= Solution
A distant satellite supplies an approximately collimated incident beam. A <concave spherical mirror> of <radius of curvature> $R_c=1.5\,\mathrm m$ has paraxial <focal length> $f_m=R_c/2$. Put the receiving feed at the principal focus, on the <optical axis> in front of the dish:
$$
\boxed{f_m=0.750\,\mathrm m\text{ from the dish vertex}.}
$$
The <radius of curvature> relation is a <paraxial approximation>. A practical dish is usually a <parabolic reflector>, which focuses an on-axis collimated beam without <spherical aberration>; a literal spherical dish has <spherical aberration>, so an exact optimum over its full <aperture> would need a specified focusing criterion.
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