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STRAYLIGHT FROM NEARBY BRIGHT SOURCES

 

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 gif 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 tex2html_wrap_inline1538 (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 tex2html_wrap_inline1540 , 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 ( tex2html_wrap_inline1540 = 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 tex2html_wrap_inline1544 m) or ISOCAM (at tex2html_wrap_inline1546 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.

  figure10

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).



ISO Science Operations Team
Tue Aug 6 11:04:33 MET DST 1996