\begin{table}%t1
\caption{The optical filtering scheme
employed
  by \bk.  In order to take advantage of the low backgrounds
  available at float altitudes, much care must be taken to reduce the
  background originating from within the cryostat.  While the metal mesh
  filters, which consist of bonded layers of polyethylene, exhibit
  in-band emissivities at the percent level, the PTFE antireflection
  coating is several times more emissive. It is crucial that these
  filters remain well
  heat-sunk and protected from infrared emission from the warmer stages.
\label{tbl:filters}}
%\centerline
 {\small 
\begin{tabular}{c|c|c|c}
\multicolumn{4}{c}{\large Optical Filters} \\
\hline\hline
Temp. & 145 GHz & 245 GHz & 345 GHz \\
\hline 77 K & \multicolumn{3}{c}{$\sim $$100~\mu$m IR blocker} \\
\cline{2-4} $\vdots$  & \multicolumn{3}{c}{540 GHz AR-coated LPF} \\ 
\cline{2-4} 2 K & \multicolumn{3}{c}{$\sim $$100~\mu$m IR blocker} \\
\cline{2-4} $\vdots$  & \multicolumn{3}{c}{450 GHz AR-coated LPF} \\
\cline{2-4} $\vdots$  & \multicolumn{3}{c}{Removable NDF$^\dagger$}
\\ \cline{2-4} $\vdots$  & &\multicolumn{2}{c}{Polarizing Grid} \\
\cline{3-4} $\vdots$  & &\multicolumn{2}{c}{420 GHz LPF} \\
\cline{3-4} $\vdots$  & &\multicolumn{2}{c}{$180~\mu$m BP/Yosh
LPF$^\ddagger$} \\ \cline{3-4} $\vdots$  & & 295 GHz dichroic LPF &
295 GHz dichroic HPF \\ \cline{2-4}\\[-3.2mm] 0.3 K   & $180~\mu$m BP/Yosh
LPF$^\ddagger$ & 360 GHz LPF & 410 GHz LPF \\ \cline{2-4} $\vdots$ &
255 GHz LPF &\multicolumn{2}{c}{ }\\ \cline{2-2} $\vdots$  & 540 GHz
LPF &\multicolumn{2}{c}{ }\\ \cline{2-2} $\vdots$  & 168 GHz LPF
&\multicolumn{2}{c}{ }\\ \cline{2-2} \hline
\end{tabular}}
\par
\smallskip
{$^\dagger$ The neutral density filter can be
mechanically rotated in and out of the beam}
{while the system is cooled down.}\\
{$^\ddagger$ A black-polyethilene (BP) Yoshinaga
filter is an absorptive filter consisting of a thallium}  
{ salt deposited on a black
polyethylene substrate, the thickness of which is} 
{tuned to minimize
reflections~(Yamada \ea \cite{yamada62}).}
\par
\end{table}