Coronagraphic Observations in Two Gyro Mode

First, coronagraphic observations will tend to be more sensitive to the jitter of the telescope line of sight, and the stability of that jitter, than other types of observations. Chapter 5 of the Two Gyro Handbook provides general information about the jitter in Two Gyro mode. The ACS (link to section in ACS handbook) and NICMOS (link to section in ACS handbook) Instrument Handbooks specifically comment on impacts to coronagraphic observations with those instruments.

A second possible impact to coronagraphic observations is due to a scheduling constraint in Two Gyro mode. Some coronagraphic observations are carried out at two different roll orientations within the same ~50 minute orbital visibility period. This can be used to try to discriminate real features from those induced by temporally varying PSFs due to effects such as breathing, or thermal changes in the instrument. In Two Gyro mode it will not be possible to obtain observations at two different roll orientations in the same orbital visibility period. This is a result of the extra steps (and time) required in Two Gyro mode to transition from a slew to FGS guiding (see Chapter 4 of the Two Gyro Handbook). It will be possible to obtain such observations in successive orbits.