Error in title. I meant refracting. At every transition between air and glass it seems to me like there would be some reflection. Each photon it seems would have a certain probability of entering a lens and reflecting off of both of the lenses interior surfaces and then continuing on into the instrument. These reflected photons are not going to end up in the right place when contributing to the image. Is this reflected light a significant design consideration for refracting telescopes? If so, what is done to minimize it's impact?