= Dither-based Alignment Sensing and Stabilization  =

<<TableOfContents>>
= Overview =
 . The idea is to dither (in pitch and yaw) all of the interferometer optics. We then read out the power (arm powers, PRC power, etc.) and demodulate at the dither frequencies. We can then feedback to control the full interferometer alignment. The concept is the same as what we use to do the initial alignment of the interferometer. It was previously tried by Valera and Bram, but never worked for some reason...
   * [[attachment:ass.png|Conceptual Diagram]]
   * See also the old [[Alignment_Sensing_and_Control]].

= Control Topologies =
== Conceptual Designs ==
=== Inline Arm Alignment ===
 . In the diagrams the dither signals to the optics are in the YAW axis of the optics, but this is done in a similar manner in the PIT axis. The labels in the colored boxes are the channels (shown in YAW, but valid for PIT as well) to which the obtained error-signals, from the demodulation, are fed back to.

 . {{attachment:DC_ASC_Inline.png}}

    .  '''Figure 1'''. Diagram of the 40m IFO in-line arm cavity (XARM). The PRM and BS are omitted for clarity.

 . '''In-line cavity legend - Figure 1:'''
 || IP_POS || Otherwise known as PZT1, located in front of the PRM. Steers the beam into the IFO. ||
 || IP_ANG || Otherwise known as PZT2, located after IP_POS and before the PRM. Also, steers the beam into the IFO. ||
 || ITMx || The ITM of the x-arm, ITMX. ||
 || ETMx || The ETM of the x-arm, ETMX. ||
 || PDH-err || The demodulated length control signal, ''C1:LSC-XARM_IN1''. ||
 || TRX || The transmitted power of the x-arm, ''C1:LSC-TRX_OUT''. ||
 || L.O. ITMx || Modulation of the ITMx in PIT or YAW using tdssine. ||
 || L.O. ETMx || Modulation of the ETMx in PIt or YAW using tdssine. ||

=== Perpendicular Arm Alignment ===
 . {{attachment:DC_ASC_Perp.png}}

    .  '''Figure 2'''. Diagram of the 40m IFO perpendicular arm cavity (YARM).

 . '''Perpendicular arm legend - Figure 2:'''

 || BS || Beamsplitter, steers the beam into the perpendicular arm cavity (y-arm).  ||
 || ITMy || The ITM of the y-arm, ITMY. ||
 || ETMy || The ETM of the y-arm, ETMY. ||
 || PDH-err || The demodulated length control signal, ''C1:LSC-YARM_IN1''. ||
 || TRY || The transmitted power of the y-arm, ''C1:LSC-TRY_OUT''. ||
 || L.O. ITMy || Modulation of the ITMy in PIT or YAW using tdssine. ||
 || L.O. ETMy || Modulation of the ETMy in PIt or YAW using tdssine. ||


 . {i} Diagram for digital dither alignment scheme of the DRFPMI [[attachment:40mASC.pdf]]



= Actual Installations and Operations =
== Realtime control model ==
 . Put a diagram of the real time control model

== Adjustment of Input/Output Matrices ==
 . Describe how the adjustment have done.
=== Input signals ===

 . The input signals to ASS are:

 || '''PD1''' ||  TRX    ||
 || '''PD2''' ||  TRY    ||
 || '''PD3''' ||  POX33I ||
 || '''PD4''' ||  POY33I ||
 || '''PD5''' ||  SP166Q ||
 || '''PD6''' ||  SPOB66 ||
 || '''PD7''' ||  AP DC  ||
 || '''PD8''' ||  PO DC  ||

 . These are displayed on the ASS MASTER display.


== How to run ASS auto-scripts ==
 . {{{ 
 How to do it
}}}

= ilog and elog entries =
 * [[http://www.ldas-sw.ligo.caltech.edu/ilog/pub/ilog.cgi?group=40m&date_to_view=02/09/2007&anchor_to_scroll_to=2007:02:10:00:50:51-valera|initial commissioning]]
 * [[http://www.ldas-sw.ligo.caltech.edu/ilog/pub/ilog.cgi?group=40m&date_to_view=02/05/2007&anchor_to_scroll_to=2007:02:05:17:53:08-rward|Input signals]] -- mapping of signals into ASS PD's

 * [[http://www.ldas-sw.ligo.caltech.edu/ilog/pub/ilog.cgi?group=40m&date_to_view=02/05/2007&anchor_to_scroll_to=2007:02:05:15:03:18-rward|compiling the front end code]]