$$ P=\frac{1}{f} $$
$$ M=\frac{F_1}{F_2}=\frac{\theta'}{\theta} $$
Parameters for a telescope
- Image scale $p=1/f$
- Collection area $A=\pi D^2/4$
- Focal ration $F=f/D$
- Diffraction limit $\theta=\lambda/D$ (single telescope) or $\theta=\lambda/D$ (interferometer)