TVo, Danny, Hang

We did a quick Finesse simulation to see how much contrast defect we should expect at the simple mich configuration, as a function of differential ITM thermal lens. Here the contrast defect was defined as the AS power at MICH dark/ power at MICH bright. 

Please see the first attached figure. We expect around 0.2% contrast defect at 10 microdiopter (100 km differential ITM lens). This should roughly serve as a requirement on TCS. 

On the other hand, to get good SRM alignment signal & small SRM detuning (which affects the low freq DARM response), we need to control the differential lens to <~ 10 microdiopter. 

The second (no diff lens) and third figures (100 km diff lens) show how the AS72 signal changes. 

The last figure show the simulated DARM TF at different levels of thermal lens.