In Operations mode the UVCS performs coronal observations (sun-center or offset-points types), roll/pointing adjustments, synoptic observations (which involve both coronal observations and roll maneuvers), disk observations (which involve frequent pointing adjustments), and occasionally observations of stars (for calibration purposes). Disk- and coronal observations requiring offset pointing will be performed only during the daily 8-hour contact periods. Coronal and synoptic observations are performed during both contact and non-contact periods. During non-contact periods the instrument axis remains pointed at sun center. Roll maneuvers may be performed during these times after TBD months of flight experience.
Sun-center type coronal observations are used for in-depth study of particular coronal regions. Each observation typically lasts between 0.5 and 5 hours. These observations are performed both during contact and non-contact periods. In the latter case, typically a series of coronal and synoptic observations are scheduled, starting near the end of one 8-hour contact period and ending near the beginning of the next 8-hour contact period.
Roll maneuvers are performed between observations to set up the instrument field of view for the next observation. Immediately following each roll maneuver there will be a fine pointing correction to ensure that the instrument axis is pointed towards sun center; this takes at most 20 seconds. The assumed roll rate is 10 degree per minute (TBC), and the assumed pointing correction rate is 0.5 arcsec per second (TBC).
Synoptic observations are performed daily. The purpose is to gather data which allow investigation of the long-term evolution of the solar corona. The synoptic observations are used for the daily image to be provided to the ECS, and they are the primary tool for planning. Hence they must be done in real time. The synoptic observations consist of 10 (TBC) sun-center type coronal observations performed at different roll-angle orientations in order to view the entire corona. Each observation lasts about 25 minutes; total duration of the synoptic observations is 6 hours. Roll maneuvers are performed every half hour and last approximately 3 minutes. The direction of roll during a synoptic observation is the same each day (clockwise or counter-clockwise). Hence, a 360 degree roll, which may be done in the course of other observations, is needed each day to return the roll mechanism to its starting position.
During the first few months of mission operations the UVCS performs synoptic observations during the 8-hour contact period. After a few months synoptic observations may be performed outside the 8-hour contact period. The time of day of synoptic observations is determined by the DSN schedule. Let T1, T2, T3 denote the three 1.3-hour contact periods used for tape dumps, where T3 is the contact just prior to the longer 8-hour contact period. Then the UVCS synoptic observation will start about 1 hour before the T1 contact, so that it will be completed just prior to the T2 contact. This allows some of the synoptic data to be captured during the T1 realtime contact and the remainder to be stored on tape recorders and transmitted in the form of playback data during the T1 and T2 contacts.
Offset-pointing type coronal observations require pointing adjustements to be performed before and after the observations, but not during the observations. These pointing offsets have to be within +/- 12 arcmin (TBC) from solar disk center. Such observations typically last a few hours and are performed once or twice per week. They are performed during 8-hour contact periods.
Disk observations are performed once or twice per week at certain roll angles. Each disk observation lasts about one hour, and requires a maximum offset of about 40 arcmin. For example, a disk observation may start with an 8 arcmin offset to place the instrument field-of-view on the solar limb (duration 1 minute), followed by 64 steps of 0.5 arcmin each to scan the field-of-view across the solar disk (duration 3.75 seconds/step). The dwell time at each pointing position is of order 1 minute. Finally, a 40 arcmin maneuver is required to reposition the field-of-view in the corona (duration 5 minutes). The main purpose of the disk observations is to acquire the data necessary for calibrating the coronal intensity measurements, both from the synoptic program and from other coronal observation programs. Such calibrations are essential for deriving plasma densities and outflow velocities from the coronal intensity data. Disk observations will always be performed during contact periods to allow such observations to be aborted by ground command in case of solar flares. Disk observations disturb the temperatures within the UVCS instrument, which may affect the optical performance of the spectrometer. Therefore, UVCS would only be able to make less demanding types of observations until the temperature is re-established.
Pointing adjustments for disk and offset-pointing observations use a nominal pointing rate of about 8 arcsec per second (TBC). A higher rate of 20 arcsec per second is also to be available, to be used in cases when disturbance to the spacecraft is not an issue.
Some observations may require more frequent interaction with the instrument in order to acquire targets which have small angular extent (e.g. stars or polar plumes) or evolve rapidly in time (e.g. coronal transients). Generally, such observations are performed by interactively positioning the mirror/occulter mechanism (every few minutes) and then starting a short observation sequence. After completion of the sequence the data is analyzed on the ground to ensure correct pointing. Such ``interactive" observations are performed only during the 8-hour contact periods, during a pre-defined time window (several hours), and require a higher than average command data rate (approximately 1 block command per minute). The UVCS does not support inter-instrument data exchange capabilities.