded Relay ME design).•Total: 68 hrs.

•Meeting to understand risks and look at the acceptance of the work and design with the WFS design team: (4 people x 1 day): 32 hrs.•Documentation of the designs with design risks: 16 hrs.

Total: Initially allocated: 240 hours. Estimate: 260 hrs.

Update:• Concepts

– LGS sensors shall have a fold mirror just before the lenslet. Since the LGS WFS is looking at a narrow field the DM (if implemented) will be located near but not at pupil.

• Demagnification issue: ½” pupil is mapped to 2 mm focal plane. One idea is to use -4f- design use two doublets (Bauman) – field curvature issue .

• A singlet and a doublet based mechanism to keep the pupil fixed as the object distance, in case of LGS observations, changes from 90 Km – 180 Km.

• Working on pick-off mechanisms for the WFSs.

• OSM concept for TT(FA) will be done along with d-NIRI, the LGS WFSs can also have the same kind of pick off mechanism. The truth sensor will have a dichroic/ beam splitter pick-off.

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Supporting effort

• Anna and the IWG are seriously starting to develop conceptual designs for the pick-off mechanism - the pick-off mechanisms for d-NIRI, the TT(FA) and the LGS WFS.

• Ralf, Chris and David are starting to think about TT(FA) performance modeling and reconciling with the error budget predictions. This may also involve re-doing the LOWFS selection KAON. As an aside, 3.1.14 (Science products) task requires new PSFs