This chapter gathers together several topics pertaining to observation planning, irrespective of focal-plane instrument and grating configuration, to serve as additional guidelines for preparing proposals. Most all of these topics are automatically addressed by the observation visualizer software discussed in Chapter 12 . Offset Pointing The offset pointing convention for Chandra is that a negative offset of a coordinate moves the image to more positive values of the coordinate and vice-versa. Example of offset pointings of the ACIS instrument are shown in Figure 3.1 .
North is up, and East is to the left.
Roll is measured positive, West of North. The roll angle shown is . Note the small circle at the location of the ACIS-S aimpoint. There are a number of factors that limit the precise time when observations can be performed. Radiation Belt Passages High particle-radiation levels are encountered as the Observatory approaches perigee.