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 25 in § 3.5.9.
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.
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 25).
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. We remark that the planets avoidance only affects OM, but not EPIC or RGS instruments.
Bright optical/UV light entering the RGS under certain angles can deteriorate the quality of the science observations. This is described in § 3.4.6.1.
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.
For more details see chapters § 3.3, § 3.3.8 and § 3.3.9.
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 3.7.1.3).
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 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. It should be noted that the count rate limits include the background.
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 program source (§ 4.4.2).