
\newpage

\section{Project strategy}

\subsection{Commissioning time}

%It is not easy to estimate the commissioning time because it depends on
%various factors. Especially, commissioning time can be affected a lot
%by unexpected problems. Since unexpected problems are unpredictable by
%definition, here we try to present our best guesses for the commissioning
%time.

The estimation of commissioning time is a critical issue in the selection of 
interferometer configuration.
Even if the sensitivity for GW signals is better, it is not favorable
if it takes a lot of commissioning time.
As described in Section\,3,
differences in the required observation time for the first GW detection 
is 2 months at most.
Thus, we evaluate the deference in commissioning time in this time scale.
The followings are the current best guesses for the commissioning time.

The behavior of the interferometer is simpler for broadband operation, 
because the sidebands are all balanced and there is no optical spring nor 
RSE peaks.  
Moreover, the peak sensitivity is slightly better for detuned configurations 
than the broadband ones. 
This means that we may need an extra commissioning time to reduce noises other 
than the optical readout noises below this peak sensitivity level.
As for the VRSE configuration,
the addition of offset to the SEC signal to detune the cavity could
introduce some technical problems since the $f_1$ sideband is not fully
resonant in the SEC. 
If we choose the SEC length to be optimized for VRSE(B) 
(i.e. $f_1$ is resonant in the SEC for broadband operation), 
the VRSE(D) configuration may suffer from this problem. 
If we optimize the SEC length for VRSE(D), VRSE(B) will suffer in turn.
We do not see any fundamental difference between BRSE and VRSE(B) if the
SEC length is optimized for VRSE(B). 
Although the detuned configurations may require some extra commissioning time, 
which we consider it to be small with a careful construction scheduling.

%but the amount of extra work
%required is unknown.

In summary, though the commissioning time will be different in candidate configurations,
the difference is not so significant (Table\,\ref{Table-tech_feas}).


%*********

%In general, the peak sensitivity is better for DRSE than BRSE. The
%better NS-NS range of DRSE owes largely to this peak sensitivity. This
%means we have to reduce noises other than the optical readout noises
%below this peak sensitivity level. This may require more commissioning
%time for the DRSE configurations than BRSE.

%It is likely that the optimal parameters for interferometer control will
%be different for different detuning phase. Therefore, the variable
%detuning is likely to require more commissioning time than others. 

%***********


\subsection{Cost comparison}

The difference in cost is another factor to
determine the optical configuration.
The differences could originate in mirror quality and additional instruments
for interferometer control (modulators and electronic devices).
However, in the current estimation, the differences in cost for candidate 
configurations will be negligible compared with the total cost of LCGT.



\subsection{Risks in noise hunting and diagnosis}

Difficulties in the noise hunting during the commissioning phase, 
and diagnosis during the observation would be an additional risk to keep 
the commissioning schedule and to realize the designed sensitivity.
The response of broadband interferometers would be simpler than the detuned
configuration, without additional offset in the resonance or anti-resonance condition
of cavities.
Moreover, the variable-band configuration has additional measurement options in noise 
hunting and diagnostics of the interferometer.





\subsection{Potential for future upgrades}

After the achievement of the original purposes to detect GW signals and 
to obtain scientific results, LCGT would have a chance to upgrade the detector%
\footnote{We assume that this upgrade is in several years after the start of observation.}.
This upgrading may contain replacements of components (mirrors and laser source),
installation of new subsystems (equipments to reduce quantum noises).
None of the candidate configurations excludes the possibilities of theses upgrades.
