In both the X-ray (EPIC/RGS) and the optical/UV regime (OM) there are restrictions with respect to the brightness of sources within the field of view, albeit of a different nature.
With increasing count rate of a target, the science quality of OM data for that source is compromised by deadtime effects. The effect is of the order of 5% for a source with a count rate of 10 counts/s, increasing to approximately 30% for sources with count rates of 50 counts/s (assuming in both cases a CCD framerate of 100 Hz).
A different kind of limitation is imposed on science observations by the OM detector photocathode. MCP-based intensifier tubes show localised gain depressions caused by bright point sources. The main cause of the sensitivity loss is related to the defects in the photocathode. From measurements made with OM test tubes, a non-linear relationship between the local sensitivity loss with accumulated count dosage was established. The brightness limits for all OM filters are tabulated in Table 19 in § 3.5.8.
OM observations of sources beyond these brightness limits are not allowed. These limits hold for any source in the OM FOV, even if it resides outside an active OM window. One way of avoiding bright sources within the FOV located near science targets is to use the magnifier (which reduces the size of the FOV, as described in § 3.5.6.3).
Since the above refers to point source limits, the quantity to be specified in XMM observing proposals for extended sources, like e.g. galaxies with active nuclei, is the maximum surface brightness (in most cases of the central object) in V magnitudes per square arcsec. The limits (at the position of the brightest emission) are the same as for point sources (see Tab. 19).
In addition, the following solar system sources must be avoided by the OM, with avoidance angles as listed:
Avoidance of planets will be ensured by the SOC and is thus not a concern to the user.
Bright optical/UV light entering the RGS under certain angles can deteriorate the quality of the science observations. This is described in § 3.4.8.1.
The X-ray count rate must be kept below a threshold (0.8 cts per full frame for EPIC in order to keep pile-up below 1%) in order to obtain high-quality data. Otherwise the energy resolution and the quality of spectro-photometry would be compromised. The count rate limit depends on EPIC observing mode, as quantified in Tab. 4.
Observations with an expected high level of photon pile-up are not per se disallowed, but a special scientific justification is required stating why and how the scientific goals of the proposal can be achieved despite the expected pile-up (and thus the deteriorated spectral response and PSF quality; §§ 3.3.10 and 3.7.1.3).
The EPIC telemetry limits are not hard-wired, making it difficult to provide universal guidelines. For EPIC MOS imaging modes the baseline count rate allowed is about 150 photons/s. For EPIC pn it is about 300 photons/s and therefore observations of single bright point sources at the pile-up limit are not constrained by telemetry. The brightest extended objects, such as the Cas A supernova remnant approach the telemetry limits, and can be accommodated by limited change of telemetry allocation depending on instrument usage, or by graceful loss of data frames. For timing modes the event rates are limited to about 1500 events per second by both the camera and telemetry constraints.
Due to the much smaller effective area of RGS compared to EPIC and the dispersion of the source spectra over many detector pixels, the source count rates per pixel will be much lower. Therefore, photon pile-up in RGS will be negligible. Only optically bright sources can impose constraints on RGS observations, as mentioned above. However, radiation nearby X-ray sources should not fall on the dispersed spectrum of the program source (§ 4.4).