ACIS CTI @ -120C
Monitoring and analysis of ACIS CTI by the instrument team. In all cases the focal plane temperature setpoint is -120C and the CTI is measured at the Mn-Kα line (5.89 keV). The average over all nodes in the I-array (I0-I3) is representative for all FI CCDs, while the average over all nodes on S3 (ACIS-7) is representative of both BI CCDs. Upper limits are 3-sigma.
Updated monthly. Includes data through June 23, 2009.
CTI Increase Summary
| Jan 2000 value | Rate of Increase |
|---|
| FI Parallel | (14.26 ± 0.05) x 10-5 | (3.32 ± 0.01) x 10-6 / year |
| FI Framestore* | ... | (7.0 ± 0.8) x 10-7 / year |
| FI Serial | < 2 x 10-5 | < 1.2 x 10-6 / year |
| BI Parallel | (1.66 ± 0.03) x 10-5 | (9.73 ± 0.04) x 10-7 / year |
| BI Framestore* | ... | (1.0 ± 1.3) x 10-7 / year |
| BI Serial | (9.02 ± 0.42) x 10-5 | (9.96 ± 0.60) x 10-7 / year |
* Assuming no change in electronic gain
Related web pages:
For more information on analysis techniques:
- "Long-term trends in radiation damage of Chandra X-ray CCDs", 2005, Proc. SPIE, 5898, 201 (astro-ph/0509035)
- "Four Years of ACIS: Monitoring Detector Performance", poster presented at Four Years of Chandra Symposium, September 2003
- "Evolution of ACIS Response", presentation for Chandra Calibration Workshop, November 2002
- "Monitoring the Charge Transfer Inefficiency of ACIS and the Impact of the Varying Cosmic-Ray Background", poster presented at HEAD meeting, November 2000
Change log:
- 09/05/03 - Data selection and analysis has changed. Some small differences between these and earlier results are possible.
- 12/03/03 - Error estimates on individual CTI measurements have been increased to better reflect the true statistical and systematic measurement errors.
- 02/28/05 - Applying new background detrending factor. This changes the absolute value of corrected CTI.
- 08/31/05 - Complete upgrade to match the analysis in 2005 SPIE paper. Analysis now includes a correction for temperature as well as background variations.
- 10/05/05 - S3 parallel and serial CTI are now computed using only G0, single pixel, events.
- 02/17/06 - added estimated upperlimits for framestore CTI increase
- 02/28/06 - updated background and temperature corrections
Plots of the data
Cosmic Ray Rate
Measured CTI is dependent on the amount of charge deposited on the CCD. For ACIS CCDs most of the charge is due to the particle background, so measured CTI is a function of the particle background rate. We use the S3 upper amplitude reject rate as a measure of the particle background rate. The variation in CTI due to the background must be modeled and removed.
Temperature Variations
Measured CTI is also dependent on the focal plane temperature. Certain spacecraft orientations allow the ACIS radiator to view the bright earth making cooling much less efficient. Observations close to perigee, such as our monitoring ones, are more susceptible than standard science measurements. The variation in CTI due to the temperature must also be modeled and removed.
So far the serial CTI of the FI CCDs has remained unmeasurable
CTI data
Measured CTI data
Corrected CTI data
Last update June 24, 2009
Web page by Catherine E. Grant (
cgrant@space.mit.edu )
MIT Kavli Institute for Astrophysics and Space Research
Massachusetts Institute of Technology