The third critical parameter of the mirror performance, as listed above, is the efficiency of the straylight rejection.
Without X-ray baffles, the XMM mirror modules would suffer from X-ray straylight entering the tubes. The effect is that the image of a 30'area of the sky (the EPIC FOV) would be contaminated by a diffuse light background produced by X-ray sources located outside this field. X-ray straylight in EPIC is produced by rays which are singly reflected by the mirror hyperbola and which reach the sensitive area of the camera unfocused. An X-ray baffle was implemented to shadow those single reflected rays. It consists of two sieve plates made of concentric annular aperture stops located in front of the mirrors at 85 mm and 145 mm, respectively. The design is such that the entrance annular aperture of each mirror remains unobstructed for on-axis rays. Most of the rays with large off-axis angle are vignetted and cannot reach the detectors anymore via a single hyperbola reflection.
The straylight collecting area in the EPIC detector as a function of
off-axis angle of a point source is anticipated to be lower than 3 cm2
for stray sources located between 20' and 
from the optical
axis. At higher off-axis angles it is completely negligible. The ratio of
the X-ray straylight collecting area to the on-axis effective area is
smaller than 0.2% at 1.5 keV for a point source located at off-axis
angles of 0.4-1.4
and negligible at higher off-axis angles.
This ratio, expressed in surface brightness values, is even smaller
since the stray-image is unfocused at detector position.
Assuming a 10 counts/s source, for EPIC MOS on-axis observations half of the incoming photon flux is distributed over ca. 150 pixels (adopting a 15'' HEW), with a central peak. The light of the same source, observed at a large off-axis angle, is distributed over about 1/4 or 1/5 of the total EPIC FOV (of order 106 pixels), leading to a mean count rate of 10-5 counts s-1 pixel-1 of diffuse straylight. However, this is only a rough approximation, because the stray radiation due to an off-axis point source is not uniformly distributed in the EPIC FOV, but produces near-annular structures in the image.
This illustrates the high straylight rejection efficiency of the XMM mirror modules. EPIC observations of isolated sources and RGS spectra are hardly affected. For bright extended sources (which are larger than the EPIC FOV, such as some supernova remnants, clusters of galaxies, stellar clusters and star forming regions), on the contrary, there might be a degradation in the ability to perform spatially resolved spectroscopy.
Simulations using SciSim (Appendix A) are strongly
recommended prior to proposing observations in case the target is within
of a bright X-ray source, or in case imaging is foreseen of
an extended object that is larger than the EPIC 30' FOV. Which level
of straylight is problematic for a given proposal depends largely on
the scientific objectives and on the ratio of the flux of the off-axis
source and the required minimum detectable source flux within the FOV.