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GBT Holography

GBT Holography. Claire Chandler Ronald J. Maddalena. Outline. What we know What we need to improve upon Aim of Holography Why we need multiple Holography experiments Plans for Out-of-Focus holography A preview… Plans for Traditional Holography. Telescope Efficiency.

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GBT Holography

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  1. GBT Holography Claire Chandler Ronald J. Maddalena

  2. Outline • What we know • What we need to improve upon • Aim of Holography • Why we need multiple Holography experiments • Plans for Out-of-Focus holography • A preview… • Plans for Traditional Holography

  3. Telescope Efficiency • Repeatable (Gravitational) Focus Tracking • Subreflector Axial Position • Require an accuracy of 1-3 mm in Ys • Currently known to ~10 mm • Subreflector Lateral Shifts • Require an accuracy of 3-6 mm in Xs and Zs. • Xs currently known to ~10 mm • Zs has yet to be determined due to limitations in subreflector motion. • Subreflector Tilts • Require an accuracy of 6’ in Xt, Zt • Currently known to 10’

  4. Telescope Efficiency • Repeatable (Gravitational) Large-Scale Surface Errors • Require 200 μm • 1.2 to 1.5 mm without FEM active surface • Probably 450 μm with FEM active • 70% at 2 GHz, 60% at 20 GHz, 35% at 42 GHz • FEM scale factor is currently assumed. • Projection to normal calculation may be wrong • Small-Scale errors • Unknown magnitude • Probably the same at all elevations.

  5. Current FEM Model

  6. Efficiencies at 20 GHz

  7. Aim of Holography • Flatten efficiency curve • Improve Focus tracking • Determine FEM scale factor, projection to normal • Measure and fix large-scale distortions not predicted by FEM • Raise high-frequency efficiencies • Improve Focus tracking • Determine FEM scale factor, projection to normal • Measure and fix small-scale surface errors

  8. Types of Holography Experiments • Phase-retrieval Holography • Requires very high signal-to-noise • Out-of-Focus Holography • Large-scale errors • Focus tracking • Multiple elevations • No special hardware • “Traditional” phase-reference holography • Large and small scale errors • Focus tracking • Small-scale errors can probably be measured over a small range of elevations. Maybe large-scale errors at multiple elevations. • Requires special hardware

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