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.
At present, there is not any problem for the star tracker on board XMM-Newton to acquire a requested pointing. However, if a star with V magnitude less or similar to 2 is within 2.5 radius of the aimed target position, special operations that imply longer slew manoeuvres are needed. Anyway, the avoidance of bright sources within the field of view of the star tracker is not a concern to the user.
With the 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 frame rate 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 26 in § 3.5.9.
Since the above refers to point source limits, the quantity to be specified in XMM-Newton 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 Table 26).
In addition, the following solar system sources must be avoided by the OM, with avoidance angles as listed:
For the EPIC pn, care should be taken to avoid the presence of a bright optical source located at the same RAWX as the target and at smaller RAWY than the target, i.e. between the target and the CAMEX. Charge would otherwise be transferred over an additional optically generated signal which would alter the CTE in a way which is not modelled, thus adversely affecting the energy reconstruction. This precaution should also be taken for windowed modes, even if the optical source falls outside of the window.
The X-ray count rate must be kept below a threshold (as quantified in Table 3) in order to keep pile-up below 1%. Otherwise the energy resolution and the quality of spectro-photometry could be compromised in the case of extended sources. For point sources, the effect is not so important and, even if the pile-up is over the threshold indicated above, good results on spectral shape and absolute flux can be obtained by excising the core of the PSF, what will obviously produce some loss of signal.
Observations with an expected high level of photon pile-up are not per se disallowed, but a special scientific justification is expected 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; §§ 3.3.9 and 22.214.171.124).
Bright optical off-axis objects can leak through the filters and generate false X-ray events that can contribute to the degradation of the telemetry bandwidth. This effect becomes important if the off-axis source is 50 times brighter than the on-axis limit, i.e. 4 or 5 magnitudes brighter.
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 115 counts/s. For EPIC pn imaging modes, it is about 600 counts/s (over all active CCDs), and therefore observations of single bright point sources at the pile-up limit are not constrained by telemetry. For Timing Mode, a conservative limit can be estimated to be 450 counts/s. Sources at the pile-up limit can still suffer a % exposure loss due to counting mode. It should be noted that the count rate limits include the background. In Timing Mode the full energy band background count rate measured during the mission is counts per second and column (99% confidence level). In most cases where the source count rate requires the usage of Timing Mode, the source largely dominates the integrated count rate in the EPIC-pn sensitive energy bandpass. It should be stressed that the exact background rate measured during a specific observation is dependent on the position of the source due to the potential confusion of serendipitous point sources as well as of truly diffuse emission, especially along lines-of-sight close to the Galactic plane.
If the total (source plus background) count rate is above a certain limit (a few hundred counts/s), either because of the source being too bright or due to very high flaring background, the EPIC cameras will switch to the so-called `counting mode', a special (non-science) instrument mode where no transmission of information for individual X-ray events occurs.
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 are much lower. Therefore, photon pile-up in RGS is usually negligible (see however § 3.4.5 for cases where pile-up may occur). Only optically bright sources can impose constraints on RGS observations, as mentioned above. However, radiation of nearby X-ray sources should not fall on the dispersed spectrum of the programme source (§ 4.4.2).