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Developer's notes

XMM-Newton Science Analysis System

omgchain (omgchain-1.8.1) [???]

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Developer's notes

Currently the omgchain checks for the presence of the complete set of sub-windows in the ODF (for the case of the Engineering-2 data). The sub-windows are combined if a single image only if all four sub-windows are present, otherwise each sub-window is processed separately.

The OM-grism photometric accuracy and spectral resolution is under investigation. In any case the spectral resolution cannot be better than 8 pixels ( $\sim 50$ Å for the visual grism and $\sim 20$ Å for the UV).

The correction for coincidence loss is not applied currently because omgchain deals with extended sources. The reliable coincidence-loss correction method for extended sources is not developed yet. However, for faint to moderately bright spectra the effect should be less than 10%. For spectra affected by the strongly visible modulo-8 patterns, where coincidence loss is important, the extracted spectra are unreliable.

Some zero- and first-order spectra may overlap (see, e.g. Fig.7). This situation is currently not treated, and the user should be aware of such a possibility. In such cases zero-order images may cause inappropriate scaling of the output plot (Fig.8).

**Figure 3:** Original grism image (no Mod-8 correction applied, no rotation); visual spectral band
$\begin{figure}\centering\epsfysize =8cm \epsfig{file=U0125320801OMS005.ps, height=15cm}\end{figure}$

**Figure 4:** Mod-8 noise corrected and rotated grism image; visual spectral band
$\begin{figure}\centering\epsfysize =8cm \epsfig{file=R0125320801OMS005.ps, height=15cm}\end{figure}$

**Figure 5:** Visual-grism image with the sources detected by **omdetect** (the source regions are marked with the green ellipses)
$\begin{figure}\centering\epsfysize =8cm \epsfig{file=S0125320801OMS005.ps, height=15cm}\end{figure}$

**Figure 6:** Spectrum extracted from the image Fig.5 by **omgrism** (corresponds to the lowest source in Fig.5)
$\begin{figure}\centering\epsfysize =8cm \epsfig{file=sp005.ps, height=15cm}\end{figure}$

**Figure 7:** UV-grism image (mod-8 noise corrected; image rotated). Note two zero-order spectra overlapping the first order spectrum (target).
$\begin{figure}\centering\epsfysize =8cm \epsfig{file=R0125320801OMS014.ps, height=15cm}\end{figure}$

**Figure 8:** Spectrum extracted from the image Fig.7 by **omgrism**. Note a zero-order spectrum oberlapping with the first-order target spectrum.
$\begin{figure}\centering\epsfysize =8cm \epsfig{file=G0125320801OMS014SPECTR0000.PS, height=15cm}\end{figure}$

XMM-Newton SOC/SSC -- 2008-10-22