What is claimed is: 1. A Fabry-Perot cavity Fourier transform microwave spectrometer comprising: a vacuum chamber; a first pair of mirrors positioned within said vacuum chamber, forming a Fabry-Perot cavity, each mirror of said first pair of mirrors having a surface with a surface finish of less than or equal to 0.25 microns rms. 2. A microwave spectrometer as in claim 1, wherein said surface of each mirror of said first pair of mirrors is coated with nickel. 3. A microwave spectrometer as in claim 2, wherein said surface of each mirror of said first pair of mirrors is further coated with either gold or silver. 4. A microwave spectrometer as in claim 1, wherein each mirror of said first pair of mirrors has an overall sphericity across its surface of less than or equal to 4 microns rms. 5. A microwave spectrometer as in claim 1, wherein one mirror of said first pair of mirrors forms one end of said vacuum chamber. 6. A microwave spectrometer as in claim 5, further comprising a receiving antenna located in said one mirror of said first pair of mirrors and a microwave amplifier directly connected to said receiving antenna. 7. A microwave spectrometer as in claim 6, wherein a microwave molecular beam is pulsed coaxially into said Fabry-Perot cavity through a pin hole in said one mirror of said first pair of mirrors. 8. A microwave spectrometer as in claim 1, wherein said first pair of mirrors forms a first Fabry-Perot cavity, said microwave spectrometer further comprising a second pair of mirrors positioned within said vacuum chamber, forming a second Fabry-Perot cavity. 9. A microwave spectrometer as in claim 8, wherein said first Fabry-Perot cavity is pre-set to propagate a first fixed microwave frequency and said second Fabry-Perot cavity is pre-set to propagate a second fixed microwave frequency. 10. A microwave spectrometer as in claim 9, wherein a microwave molecular beam is pulsed coaxially into each of said first and second Fabry-Perot cavities through a pin hole in one mirror of the pair of mirrors which forms each cavity.