Observations close to a bright source are limited by straylight. Major
contributors are the diffraction, from the struts near the edge of the field
of view and, between 3 arcmin and 1 degree 
off-axis angle, from the edge of
the secondary mirror, and, further away, the diffusion of the primary mirror
which is directly illuminated by the bright source.
As a measure of the telescope's rejection of radiation from a source
from outside it's field-of-view, the quantity NDP 
(Normalized
Detector Power) is defined as the Total Power (in W) as seen
in the 3 arcmin field-of-view for a source at off-axis angle 
,
divided by the total power (in W) incident on the telescope, when the same
source is placed on-axis.
As an illustration of the size of the effect, Fig. 10
gives the NDP ( = 3 arcmin ... 2 deg) for a point source
obtained by computer models for the case
of Jupiter (assumed to be a point source when outside the field-of-view)
observed by ISOPHOT (at 
m) or ISOCAM (at 
m).
Observation programmes dedicated to measure the actual effect of straylight and their data analysis are ongoing. The ISO Data User's Manuals detailed descriptions of the effects of straylight.
Figure 10: Straylight from near-field bright point-like source, along azimuthal angle 0. (adapted from BRO Report # 2270 ``In-field and near-field illumination by Jupiter'', Breault Research Organisation, Inc., Tucson, USA, 1993).
