Helios did not cast a kind eye upon ACIS in 2000: we had four strong solar radiation events for which we had to safe ACIS by taking it out of the focal plane. The strongest solar flare since 1974 actually was well-timed with the High-Energy Astrophysics Division conference in Hawaii in November where a good many folk from the CXC were in attendance. (Those in the know there were all using at least SPF 64 of course!)
Why do we worry about such solar events? From our early on-orbit experience we learned that the strong low energy proton flux in the radiation belts harmed the Front-Illuminated (FI) CCDs of ACIS by increasing their charge transfer inefficiency (CTI) -- the belts contain protons of about 100 keV at intensities of 108 protons cm-2 sec-1. From doing experiments and simulations on the ground it became known that protons of a few hundred keV could bounce off of the Ir coated mirrors and make it to the focal plane, much as the x-rays do. We were able to isolate the energies which were harming the FI CCDs by noticing that the Back Illuminated (BI) CCDs suffered no damage -- due to their ``upside-down" nature they have enough extra bulk Si protecting their gate structures to stop a few hundred keV protons. Since energetic particle ejections from the Sun sometimes contain large fluxes of such low energy protons we have initiated a procedure whereby we take ACIS out of the focal plane and tuck it away in a well-shielded compartment off to one side if the solar low energy proton flux becomes too great. (Of course, we do the same for every perigee passage when we cross the radiation belts). So far this procedure has worked well: the CTI increase has been barely detectable at a level of a few10-6 year-1 for the FIs.
The instrument and operations teams have also been busy trying to devise and implement clever ways of recovering some of spectral performance of the FIs. A row dependent energy resolution degradation (aka CTI degradation) corrector has already been published: ``Mitigating Charge Transfer Inefficiency in the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer" by L. Townsley et al., ApJ Letters 534, 139. In addition a novel clocking procedure called `Squeegy' mode is being researched to try to improve the resolution of the FI devices. This mode periodically sweeps a large amount of charge, which is continually deposited by cosmic rays in the CCDs, across the CCD frame thereby partially filling the charge `traps' which have resulted from the proton induced damage, and which are the source of the FIs degraded resolution. For updates on this and possibly other novel methods of improving the performance of the FIs, keep an eye on Chandra web pages.
Following an observation of Jupiter it was noticed that the ACIS sensitivity to optical light is higher than expected. Observations of Vega show the ACIS-S sensitivity to be 1.5 magnitudes higher than expected for blue objects. Observations of Betelgeuse show the sensitivity 2.5 magnitudes higher for red objects. The new sensitivity curves are available on line at:
You may have heard that this ``light leak" makes solar system observations impossible. THIS IS NOT TRUE. In the last 6 months, working closely with the instrument teams, we have successfully observed Jupiter, Venus, Comet Linear 1999/S1 and Comet McNaught-Hartly 1999/T1. A combination of strategies have been invoked to ameliorate the light leak including: observing with ACIS-I (which has a thicker optical blocking filter than ACIS-S -- see the POG) using continuous clocking mode, using very faint mode to monitor local background, using gratings to scatter X-rays away from the optical light, and changing the ACIS event thresholds. Getting the softest energies is a challenge with a bright optical source. If you are concerned about the impact on your science read the on-line POG carefully for the most up-to-date information. If you still have questions please contact us at the CXC Helpdesk.
On the plus side, an intermittent problem with one of the six Front End Processors (FEPs) which process pixel data from the CCDs, the so-called FEP-0 problem, seems to have mysteriously, and thankfully, disappeared.
- Yousaf Butt on behalf of the extended ACIS Instrument and Operations Teams