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Refined Scientific Requirements for the PTCS

Refined Scientific Requirements for the PTCS. Jim Condon. GBT Science Summary – Frequency Coverage. Tracking errors: sensitivity loss and flux uncertainty. Distributions of flux uncertainties from tracking errors. Pointing requirements. 2003 October: improved blind pointing

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Refined Scientific Requirements for the PTCS

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  1. Refined Scientific Requirementsfor the PTCS Jim Condon

  2. GBT Science Summary – Frequency Coverage

  3. Tracking errors: sensitivity loss and flux uncertainty

  4. Distributions of flux uncertainties from tracking errors

  5. Pointing requirements • 2003 October: • improved blind pointing • usable pointing at 52 GHz σ = 2.8 arcsec • Early 2005: • good pointing at 52 GHz σ = 2.0 arcsec • usable pointing at 86 GHz σ = 1.7 arcsec • 2005 October: • good pointing at 86 GHz σ = 1.2 arcsec • usable pointing at 115 GHz σ = 1.3 arcsec

  6. GBT pointing calibrators NVSS calibration sources 45 arcsec resolution σ = 0.7 arcsec σ²(cal) < σ² / 16 at 52 GHz New calibration images 5—10 arcsec resolution σ< 0.3 arcsec σ²(cal) < σ² / 16 at 115 GHz

  7. Sky distribution of GBT pointing calibrators

  8. Tracking time • Operational efficiency = (1 – β) • Calibration overhead β = 0.2, t(cal) = 5 min • Required tracking time Δt = t(cal) / β ≈ 0.5 hours • We may be able to reduce Δt or • increase operational efficiency by reducing β, • but Δt < 12 min (3 deg at sidereal rate) not useful

  9. Wind and short-term (40 s) pointing changes σ(wind) = 1"[ v / 2.5 m/s ]²

  10. GBT cumulative wind-speed distribution Extreme wind Normal wind Benign wind

  11. Wind speed conditions • Operational efficiencies: • benign conditions (2.5 m/s): 60% • normal conditions (3.5 m/s): 75% • extreme conditions (5.0 m/s): 90%

  12. Wind speed requirements • σ²(wind) = σ² / 4 (1/4 of variance allocated to wind) • 2003 December results: • usable 52 GHz pointing at v = 3.0 m/s • Early 2005 requirements: • usable 52 GHz pointing at v = 3.5 m/s • usable 86 GHz pointing at v = 2.5 m/s • 2005 October requirements: • good 86 GHz pointing at v = 2.5 m/s • usable 115 GHz pointing at v = 2.5 m/s

  13. Thermal requirements • As with wind, the main scientific requirement is • good operational efficiency. • Thermal corrections can be made continuously. • Thermal stresses may be too large for good pointing • around sunrise and sunset • while slewing around the sky on a sunny day

  14. Blind pointing • Calibrator acquisition: • σ(az) = 0.30 θ for 90% reliability on first try • 2003 October: • 52 GHz requirement σ(az) = 4.3 arcsec • Early 2005: • 86 GHz requirement σ(az) = 2.6 arcsec • 2005 October: • 115 GHz requirement σ(az) = 1.9 arcsec

  15. Blind pointing gradients |ΔE| / rad

  16. Requirements for blind pointing gradients • 2003 December results: • good pointing at 52 GHz |ΔE| = 10.8 arcsec/rad • Early 2005: • usable pointing at 86 GHz |ΔE| = 9.4 arcsec/rad • 2005 October: • good pointing at 86 GHz |ΔE| = 6.5 arcsec/rad • usable pointing at 115 GHz |ΔE| = 7.0 arcsec/rad

  17. Aperture surface accuracy • good efficiency ε = λ / 16 η = 0.54 • usable efficiency ε = λ / 4πη = 0.37 • 2003 October: • usable efficiency at 52 GHz ε = 0.46 mm • Early 2005: • usable efficiency at 86 GHz ε = 0.28 mm 2005 October: • good efficiency at 86 GHz ε = 0.22 mm • usable efficiency at 115 GHz ε = 0.21 mm

  18. Focus tracking stability

  19. Focus tracking requirements • Good focusing Δy = | λ / 4 | • Usable focusing Δy = | λ / 2 | • 2003 October: • usable focusing at 52 GHz Δy = 2.9 mm • Early 2005: • usable focusing at 86 GHz Δy = 1.7 mm • 2005 October: • good focusing at 86 GHz Δy = 0.9 mm • usable focusing at 115 GHz Δy = 1.3 mm

  20. Summary of PTCS requirements: • 2003 October: • usable pointing at 52 GHz σ = 2.8 arcsec • improved blind pointing • calibrators OK for 52 GHZ σ(cal) = 0.7 arcsec • tracking time at 52 GHz Δt = 0.5 hours • 60% of time (benign wind) v = 2.5 m/s • blind pointing at 52 GHz σ(az) = 4.3 arcsec • blind gradient at 52 GHz |ΔE| = 10.8 arcsec/rad • usable efficiency at 52 GHz ε = 0.46 mm • usable focusing at 52 GHz Δy = 2.9 mm

  21. Summary of PTCS requirements: • Early 2005: • usable pointing at 86 GHz σ = 1.7 arcsec • calibrators OK for 86 GHz σ(cal) = 0.3 arcsec • tracking time at 86 GHz Δt = 0.5 hours • 60% of time at 86 GHz (benign wind) v = 2.5 m/s • 75% of time at 52 GHz (moderate wind) v = 3.5 m/s • blind pointing at 86 GHz σ(az) = 2.6 arcsec • blind gradient at 86 GHz |ΔE| = 9.4 arcsec/rad • usable efficiency at 115 GHz ε = 0.28 mm • usable focusing at 86 GHz Δy = 1.7 mm

  22. Summary of PTCS requirements: • 2005 October: • good pointing at 86 GHz σ = 1.2 arcsec • calibrators OK for 115 GHz σ(cal) = 0.3 arcsec • tracking time at 115 GHz Δt = 0.5 hours • 60% of time at 115 GHz (benign wind) v = 2.5 m/s • 75% of time at 52 GHz (moderate wind) v = 3.5 m/s • blind pointing at 115 GHz σ(az) = 1.9 arcsec • blind gradient at 86 GHz |ΔE| = 6.5 arcsec/rad • usable efficiency at 115 GHz ε = 0.21 mm • good focusing at 86 GHz y = 0.9 mm

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