For a dish:
F = (F/D) x D
where:
F = focal length (support length) F/D = focal ratio D = dish diameter
Example for a 1.0 m dish:
+———–+—————-+ | F/D Ratio | Support Length | +———–+—————-+ | 0.25 | 250 mm | | 0.30 | 300 mm | | 0.35 | 350 mm | | 0.40 | 400 mm | | 0.45 | 450 mm | | 0.50 | 500 mm | | 0.55 | 550 mm | | 0.60 | 600 mm | | 0.65 | 650 mm | | 0.70 | 700 mm | | 0.75 | 750 mm | | 0.80 | 800 mm | +———–+—————-+
Common dish sizes:
+———–+———-+———-+———-+———-+ | Diameter | F/D=0.35 | F/D=0.40 | F/D=0.45 | F/D=0.50 | +———–+———-+———-+———-+———-+ | 600 mm | 210 mm | 240 mm | 270 mm | 300 mm | | 800 mm | 280 mm | 320 mm | 360 mm | 400 mm | | 1000 mm | 350 mm | 400 mm | 450 mm | 500 mm | | 1200 mm | 420 mm | 480 mm | 540 mm | 600 mm | | 1500 mm | 525 mm | 600 mm | 675 mm | 750 mm | +———–+———-+———-+———-+———-+
If you know the dish diameter (D) and depth (d), the focal ratio can be found from:
F = D^2 / (16 x d)
and then:
F/D = D / (16 x d)