D. Measurement Approach

Phil HinzPrincipal Investigator

Measurement and Analysis Compliance

• This presentation discusses the approach to measuring and analyzing HOSTS observations and demonstrates performance

• Relationship to ORR Success Criteria: The presentation demonstrates that the 12-zodi sensitivity is satisfied

• Concerns: None• Liens: None

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1.a LBTI demonstrates 12-zodi sensitivity (1, single star measurement, PLRA zodi model)Green

A Quick Tour of the LBT

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Adaptive Secondary (M2)

Prime Focus Camera(stowed)

Swing Arm

Tertiary (M3)

DX (right)Primary (M1)

Multi-Object DoubleSpectrograph (MODS)

C-Ring

Azimuth Bearing

LBTI

Bent GregorianFocal Station

SX (left)Primary (M1)

Uprights WindBracing

ARGOS LaserLaunch Telescope

Dynamic BalanceElement

Rear View of LBT

DX Mirror Cell

All telescope optics outfitted with seismic accelerometer to completely define their motion in all 6 DOF at 1 kHz

Key Parameters of the LBT

• Sensitivity

• Wavefront precision

• Simple, cryogenic beam-combination

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LBT AO SecondaryHigh-StrehlAO-corrected image inJ-Band

LBTI Layout

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Beam-combiner optics are at 80 K

Nulling Optical Path

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Nulling Optics are at 80 K

Phase sensing is completely common-path

Control System

• Three main loops:– Left AO system– Right AO system– Phasing system

• All coordinated by the science camera (NOMIC)

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Manual Set-Point Adjustment

Phase Sensing Approach

• Pupil image is formed at 2.2 µm wavelength– Wedge in the beamsplitter creates a tilt at K band for overlapped

11-µm images– Argument from Fourier Transform gives phase error signal at 1 kHz– Group delay is encoded in the visibility of the fringes across the pupil

• PID and discrete vibration filtering used for closed loop control– Feed-forward from accelerometers gives error signal at 1 kHz

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1 kHz

1 Hz

Accelerometer Inputs

Overall Observing Sequence

• The HOSTS observing sequence for each science object is set up as follows:

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Acquisition Process (Once per Pointing)

• Each pointing requires– Object acquisition – AO setup– Fringe acquisition– Phase sensor setup– Null acquisition

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Set-point adjustmentwill be automated

during SV

Offset Process (Once per Nod)

• Each offsetrequires– Telescope offset

command– AO pause and

resume– Manual phase-loop

closing

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Phase-loop automatedpause and resume willbe implemented during SV

Survey Optimization

• Why not observe, say, 150 stars?– The sample of “good” stars for LBTI’s observational parameters are 50-

100. Detectability drops off for additional stars– The Mission Success Analysis, suggests sensitivity is preferred to

number of stars (although both help)

• So, why not observe 20 stars more deeply?– Improvements using very long exposures may not be realizable

• Have you considered more/less dwell time per nod?– This is set by an attempt to balance efficiency and low frequency

background effects– Less would be preferable and will be explored during SVP

• Have you considered more/less dwell time per pointing?– This is set by an attempt to measure slow drifts– No drifts are seen in data. Could perhaps be lengthened

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