Reduced Gyro Mode Tips and Resources

On June 4, NASA announced that Hubble has transitioned to Reduced Gyro Mode (RGM). This page offers advice to program PIs and proposers for how to make their programs observable and schedulable under RGM.

Reduced Gyro Mode Tips

  • It will be useful to read the RGM Primer and the RGM presentation to the STUC. This provides both some background as to how HST is now being operated and also indications as to which type of observations are possibly more difficult to schedule.
  • The largest impact of RGM is that scheduling observations is significantly more constrained as HST can only acquire targets when BOTH a) the target is in the telescope's field of view, AND b) the fixed head star trackers (which point at a large angle away from the telescope's line of sight) have a view of the sky not blocked by the earth or moon. This leads to the reduced field of regard at any one time and the limited range of orientations available at any point on the sky.
  • Be certain that you have the most recent version of APT (check for updates frequently as changes may happen as operations in RGM are optimized).
  • Visit Planner in APT provides insight into potential opportunities to schedule your observations and has been updated for RGM. If zero opportunities exist, we cannot execute the observations. It is necessary that you relax some constraints on your observations.
  • Only impose special requirement constraints on your observations that are absolutely required to accomplish the proposed science. The more constrained your program is, the longer it will take to schedule. Constraints that are most likely to cause your program to have fewer or even zero scheduling opportunities are:
    • Timing links (i.e. use of special requirements AFTER, BEFORE, BETWEEN, GROUP WITHIN, SEQ WITHIN).
    • Orientation constraints: ORIENT. Note that SAME ORIENT AS, ORIENT TO FROM are additionally demanding from a scheduling perspective as they impose timing requirements linked to other visits.
  • In most cases, the available observing time within each orbit in RGM will be shorter due to the extra time required for slewing and guide star acquisition each orbit. This means that there will be fewer orbits in RGM in which your observations can schedule during the cycle. Previously in 3-gyro mode, programs were normally scheduled using the 30% most appropriate orbits. In order to improve the overall scheduling efficiency and opportunities in RGM, this default is now 70%. In other words, observations must be trimmed slightly (with respect to 3-gyro mode durations) in order to optimize overall observatory scheduling efficiency and flexibility in RGM. Additionally, in some cases, your Program Coordinator (PC) may need to increase this to 100% (SCHED100) to make it possible to schedule your observations. That will result in additionally shortened exposure times.
  • APT now incorporates knowledge of the complex viewing geometries used in Reduced Gyro Mode. However, as before, it cannot account for phase shift in the orbit due to variable atmospheric drag so observations requiring absolute time (e.g. exoplanet transits) still cannot be scheduled more than ~10 weeks in advance.
  • Observations that require HST to operate without FGS guiding (e.g. DROP TO GYRO, PCS MODE GYRO) are no longer permitted. This precludes DASH mode, multiple acquisition within a single orbit, and guide star hand-offs.
  • The solar exclusion angle is now 60 degrees.