ACIS Instrument Support - Monthly Progress Report for March 2000

1. ACIS Characterization

1.1 Characterize and model radiation damage in ACIS CCDs

  ...] Completed a series of faint mode observations of the ext. cal. source with a frame time of 9.9 seconds which replaced one set of standard CTI measurements around the radiation belts.  This tests the utility of sacrificial charge to decrease measured CTI.

 [...] Conducted two additional raw frame mode measurements on I3 and S2.  Preliminary results indicate that I3 trap behavior is similar to S2.

 [...] Conducted CTI measurement in alternating exposure mode with frame times of 4.0 and 3.2 seconds.  This test confirms the anti-correlation of particle events per frame and measured CTI.

  ...  Continued analysis of raw mode flight data trying to determine electron trap characteristics.  Results presented in paper at SPIE meeting in Munich entitled "Radiation damage in Chandra CCDs". 

  ...  Continued characterization of radiation damaged CCD, w459c1

        * Confirmed importance of using a cosmic ray background simulator during lab testing.  We settled upon a 60Co source - but it is unclear if this is really the best item to use. In general, the presence of a relatively large background charge improves resolution at higher x-ray energies. This is something we already infer from the long exposure time flight data.

       * A CTI vs. energy curve was completed at several temperatures using characteristic lines from the IFM. The result here is that CTI scales with the square root of the energy which is consistent with our models of charge cloud size.

       * Studied the effect of radiation damage on the quantum efficiency as a function of photon energy and row number.  Furthest from the readout node a substantial QE degradation can be seen at very low energies and high energies due to CTI degradation. (ACIS Memo #182)

  ...  Continued testing of squeegee readout mode using w459c1

       * The squeegee technique was re-examined, particularly after realizing the importance of the cosmic ray flux. The technique will improve both CTI and fwhm energy resolution if the number of "saved" rows is increased. Oddly, the same performance can be achieved if the transfer time is slowed, so the important parameter is the product of saved rows and inverse transfer speed (e.g. saving 16 rows is the same as saving 2 rows but clocking the parallels 8 times slower) This results suggests that there is some sort of effective capture time which is a whole lot slower than one might imagine.

        * Testing a flight candidate squeegee mode for various characteristic energies suggest that the fwhm and CTI improvement is only going to happen at x-ray energies above 2 keV.

        * Trailing pixel analysis was also performed on the squeegee mode data and results were compared to no-squeegee data. Emission time constants were found to be the same. This confirms that our understanding of the detrapping process is correct.

1.2 Define and characterize alternative CCD readout modes

  ...  Continued laboratory testing of squeegee mode (for analysis see above)

1.3 Monitor scientific on-orbit performance of ACIS

  ... Continued monitoring of ACIS CTI.  No additional radiation damage detected in either FI or BI CCDs.


2. Engineering

2.1 Flight software maintenance

 [... Created and tested S/W patch to report event filtering statistics within software housekeeping packets.

 [... Created and tested S/W patch to run flight CCDs in "squeegee" mode.

2.2 Instrument science performance

  ...  Continued quick-look processing of all ACIS science runs, using EHS telemetry supplied by the GOT.

 [...] Reprocessed all ACIS XRCF data to parallel the current quick-look
    processing archive, spreadsheets, etc.

  ...  Continued to publish the "annotated DOT" timeline reports.

  ...  Monitored performance of new ACIS Flight Software patches including FEP bias parity error fix.  FEP bias parity errors never exceeded trigger value.