The product of these spectra give the curves shown above. This suggests that small optical imperfections in the ITMs can lead to big changes in the IFO operation. At the moment, I don't know how to make those mirror maps, so I've only run FFT with perfect optics. We want to be able to use existing LIGO designs for the suspension controllers. We want to keep the size of the optic as small as is possible while giving acceptable suspension thermal and radiation pressure noise.
Smaller optics cost less.
This is perhaps the most important consideration driving us towards smaller optics. The following output from the fft program shows the beam spot on a FP arm end mirror. The mirror aperture extends almost to the edge of the mesh area. To 40 meter Upgrade page . To LIGO home page at Caltech , and to LIGO home page at MIT .