\begin{table}%t1
\caption{\label{tab1}Summary of the in-flight calibration targets used since the launch on 2004~November~20.}
\small%\centerline
 {
\begin{tabular}{lllll}
\hline  \hline
%&&&& \\[-8pt]
Object$^\ddagger$  & Type & Mode & Purpose &  Exposure (ks) \\
\hline
%&&&& \\[-8pt]
RXJ 1856.5-3754$^{x,c,s}$ & Neutron star & PC/WT & Low energy response & 81/70 (31/40)$^\dagger$ \\
PKS 0745-19 & Cluster of & PC & Effective area & 63 (20)\\
            & galaxies  &    &                &    \\
2E 0102-723$^{x,c,s}$ & Line-rich SNR & PC & Gain, energy resolution and    & 119 (28)\\
            &    & WT & shoulder    & 76 (26) \\
Cas A$^{x,c,s}$  & Line-rich SNR & PC & Energy scale offset, gain, shoulder, &
235  (67)\\
            &    & WT & CTI and energy resolution  &  62 (39)  \\
3C 273$^{*/x,r,c,s}$      & Quasar & WT & Effective area & 18  \\
PSR 0540-69$^x$  & Pulsar & PC/WT & Effective area    & 56/26 (25 in PC)\\
PKS 2155-304$^*/x$ & Blazar & PC/WT & Effective area  & 14/13 (5/9)\\
NGC 7172$^{*/x}$    & Seyfert 2 & PC & Redistribution & 15  \\
G21.5$^x$        & Featureless SNR    & PC & High-energy shelf  & 43 (78 in PC
\& 77 in WT) \\
Mkn 421$^*$      & Blazar & WT & Effective area  & 33 (10) \\
Crab$^{x,c,s,r,i}$      & Pulsar/SNR & WT & Effective area  & 46 (5) \\
\hline
\end{tabular}}
\medskip
$^\ddagger$ The letter indices $x,~c,~s,~r,~i$ correspond to sources observed by other X-/Gamma-ray instruments for calibration purposes: {\it XMM-Newton}; {\it Chandra}; {\it Suzaku}; {\it RXTE}; {\it INTEGRAL}, respectively. \\
$^\dagger$ The numbers in parentheses correspond to the exposure time collected from September~2007 to the end of December~2007 and dedicated to the re-calibration of the XRT after the substrate voltage change (see  Sect.~\ref{Vss}), while the other numbers correspond to the calibration
data collected prior to the substrate voltage change on 2007~August~30. \\
$^*$ This symbol corresponds to the sources for which simultaneous calibration observations were performed with the {\it XMM-Newton} EPIC~cameras.
\end{table}