XMM-Newton Users' Handbook

3.5 OPTICAL MONITOR (OM)

Besides its three X-ray telescopes, XMM-Newton also has a co-aligned 30-cm optical/UV telescope (OM), providing for the first time the possibility to observe simultaneously in the X-ray and optical/UV regime from a single platform. A summary of OM's salient properties is provided in Tab. 12. Although small in size, the OM is a powerful instrument because of the absence of atmospheric extinction, diffraction and background. It can perform optical/UV observations simultaneously with the X-rays observations, in the waveband from 180 to 600 nm. Images of the central part of the X-ray field of view with a resolution of ca. $1''$ (depending on instrument configuration), low-resolution grism spectra of the optical counterparts of X-ray sources or high time-resolution photometry can be obtained. Due to the extreme sensitivity of its detectors, the OM is very well suited for observing faint stars. It cannot be used for observations of optically bright sources which would damage the detectors permanently ($m_V \leq$ ca. 7.4 mag, for an A0 star with the V filter). The OM is a photon-counting instrument, therefore, as explained below, objects with high photon rates, even within the safety limits, will produce poor quality data.

Table 12: OM characteristics - an overview

Total bandwidth$^1$	180 - 600 nm
Spectral bandwidth$^2$	180 - 600 nm
Sensitivity limit$^3$	20.7 mag
Field of view	ca. $17'$
Pixel size	0.476513 arsec/pxl
PSF (FWHM)	$1''.4$ - $2''$$^4$
Timing resolution$^5$	0.5 s
Resolving power$^6$	$\sim$ 250
Brightness limit$^7$	$m_V = 7.4$ mag

Notes to Table 12:
1) See Fig. 87 for filter bandpasses.
2) Covered by two grisms.
3) 5-$\sigma$ detection of an A0 star in 1000 seconds (B filter; see Table 20).
4) Depending on the filter
5) In the Fast Mode.
6) $\lambda$/ $\Delta\lambda$ in the optical and UV grisms.
7) Highest magnitude of a star, which can be observed with the OM without exceeding the instantaneous maximum flux. This brightness limit is dependent on the filter and the spectral type. The number quoted in Tab. 12 refers to an A0 star with the V filter; see also Tab. 23. The dose limit is instead related to the total amount of photons detected during a whole observation.