\begin{table}%t1
%%\centering
\par
\caption{Optical requirements and environmental conditions.\label{tab1}}
\begin{tabular}{ll} 
\hline \hline Encircled energy & $>$70\% in a pixel ($13\times 13$~$\mu\rm  m^2$) \\
Modulation Transfer Function (MTF) & $>$70\% at Nyquist Frequency (38.5 c/mm)\\
 Backfocus & $\approx$50 mm\\
Transmission (at 550 nm) & $>$70\% at the beginning of life\\
& $>$60\% at the end of life (5 years mission)\\
Spectral range & Johnson $V$ band centred at 550~nm\\ \hline Lens barrel
material &Titanium alloy (Ti-6Al-4V)\\ Backfocus (part of Focal Plane
Assembly) material &INVAR\\ Radiation dose &42 krad over 5 years (worst
case)\\ Nominal conditions &0~C (in vacuum)\\ Thermal range (operational
conditions) &$-$20~C to $+$20~C\\ Thermal range (non-operational
conditions) &$-$80~C to $+$45~C\\ Vibrations (non-operational conditions) &
Random: 27 g RMS in the range 20--2000~Hz\\ \hline
\end{tabular}
\end{table}