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 strictly simultaneous observations in the X-ray and optical/UV regime from a single platform. A summary of OM's salient properties is provided in Table 15. Although small in size, the OM is a powerful instrument because of the absence of atmospheric extinction, diffraction and background. It has three optical and three UV filters over the wavelength range 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 the detector, the OM is well suited for observations of faint sources. However, observations of optically bright sources would cause permanent damage to the detector ($m_V \leq$ ca. 7.4 mag, for an A0 star with the V filter), thus, the OM can not be used if a bright optical source is in the field. 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 15: 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 arcsec/pxl
PSF (FWHM)	$1''.4$ - $2''$$^4$
Timing resolution$^5$	0.5 s
Resolving power$^6$	$\sim$ 180
Brightness limit$^7$	$m_V = 7.4$ mag

Notes to Table 15:
1) See Fig. 102 for filter bandpasses.
2) Covered by two grisms.
3) 5-$\sigma$ detection of an A0 star in 1000 seconds (B filter; see Table 22).
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 Table 15 refers to an A0 star with the V filter; see also Table 25. The dose limit is instead related to the total amount of photons detected during a whole observation.