The picture defines the names of all relevant lengths in a dual recycled interferometer. The physical degrees of freedom are defined as follows:

• DARM = L_X - L_Y
• CARM = (L_X + L_Y) / 2
• MICH = l_X - l_Y
• PRCL = l_P + (l_X + l_Y) / 2
• SRCL = l_S + (l_X + l_Y) / 2

Mirror motions are defined to be orthogonal to the mirror surface, positive in a direction selected by Finesse convention:

• PRM motion is negative in BS direction
  
• SRM motion is positive in BS direction
  
• BS motion is positive in PRM direction
  
• EMX and EMY motions are positive when cavities get shorter

Optickle convention is the same except for the reversed sign of PRM and possibly a global sign change.

The simplest extension of Virgo driving to AdVirgo consists in controlling MICH / PRCL / SRCL using PRM / SRM / BS. The following relations between mirror motions and lengths hold:

• delta_l_P = dz_PRM - sqrt(2) dz_BS
• delta_l_X = sqrt(2) dz_BS
• delta_l_Y = 0
• delta_l_S = - dz_SRM

Simple inversions give the relations between degrees of freedom and mirror displacements:

• dz_EMX = - DARM / 2 - CARM
• dz_EMY = + DARM / 2 - CARM
• dz_BS = 1 / sqrt(2) MICH
• dz_PRM = PRCL + 1/2 MICH
• dz_SRM = - SRCL + 1/2 MICH

The following table summarized the signal amplitudes to be applied to all mirrors to produce one degree of freedom motion:

	DARM	CARM	MICH	PRCL	SRCL
EMX	-0.5	-1
EMY	0.5	-1
BS			1/sqrt(2)
PRM			1/2	1
SRM			1/2		-1

These are the correct amplitudes and signs to be used in Finesse simulations. Note that Finesse static tuning are in degrees (360 degrees = 1 lambda) while transfer functions are given in W/rad (2 pi radians = 1 lambda).