Orbital Visibility and Scheduling

Orbital visibility is the time during the HST orbit when the target to be observed is not occulted by the earth. This is the time that is available for guide star acquisitions (6 minutes), target acquisitions, science exposures, calibration exposures, and instrument overheads.

For Phase I proposal development, use the Orbital Visibility Table in the Orbital Visibility section of the HST Primer to determine the available orbital visibility time for your observations based on their target declination. There are separate columns in this table for Large and Regular GO proposals, and Low (sky) and Shadow observing. This table provides a close approximation for how much time will be available based on your target declination.

During Phase II, the APT Orbit Planner must be used to construct your observation by organizing exposures into orbits within visits. The Orbit Planner will compute a value for the amount of time available specifically for your targets based on current HST orbit predictions.

Roughly 80% of the sky is available on any given day in three-gyro mode for observations whose scheduling opportunities are unconstrained by science requirements. There are three proposal dependent factors which determine whether or not a given target can actually be observed on a day (i.e., when its view from HST is not obstructed by the Sun or Moon). These factors are:

• The amount of orbital visibility time that is required per HST orbit,
• Restrictions on the orientation of the telescope (which impose implicit time restrictions).
• Restrictions on the time/date that the target(s) may be observed.

Orbital Visibility: It is possible to request specific, non-default orbital visibility durations for an observation. Specifying long orbit visibility times will result in restricting the number of days that your target can be observed, while specifying short orbit visibility times may increase the number of observing days available. However, shortened orbit visibility may result in less than optimal use of HST, since there will be potential observing time left unused in an orbit. The default values have been chosen to provide the best balance between observing opportunity and telescope efficiency. We use the term EFFiciency level to describe this variable. If your program requires a non-default orbit visibility time, you must justify the need (in Phase I) or contact your Program Coordinator (in Phase II). In summary, the orbit visibility time determines the total number of days that can possibly be available for unconstrained target observing.

Time/Date Restrictions: Based on the characteristics of a target, it may be necessary to restrict the total number of scheduling opportunities to occur within absolute start and end dates or require that a series of observations occur at certain relative times. The use of such absolute and relative (links) timing visit special requirements should be based strictly on science justifications and should be specified with as much tolerance as possibly can be provided without compromising the goals of the observation.

Telescope Orientation: Science goals may dictate the need to either explicitly orient a target within the science aperture or to align separate exposures relative to each other. Such orientation requirements impose an implicit timing restriction on each visit that has the potential to severely reduce the number of scheduling opportunities for the entire set of observations.

Whenever possible, observers should minimize the number of scheduling requirements placed on your observations and maximize the tolerances on such scheduling requirements. This will:

• Improve schedulability by allowing execution to occur at all times that your targets are viewable (thereby reducing the chance of scheduling conflicts with other programs) and
• Improve overall telescope efficiency by allowing execution at times of optimal orbital visibility.

PIs of programs who are requesting CVZ observations can use the APT Visit Planner to identify CVZ opportunities for their targets. Potential CVZ opportunities (number and duration in days) will be shown in the resulting Visit Planner Scheduling Suitability display for CVZ flagged observations or visits that are processed in the Visit Planner. Follow the discussion in the Verifying Scheduling Feasibility section for more information on running the Visit Planner and evaluating its displays and reports.

PIs of Large and Regular GO programs are encouraged to verify the scheduling feasibility of their observing program where timing and orient constraints are required. Taking the time to do this in both the Phase I and Phase II processes will facilitate the proposing, development and implementation of technically feasible HST programs. Scheduling feasibility is verified by running the APT Visit Planner on each Observation (Phase I) or Visit (Phase II). The Visit Planner produces the following outputs:

1. Scheduling suitability display
2. Available number of observing Days vs Orientation plot
3. Available orientation range vs Day of Year plot

Initial Verification Steps

In Phase I, you will verify your 'observations'. In Phase II, the entities verified are 'visits'. For simplicity, the following steps will refer to Phase I observations only, but the steps are identical for Phase II.

1. In APT, once you have defined your observations, click on the Visit Planner icon.
2. Run the Visit Planner on the observations by either selecting individual observations in the Tree Editor, or by selecting the Observations container (which will process all observations at once), and then clicking on the "Update Display" button. Note that in either case, observations that are linked together by timing constraints will be updated at the same time.
3. Click on the "Roll Range Report" button, and in the popup, select 'Plot Data' to generate the 'Days vs Orient' and 'Orient Range vs Day of Year' plots.

