## Interferometers

Big telescopes are expensive, and doubling the size of a telescope increases the cost by a much bigger factor. In addition, bigger telescopes suffer from deforming under their own weight, so there is a limit to the useful size of a single telescope. There is however, a way to combine telescopes so that the effective size of a telescope is equal - in one sense - to the distance between the telescopes. Combining telescopes in this way is called interferometry.,BR /. The light gathering power of a telescope is proportional to its area. If we combine telescopes, the light gathering power of the set of telescopes considered as a single instruments is proportional to the sum of the areas.
The resolving power of a telescope with a primary lens/mirror of diameter
$D$
$\lambda$
$\theta \simeq \frac{1.22 \lambda}{D}$
$\theta$
$D$