Formulas areYou wish to form an image of the moon, diameter 3500 km, a distance of 380 000 km away, with a reflecting telescope. If the image needs to be 4.5 cm in diameter, find the required focal length of the mirror in the telescope.

$\displaystyle \frac{1}{f}=\frac{1}{di}+\frac{1}{do}$

$\displaystyle \frac{hi}{ho}=\frac{-di}{do}$

F stands for focal point, di is distance to the image, do is distance to the object, hi is height of the image and ho is height of the object.

$\displaystyle \frac{4.5*10^-5}{3500}=\frac{-di}{380000}$

$\displaystyle f=-4.88*10^-3$

$\displaystyle \frac{1}{f}=\frac{1}{380000}+\frac{1}{-4.88*10^-3}$

$\displaystyle f= -4.88*10^-3 km $

is this correct???