\begin{table}%t1
\caption{{\it Beppo}SAX observation log of Mkn 421.}
\label{tab1}
%\centerline
{
\begin{tabular}{lcccccc}
\hline  \hline
 Date & Start UT & End UT & LECS exp. (s) & MECS exp. (s) & PDS exp. (s) & PDS count rate$^1$ (s$^{-1}$) \\
\hline
1997/04/29     & 04:02 & 14:42 & 11~562  & ~23~283 & ~18~388 & 0.21 $\pm$ 0.04 \\
1997/04/30     & 03:19 & 14:42 & 11~379  & ~23~975 & ~22~079 & 0.16 $\pm$ 0.06 \\
1997/05/01     & 03:17 & 14:42 & 11~171  & ~23~713 & ~21~748 & 0.13 $\pm$ 0.06 \\
1997/05/02     & 04:10 & 09:41 & ~4417  & ~11~368 & ~10~594 & 0.31 $\pm$ 0.08 \\
1997/05/03     & 03:24 & 09:41 & ~4326  & ~11~672 & ~10~734 & 0.10 $\pm$ 0.08 \\
1997/05/04     & 03:25 & 09:45 & ~4880  & ~12~177 & ~11~168 & 0.16 $\pm$ 0.08 \\
1997/05/05     & 03:32 & 09:45 & ~4971  & ~11~911 & ~10~946 & $<$0.08 \\
 %              &       &       &         &         &         &  \\
1998/04/21-22  & 01:52 & 03:13 & 23~620  & ~29~547 & ~26~676 &  0.51 $\pm$ 0.05\\
1998/04/23-24  & 00:27 & 06:37 & 27~188  & ~34~656 & ~30~923 &  0.41 $\pm$ 0.05\\
1998/06/22-23  & 07:16 & 02:21 & 11~279  & ~32~516 & ~27~824 &  0.45 $\pm$ 0.05\\
%               &       &       &         &         &         &  \\
1999/05/04-08  & 18:16 & 03:21 & 62~868  & 121~813 & 114~693 &  0.14 $\pm$ 0.02\\
 %              &       &       &         &         &  \\
2000/04/26-/05/03 & 17:36 & 02:13 & 140~374 & ~127~702$^2$ & 151~251 & 3.34 $\pm$ 0.03 \\
2000/05/09-12  & 04:09 & 07:06 & 70~070  & 139~376 & 131~768 & 2.55 $\pm$ 0.02 \\
\hline
\end{tabular}}
\medskip
$^1$ 15--90 keV energy band.\\
$^2$ MECS not operating in the second half of the observation.
\end{table}