CHANGING ORBIT NUMBERS AND ACTUAL DURATIONS IN SUB-EXPOSURES
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
documentation and movie. Initially, the orbit number field is
blank, and can be populated either by explicitly setting the
values, or by running the
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
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 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
2615s, thus splitting the additional time between the 2 sub-exposures
of the CR-SPLIT.
There is a visibility overrun, which is due to an error in inputting
the exposure time (entering 2615s instead of 2515s). 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 (55s for all sub-exposures) and
equally splitting the unused visibility (220s), 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
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
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.