2.4 AE Version 2008-06-09

• After discovering that the standard procedures for calling MARX (e.g. those listed on the ChaRT web page in May 2008) do not account for off-nominal SIM positions which occur in some observations, we have attempted to improve the algorithm AE uses to call MARX. We have also attempted to implement the published MARX workaround for simulating S-array sources with an I-array aim point, and vice versa.

  These changes to the MARX calling procedure were deduced by a couple of AE users, with very limited assistance from the CXC and MARX teams. I am not confident that the method accounts for evolution of the observatory geometry and default operating parameters. As has been the case throughout the mission, I encourage you to always suspect the accuracy (size, position, and normalization) of the PSFs that AE generates using the mission tools (originally mkpsf, now MARX). If you find flaws in the PSFs that concern you, I encourage you to reproduce the problems outside the context of AE and to report your concerns to the CXC.

• Note that application of the CXC thread ``Improving the Astrometry of your Data: Correct for a Known Processing Offset'' will induce an offset between your data and the MARX PSF. We do not understand the header magic involved here, but fortunately this thread in not applicable to data re-processed after 2004.

• As of June 2008 the CXC's afterglow cleaning tools have significant problems. See the CIAO web page on afterglows for a discussion of the afterglow issue. The first afterglow tool, acis_detect_afterglow, finds many afterglows, but eats out the cores of bright sources. The second tool, acis_run_hotpix, is gentle on sources, but misses many afterglows, leading to lots of spurious source detections for many projects. We recommend an analysis work flow which is bifurcated--a source detection branch uses both CXC afterglow tools for aggressive afterglow cleaning, and a source extraction branch omits the acis_detect_afterglow in order to preserve the cores of bright sources.

  For projects in which you have already extracted a catalog that may be contaminated with lots of spurious ``afterglow sources'', we have written a simple tool, ae_afterglow_report, that looks for suspected afterglow events in the extracted data for each AE source. Calls to this tool are shown in our personal recipe (§7.1), but it is not documented further in this manual. Obviously, this tool is a desperate workaround to the unpleasant situation where you have extracted data that are contaminated with afterglows; no afterglow algorithm run this late in the analysis can be as effective as an algorithm run on the L1 data.

• The AE package now includes a convenient tool for constructing exposure maps (§7.20).