Big Dish Telescope: Reflector and Feed

Most details here are for the original 10-foot TVRO fiberglass dish. Where noted, a few are for an alternative 10-foot metal mesh dish.

• Diameter D = 3.05 m (120 in) (interior surface, excluding outer rim)
• Depth d = 0.61 m (24 in) (from dish bottom to plane of rim, along axis of symmetry)
• Focal Ratio f / D = D / (16 d) = 5/16 = 0.31 (assuming parabolic cross-section)
• Focal Length f = D (f / D) = D² / (16 d) = 0.95 m (37.5 in) (focus distance from dish bottom)
• Shape: manufactured as circular paraboloid (spider test shows good symmetry, converging within 1 mm)
• Composition: solid metal foil reflector surface embedded within fiberglass shell
• Mass = 91 kg (200 lb) for fiberglass dish
  • Alternative metal mesh dish mass is 45 kg (100 lb).
• Shell Integrity: minor damage to part of outer rim; surface ok after fiberglass patching
  • May paint for weather protection; investigating options.
• Reflector Surface Discontinuties = 3 mm occasionally between overlapping layers (usually smooth)
  • Estimated from visual inspection of exposed area before patching.
  • Alternative metal mesh dish hole size to be deterined.
• Reflector Surface Accuracy ε = 1 mm mostly = λ / 25, λ / 75, or λ / 210 for λ = 2.5, 7.5, or 21 cm
  • Defined formally as the root-mean-square (RMS) of deviations from a smooth surface.
  • Estimated visually here from exposed area before patching.
• Reflector Surface Efficiency η = exp [ - (4 π ε / λ)² ] = 0.78, 0.97, or 0.996 for λ = 2.5, 7.5, or 21 cm
  • Effective area divided by geometric area; computed for different wavelengths using Ruze equation.
• Feed Position: Prime focus centered in front of the dish, distance f = 0.95 m (37.5 in) from dish bottom.
• Feed Support: metal ring mounted on 1.52 m (60 in) hollow stalk through sleeve in bore site (dish center)
  • Ring positioned in focal plane at prime focus.
  • Stalk tube used both as support and as conduit to carry signal and power cables from feed to back-end.
• Feed-Reflector Angle ψ = 2 arctan (D / (2 f)) = 116° (dish angular diameter as seen from feed)
  • To minimize ground spillover, it is desirable to taper feed sensitivity to 10% at the dish edges.
• Matching Feed Beam 50% Power Width φ ≃ (0.6 ψ) ≃ 70°, depending on feed beam shape.
  • A sinc² untapered beam has a 50% / 10% power width ratio of 0.60, implying φ = 70°.
  • A cos² tapered beam has a ratio of 0.63, implying φ = 73°.
  • A Gaussian tapered beam also has a ratio of 0.60, implying φ = 70°.