start of a note summarising the OP finding proceeduce in zero_locks

playground/anna/file_review/LockingLigo.tex

+\documentclass{article}    
+    \usepackage{graphicx} % Used to insert images
+    \usepackage{xcolor} % Allow colors to be defined
+    \usepackage{geometry} % Used to adjust the document margins
+    \usepackage{amsmath} % Equations
+    \usepackage{amssymb} % Equations
+    \usepackage[mathletters]{ucs} % Extended unicode (utf-8) support
+    \usepackage[utf8x]{inputenc} % Allow utf-8 characters in the tex document
+    \usepackage{hyperref}
+\geometry{verbose,tmargin=1.5cm,bmargin=1.5cm,lmargin=1.5cm,rmargin=1.5cm}
+
+\newcommand{\tcg}{\textcolor{gray}}%%%%%%% for notes
+
+\newcommand{\ZL}{\texttt{zero\_locks.py} }
+
+\begin{document}
+    
+% Document parameters
+\title{Locking LIGO}
+\author{Anna Green,  Andreas Freise, Dan Brown}
+\date{December 2016}
+\maketitle
+  
+% introduction:
+This document will describe the process of using \ZL to find the operating point of 
+a dual-recycled Michelson interferometer with Fabry-Perot arms (DR-FPMi), and implementing \texttt{lock} 
+commands to maintain this operating point in response to smooth changes to optical parameters.
+    
+%\tableofcontents
+%\vspace{1em}\hrule\vspace{1em}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+ \section{Procedure }
+ 
+ \tcg{What \ZL requires as inputs, and what is does with them by default (i.e. when run as 
+ \texttt{main()}, in as short a form as possible (i.e. an enumerated list, with description of what to 
+ look for at each stage).}
+ 
+ \begin{enumerate}
+ \item Pre-tuning. Set all cavities to co-resonate by maximising or minimising the power, or using transfer functions, as appropriate. 
+ 
+ 	\tcg{How to know it's worked}
+ 
+ \item Apply a DARM DC offset by detuning the ETMs differentially.
+ 
+ \item Error Signals. Make the Lock block.
+ \end{enumerate}
+ 
+ %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+ \section{\ZL functions}
+ 
+ This code includes several functions, and calls several others from PyKat (and \texttt{AG\_tools.py})\tcg{move?}. 
+ Here, we will run through some of  the key functions in the order they are used and describe what they do in 
+ a little more detail, including example plot outputs.
+ 
+ \subsection{\texttt{zero\_locks()}}
+ A misnomer! \tcg{change?}
+ 
+ This take a DR-FPMi and performs a preliminary tuning to ensure that the cavities are all on resonance. This is 
+ a hierarchical process:
+ 
+	 \begin{enumerate}
+	 \item Set the individual arm cavities on resonance by tuning the ETMs to maximise the circulating power
+	 \item Set the Michelson to a dark fringe by tuning the beamsplitter to minimise transmitted power
+	 \item Tune the PRM to maximise the power circulating in the PRC
+	 \item Tune the SRM to signal recycle using the DARM transfer function \tcg{to do what?}
+	 \item Move from signal recycling to RSE mode by shifting the SRM tuning by 90$^\circ$
+	 \end{enumerate}
+ 
+ \subsection{\texttt{DC\_Offset()}}
+ \subsection{\texttt{make\_locks()}}
+ 
+ \subsection{\texttt{diagnostics()}}
+ Can be called at any time to produce the following outputs, useful for checking the file's current status;
+ \begin{itemize}
+ \item \texttt{print} the input, cavity circulating and transmitted powers, and ratios of these powers
+ \item plot the power and RF sideband amplitudes in response to each degree of freedom \tcg{see plot}
+ \item plot the error signal for each degree of freedom \tcg{see plot}
+ \item (option - disabled currently) plot the quantum noise limited sensitivity of the interferometer \tcg{see plot}
+ \end{itemize}
+ 
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%\newpage
+%\appendix
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%\begin{thebibliography}{99}
+%\addcontentsline{toc}{section}{References}
+%\bibitem{T0900511} Matt Evans and Lisa Barsotti. Modeling of Alignment Sensing and Control for 
+% Advanced LIGO. \textit{LIGO DCC Technical Note T0900511}, 2010
+%\end{thebibliography}
+
+\end{document}