Aimpoint Trending

Chandra Aimpoint Trending

The observation aimpoint is defined as the position on the focal plane where an on-axis target is located, assuming that the SIM offset is zero and both Y Offset and Z Offset (aimpoint offset) are zero. Over the course of the mission the mean aimpoint has drifted over 30 arcsec, due primarily due to the steadily increasing temperature of the Aspect Camera Assembly (ACA). See the Details and References sections for further information.

In addition to a mean drift there has been a steady increase in the scatter on time scales of days. This term is directly related to the absolute pointing accuracy which describes the positional accuracy with which a target can be placed on the detector. Starting around 2015 the amplitude of this scatter has been large enough to potentially impact "pointing-sensitive" observations in which an absolute pointing error might affect science or instrument safety. This includes ACIS windowed or subarray observations, or grating observations of unusually bright sources that must be kept just off the detector.

The mean aimpoint drift adds to pointing uncertainty by introducing a discrepancy between the predicted median aimpoint (used in the planning process) and the actual median aimpoint around the time of observation. The predicted aimpoint for each detector fixed at a preset value since cycle 18 (2017) as stated in the Proposers' Observatory Guide. The temperature-dependent ACA alignment shift is compensated by means of a dynamical pointing offset that is introduced into the planning process. See the FDB transition to dynamic aimpoints for an overview and the Aimpoint transition plan for implementation details.

Legacy aimpoint monitor page and details

For the historical version of the aimpoint trending page that was used as the basis for understanding the aimpoint issue and developing the dynamical offsets concept, see the Legacy aimpoint monitor page. That page is no longer being updated, but includes key details of processing that are still relevant. The following plot shows the difference in CHIPX and CHIPY between the planned observation aimpoint and the actual aimpoint. The planned aimpoint is computed using the planned aimpoint chip coordinates (CHIPX/Y) and observer target offsets and the SIM-Z position. The actual aimpoint is computed using dmcoords and keyword values from the CXC archive L2 X-ray event file. The plot shows up to 6 months of data starting from when dynamic aimpoints were initially put into use (AUG2916 schedule).

The data values are stored in the observed aimpoints table (HTML or ASCII).

Intra-observation aimpoint drift

During an observation the aimpoint can drift, and this is illustrated in the plot below. However, from the perspective of planning observations this need not be considered because it is already included in the Aimpoint Trending plots. This is because those plots sample from 1 ksec intervals within every science observation (instead of per-observation means), thus picking up the extremes.

ACA housing temperature

The trend of ACA housing temperature is shown below, and one easily sees the qualitative similarity with the aimpoint trending data.

Analysis code

The scripts used in this analysis are in available on GitHub in the aimpoint_mon repository.

References

Last modified:05/21/25

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