ATA/SKA Issues

1. ATA/SKA Issues Jack Welch

2. ATA Log-periodic Feed 0.5-11 GHz

3. ATA Feed Gain vs. Frequency G0 = 11.5 ± 1dB

4. Cross-Polarization Both Models

5. Expected Tsys for ATA Tsys ˚K = 8 + 6.3√f(GHz) +7 + 7 +2.7 + 3 √f(GHz) (f(GHz))2.7 Galaxy Diffractive spillover Ohmic losses LNA CMB Atmosphere + geometric spillover

7. Optimum Antenna DiameterSKA Memo #78 • Array of n antennas, each with diameter D total observing time T, point source sensitivity Sfor single pointing for survey • Array cost for single pointingfor survey

8. Optimum Antenna Diameter (more)SKA Memo #78 • Differentiate with respect to D to find the minimum cost for the array for single pointing for survey

9. ATA Antenna Constraints • Feed Frequency Range: .5 GHz–11.2 GHz • Feed Focus: s = 1.4 λ • Feed focal ratio: .65 • Feed Gain: 11.5 db • Antenna Diameter ~ 6m

10. Optical Design • Key reference paper by Per-Simon Kildal PGAP, AP-31, #6 November 1983 • Aperture efficiency for two mirror system is • ηIllis the illuminating efficiency; d is the secondary diameter; D is the primary; Cb is 1 for uniform illumination, ~1.5 for -10 dB taper. Cd  Cb/π. The middle term is the aperture blockage. The third term is the edge diffraction loss. A0 is the amplitude edge illumination.

11. Optical Design (continued) • An optimum primary diameter for array survey sensitivity S∞ ND, with a modern wideband, single pixel feed system is ~ 6 m • The secondary mirror should be no smaller than about 4 λ, 2.4 mfor the longest wavelength of 60 cm. • The blockage is large, and the diffractive sidelobes are high for symmetric Cassegrain with these reflector sizes • An offset Gregorian is the better choice, which removes the middle term from the aperture efficiency expression and gives the low sidelobes of a clear aperture.

12. Paraxial Patterns Prime Focus Cassegrain Offset Gregorian Offset Gregorian Beam Angle Beam Angle

13. ATA Optics: Offset Gregorian 6m primary Radome Shroud Log-periodic Feed with Actuator 2.4m secondary

14. Gain Tables

15. Shroud Effects • Echoes from the feed Pr = G2(90)  Pi 10 radius = -42 dB @ =500 MHz • Primary Beam Waste: 4 bw  2 • Secondary Beam Waste: 4 bw  3

18. Feed Patterns In And Out Of Focus

19. Antenna properties • Surface RMS: 0.7mm (ok for 24 GHz) • Aperture Efficiency: 0.6 • Low side lobes from clear aperture • Feed at F/.65 • Good overall gain with feed at 6 GHz focus

20. Holographic Measurements at 4 GHz Phase Amplitude

21. The Antenna Pattern At 2.3 GHz

22. Spillover vs. Zenith Angle Tsys Zenith Angle

23. Antenna PropertiesOffset Gregorian ATA • Surface accuracy 0.7 mm RMS (λ/20 @ 21GHz) • Primary field of view 3.5º/f (GHz) • Primary reflector blockage by secondary ≤ 2% • Secondary diam = 2.4 m or 4 λ @ 500 MHz • Optical pointing 10” RMS • Assembly: 8 person days • Tipping curve 20 10 0 * Excess Temperature (K) * See slide show Elevation

24. Pointing Requirements • Basic interferometry • Single Antenna • Multiple pointing interferometry (mosaicing)

25. Single antenna transfer function Zero-spacing Problem Multiple-pointing array transfer function β= √u2+v2 D/λ

26. Pointing Errors Create Phase Errors

27. Effect Of Pointing Errors • With pointing errors δx and δy • The effective transfer function is now • At β = D/2λ, the expected value of • is 0.93 for σθ/Θ=0.1, a 0.1 beamwidth pointing error • The same error results from the interferometry • Altogether, 1/30th beamwidth pointing is required for map errors to be less than 10%

28. Single antenna transfer function Filling In The Zero-spacing WithLarger Antennas 4d 3d 2d D = d array antenna diameter β= √u2+v2 D/λ

29. Night Time Pointing

30. Long Term Tracking

31. GPS Pointing in the Daytime

32. ATA-42 from the Hat Creek hang glider launch

33. The ATA-42 Today

34. ATA Antenna Requirements