What is the bottom limit of telescopes and why can't they detect visible light?

[aster79]

"Bottom" limit of telescopes

I know this is a very elementary question, but I was never quite able understand why there was a bottom limit to what telescopes could "see". For example, I have not quite been able to grasp why telescopes can only see in the radio, but are invisible to optical rays, considering that they are large enough to be able to resolve objects emitting light at radio wavelengths. I understand why there is an upper limit to the wavelengths a telescope is able to "see" at, but for some reason I can't understand why there is a lower limit as well.

 

[tiny-tim]

Welcome to PF!

Hi aster79! Welcome to PF!

Do you mean why can't a big radio telescope also be used for seeing in ordinary light (presumably, if they were covered with something smooth and reflective, like silver)?

It's because the dish has to be accurate to within a fraction of a wavelength, which is easy for radio wavelengths, but beyond our engineering capability for the extremely short light wavelengths.

 

[ideasrule]

Radio telescopes also have sensors that are basically electronic oscillators resonating at the frequency of the detected waves. It's not practically possible to make a visible light electronic oscillator, so radio telescopes can't detect visible light.