This document describes how to use orbit numbers and actual durations; see the training movie.

APT gives a proposer the ability to efficiently pack their orbits, and to support this two concepts have been implemented - orbit numbers and actual durations for sub-exposures. Sub-exposures are the components of single-line exposures, such as the 2 components that make up a CR-SPLIT=2 exposure, the 3 components when there is a 3-step PATTERN, or the 4 components when the number of iterations=4. Also note that even exposures with 1 component will have a sub-exposure, as that is the only way to allow setting the orbit number.

Orbit numbers, which are strictly for planning purposes, are used to specify which orbit an sub-exposure will be placed. If there is a scientific need need to have a group of exposures execute in an orbit, then this must be explicitly requested with an Sequence Non-Int; see separate Exposure Sequence separate documentation and movie . Initially, the orbit number field is blank, and can be populated either by explicitly setting the values, or by running the Orbit Planner. Once the Orbit Planner has been run, the assigned orbit numbers will be used to place exposures in orbits, unless they are explicitly reset.

Actual durations, which are the exposure times for the individual sub-exposures, are used to change the exposure time of one sub-exposure without affecting the others. This is different from changing the overall exposure time, where all pieces are equally impacted. Use of actual durations therefore overrides the value of the exposure time. If you have set an actual duration (either by filling in a value or using the Auto-Adjust feature (see separate Auto-Adjust documentation and movie)), the exposure time keyword will be rendered unedittable, since it is no longer being used by the software. Clearing the actual durations will allow the exposure time to be active again. Initially, the actual duration field is blank, and will remain blank unless explicitly set. You should only populate this field if you wish to change the value.

Looking at the results

With the visit specified, run the Orbit Planner to examine the results; see separate Orbit Planner documentation and movie . Note the "Clear Orbit Numbers and Actual Durations" and "Clear Actual Durations" buttons, which can be used to reset all values in the visit.

Packing the orbits

Since there is unused orbital visibility in orbit 1, there is a need to better pack the orbit. In this example, we will manually pack the orbit, but see the separate documentation and movie for a description of the Auto-Adjust function. For this orbit, we will pack the orbit by changing the exposure time from 2300s to 2610s, thus splitting the additional time between the 2 sub-exposures of the CR-SPLIT.

There is an orbital visibility overrun, which is due to an error in inputting the exposure time (entering 2610s instead of 2510s). The reason the sub-exposure did not drop down to the second orbit is that it was assigned an orbit number of 1. Thus, the orbit number prevented an accidental error in updating the exposure time from causing a cascading effect on the exposures (e.g. Exposure 2, split 2 going to orbit 2, and Exposure 3, copy 2 starting a third orbit). By reducing the exposure time, the orbit can be properly packed. Note that overheads can scale with exposure size so that adding or subtracting exposure time will not necessarily add or subtract that exact amount of time from the total duration of the exposure.

For the second orbit, the actual duration field will be used. Instead of updating the exposure time (53s for all sub-exposures) and equally splitting the unused orbital visibility (215s), the unused time will be added to the last two sub-exposures.

As previously noted, the use of the actual duration fields has made the exposure time field unedittable.

Common Orbit Number diagnostics

A new exposure, to execute in the third orbit, is now added. After populating the exposure, there is a diagnostic on the sub-exposure.

This message will occur whenever a new exposure is added. The field can either be populated manually, or the Orbit Planner will populate it during processing; if the latter is chosen, you should reset all orbit numbers prior to running the Orbit Planner.

Another common orbit number error occurs when you drag exposures, which results in the orbit numbers no longer being monotonically increasing. Also, when you duplicate an exposure, the new exposure keeps the same orbit number as the one it is a copy of. Thus, if the original exposure filled an orbit, then when a copy is made, a visibility overrun will occur since the duplicated exposure will be placed in the same orbit which was already packed.

Last modified: May 13, 2014.