I have not had any success the past two days in getting an interferometric measurement of the SRC length. 

So, the question posed at today's meeting was: "How precisely do we need to change the SRC length to be able to lock the DRMI on 3F"

The two ways I could think to quantify this are:

• How much MICH -> [S,P]RCL cross coupling is ok?
• How much [S,P]RCL ->  MICH cross coupling is ok?

REFL33 should have its phase set to put PRCL along I, and REFL165 should have SRCL along I, so the simulation result that matters is the angle of MICH in these planes. The cross couplings are then given by the appropriate trigonometric projections. In the following plots, I used 10% as the acceptable cross coupling in either direction. 

Result:

Thus,

• To limit the MICH -> [S,P]RCL coupling to 10%, we must hit the ideal length within +- 1.2cm.
• To limit the SRCL -> MICH coupling to 10%, we must hit the ideal length within +- 2mm.
• It doesn't look like we can get the REFL33 angle totally to 90 degrees, REFL165 looks more promising.

Code (finesse + pykat + ipython notebook) and plots are attached.