Chandra Aspect Operations

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. Currently the predicted aimpoint for each detector is updated once per year, typically in late November prior to issuing an update to the Proposers' Observatory Guide for the subsequent Call for Proposals. Once the yearly update is made aimpoint is assumed to be static (for planning purposes) until the next update. As of 2015 this approximation is not adequate for pointing-sensitive observations.

Webpage contents and usage

This webpage serves as a reference for both the long-term and current trend in the Chandra aimpoint. It is updated daily and gives the effective bounding box of observed aimpoint positions in chip coordinates over the last 6 months. At this time only ACIS-S and ACIS-I are included because there have been no pointing-sensitive observations on HRC to date.

Many users with pointing-sensitive observations will only need to reference the 6-month aimpoint bounds and median and compare with the latest predicted aimpoint values. This will provide guidance for adjusting the target offset with assistance from your USINT contact.

Going deeper

To dig a little deeper and understand how this issue has evolved, the plots in this page are live plots, powered by the Python mpld3 package and ultimately by the Javascript D3 plotting engine. The most insight can be gained by using the linked brush feature which allows selecting points in one panel and seeing the same points highlighted in the other panels.

To do this, left-click in the lower-right panel (CHIPY vs. Year) and drag the mouse up and right to select a box that covers the full CHIPY range and about two years of data. (You might need to click once in the plot first to get a + cursor). Once you have a gray selection box, carefully move your mouse into the box so that it turns into the normal Move icon (four arrows pointing outward). Now click and drag the box all the way to the left and then slowly pan it rightward to mimic the progression of time. Now you will see a virtual movie of how the aimpoint has been dynamically changing.


Observed aimpoint differences trend

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.
The plots below show a representative sampling of aimpoint positions during Chandra science observations. This includes points corresponding to the minimum, 10th percentile, median, 90th percentile, and maximum during one-month bins. This sampling is complete with regard to outliers and does not distinguish between individual observations. See the Details section for a full description of this process and links to the actual code.

In each plot there is a box which highlights the maximum range of aimpoint CHIPX and CHIPY within the last 6 months. In addition there is a red star which shows the current aimpoint used for planning.

The numerical table values shown in this page are available in machine-readable JSON format as info.json.


A key point to highlight for ACIS-S is that the planning aimpoint is at the extreme corner of the observed aimpoint extent. This means that the aimpoints for some observations in the last 6 months have been offset by nearly 40 pixels in CHIPX and over 20 pixels in CHIPY. Another important point is that there is a strong correlation between CHIPX and CHIPY which is most apparent when viewing the data dynamically using the linked brush, but is also visible noting that the darkest red points are all from the last year of observations.

The coordinates of the 6-month bounding box (shaded box in the plot below) and the 2015.0 planning aimpoint from the POG (red star) are:

Min Midpoint Max POG
CHIPX 204.6 241.9 279.3 200.7
CHIPY 464.8 483.1 501.4 476.9

To account for the difference between the current POG value and the current aimpoint, ADD the following values to the existing observation target offsets DY and DZ:

DY +20.3 arcsec+0.338 arcmin
DZ +3.0 arcsec+0.051 arcmin