Comparison between the control channels used for the signal recycling cavity (SRC) alignment, H1 and L1:
| DoF | Sensor | Controlled optic | |
| H1 | SRC1 | AS_B RF36I | SRM |
| SRC2 | AS_C DC | SR2 & SRM | |
| L1 | SRC1 | AS_C DC | SR2 |
| SRC2 | AS_A RF36I | SRM |
Both IFOs use a AS port, 36 MHz WFS to control SRM alignment, though H1 uses WFS B and L1 uses WFS A. I don't know if this difference is arbitraty or reflects some real difference in sensing. Both IFOs use the AS port QPD, AS_C, to control the alignment of SR2, though H1 also feeds this signal to the SRM, to decouple the action of this loop (SRC2) from the SRC1 loop.
The other thing to notice is that the DoF (degree-of-freedom) naming between the H1 and L1 is swapped. This is just unfortunate and should be fixed.