Note that the satellite lower limit of 2 arcsec for 
and 
is a limitation to the gain which can be
achieved with this method.
The area of the sky covered by a raster (M, N, 
M,N)
is in 
:
But the area of the sky covered with the improved flat-field accuracy is only the central portion:
For this area, each point of the sky is seen by 
pixels:
The 3 pixels wide border zone of Area1, around Area2, is covered with a flat-field accuracy degrading outwards.
In order to compute the number 
of exposures per
pointing, proceed as in paragraph 
.
The lower limit of 
4 applies also for a raster