4.2.3 Sky visibility during the XMM-Newton mission

Target visibility should always be checked using the Target Visibility Checker available at URL http://xmm2.esac.esa.int/external/xmm_sched/vischeck/.

The Target Visibility Checker allows to input the RA and Dec of a source and find the orbits and times when it is visible to XMM-Newton subject to viewing constraints. The constraints applied in the visibility analysis are actually used for satellite operation. More details are provided by the Target Visibility Checker online help.

Fig. 102 displays a first-order approximation of the sky visibility during AO-9. It is shown for illustrative purposes and can be used as a rough guideline to assess whether any visibility problem exists, but it is not intended to be used for quantitative statements.

**Figure 102:** *Approximate sky visibility, as a fraction of the total time available for science*
$\begin{figure}\begin{center} \leavevmode \epsfig{height=0.41\textheight, file=figs/xmmskyfrac.ps} \end{center} \end{figure}$

**Figure 103:** *Maximum target visibility within a single orbit of XMM-Newton during AO-9*
$\begin{figure}\begin{center} \leavevmode \epsfig{height=0.41\textheight, file=figs/xmmskymax.ps} \end{center} \end{figure}$

**Figure 104:** *Evolution of maximum target visibility within a single orbit for future AOs*
$\begin{figure}\begin{center} \leavevmode \epsfig{height=0.9\textheight, file=figs/xmmskymax_ao09_ao14.ps} \end{center} \end{figure}$

Fig. 103 shows sky areas for which a certain maximum continuous target visibility is reached within a single revolution in AO-9. Due to the evolution of the XMM-Newton orbit, this diagram is in fact changing with time; the visibility of some sky regions has become better along the XMM-Newton lifetime, while it is now significantly worse in some other regions. The same Fig. 103 has been generated for next AOs (Fig. 104), based on orbit evolution forecast. Absolute numbers on the visibility of a given region must be taken just as rough estimates, but the overall trend is correct: the poorer visibility occurs near the Earth location at spacecraft apogee and this position moves to higher declination and lower right ascension.

We recall that to obtain accurate visibility information for a given target coordinates, the Target Visibility Checker must be used.

XMM-Newton Users Handbook

4.2.3 Sky visibility during the XMM-Newton mission