In planning observations it is possible to reduce pile-up effects
by pointing the axis of the telescope away from the target of interest
by a small amount. As the PSF gets larger with increasing off-axis angle
the detected flux from a point source will be spread over many more pixels,
hence decreasing the probability of photon pileup within a single CCD frametime.
Naturally the telescope used off-axis has degraded spatial resolution,
so this technique is best used for relatively isolated point sources. Note
also that the effective area of the HRMA decreases with off-axis angle,
but in these cases the desire is to limit the rate of photons detected,
so the area decline is not usually a problem. The user should also take
care that the offset pointing does not place the target within the `gaps'
between CCDs, or off the active chip array.
Figure 6.40 shows the off-axis
angle required to lower the pile-up fraction to 1% versus incident flux.
By using this figure, an observer can request an off-axis angle which reduces
the source flux at which pile-up occurs by two orders of magnitude.
Figure 6.40: Flux limit for 1%
pile-up as a function of off-axis angle.
