If you take this picture of a telescope 
It has several gaps through it. My lecturer was mentioning how as long as the holes and imperfections in your disk are less than the wavelength of light you are observing, they can be ignored. He said the telescope shown above was used to measure hydrogen line emissions with wavelength of about 20cm. My question is, if the gaps are about a few cm wide, wont the incoming light diffract?
Is it just that the gaps and imperfections are significantly smaller than the the wavelength so there is no diffraction and just reflection?