Analyzing the Visit Planner Results

The primary goal of running the Visit Planner is to identify those observations with problems that make them infeasible (e.g. a timing constraint that places the observation when the target is in the Solar Exclusion Zone), so that the PI can make the appropriate changes. A secondary goal is to identify those observations that will restrict their execution to a very narrow timeframe (e.g. - days vs weeks), since this could impact the overall efficiency of the science program. In cases where only limited opportunities are discovered to exist for an observation, PIs are asked to document (in Phase I) the scientific need for the constraints causing the narrow scheduling timeframe (e.g. why is a 2 degree ORIENTation range necessary?).

The Scheduling suitability Display presents a horizontal bar which spans the timeframe over which the target of an observation may be observed assuming the default orbital visibility times for your program type (Regular, Large or CVZ). Basically, you want to verify that there are a reasonable number of days available on which to execute your observation. In general, there are nominally 5 orbits on any given day that are available for observing a particular target with HST. Dividing the number of orbits that your observation requires by 5 gives a minimum number of days required to execute that observation.

The more days available, the less likely we are to have difficulties with scheduling conflicts between your observation and those of other programs. If you find that the number of available days is about equal to the number of days needed, it would be advantageous to explore what can be done to modify or relax constraints to gain scheduling flexibility. Obviously, if you need more days to execute than are available, your observation is technically infeasible with respect to scheduling. In that case, you can use the plots generated previously to evaluate options for modifying your observing constraints.

Using the Visit Planner Scheduling Suitability Display

Information about why an observation has limited or no suitability can be found by selecting the key to left of the suitability plot for each observation. This will expand the plot to show the details of the constraints that factor into the overall suitability (Target Viewing, Sun, BETWEEN, ORIENT, AFTER BY, etc.). This information can be used to see which constraints are restricting suitability.

When a potential problem is identified, the accompanying data plots provide information needed to analyze the orientation related components of the suitability display. This analysis will help you to modify special requirements to effect a more desirable scheduling suitability result.

• Available Number of Observing Days vs Orientation: This plot shows how many days are available in the cycle for a given spacecraft orientation. Use this information to balance science aperture orientation requirements with orbit number requirements.
• Available Orientation Range vs Day of Year: This plot shows the range of orientations that is available on each day of the cycle. This is especially useful if you require many orbits at a given orientation (ORIENT, SAME ORIENT), if you have two or more observations that must be offset from one another in orientation (ORIENT FROM), and if your observation combines a timing constraint (BETWEEN, AFTER BY, etc.) with an orientation constraint.

How to Save Unschedulable Observations

If in Phase I there is no scheduling suitability for an observation even after you have made modifications to observing constraints, you may rerun the Visit Planner using the 'Increase Scheduling Flexibility' option. When run in this mode, the Visit Planner will assume that only the mimimum orbital visibility time is available and therefore it will identify the absolute maximum number of days available for that target. If the subsequent result still indicates that there is no scheduling suitability, then the observation is indeed infeasible as defined and should not be proposed.

If on the other hand there is scheduling suitability, then you may propose for that observation. However, since you are basing the feasibility of that observation on the assumed use of mimimum orbital visibility ('Increase Scheduling Flexibility'), then you should indicate this fact in your proposal.

Special Instructions for Computing Your Orbit Request for Observations that Require the 'Increase Scheduling Flexibility' Option.

As stated, using the 'Increase Scheduling Flexibility' option implies that an observation must be scheduled based on the minimum orbital visibility times for its target's declination range. Accordingly, your orbit request should reflect this assumption. Therefore, when following the instructions for calculating your Orbit Request in the proposal instructions, you should use the values in the Large visibility column rather than the default Visibility column. It is important to use these alternate values to be sure that the appropriate number of orbits are requested. An observation that requires the minimum orbital visibility will likely require more orbits to execute than an observation on the same target that can use the default orbital visibility.

The following figure is a graphical representation of the general decision process for determining the scheduling feasibility for fixed targets. The decision process for unconstrained observations involves minimal effort, whereas constrained observations require careful consideration of how orientation and timing specifications effect scheduling opportunities.