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 (I1-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.
NEW! To combat decreasing count rates, CTI is also being measured in 0.4 year time bins. Corrections are being made for background changes. Only observations with average temperatures of -119.4C or less are included in the bins.
Updated monthly. Includes data through March 17, 2025.
| Jan 2000 value | Rate of Increase | |
|---|---|---|
| FI Parallel | (14.79 ± 0.01) x 10-5 | (2.12 ± 0.01) x 10-6 / year |
| FI Framestore* | ... | (4.3 ± 0.8) x 10-7 / year |
| FI Serial | < 7 x 10-6 | < 9 x 10-7 / year |
| BI Parallel | (1.51 ± 0.01) x 10-5 | (8.43 ± 0.01) x 10-7 / year |
| BI Framestore* | ... | (4.1 ± 0.3) x 10-7 / year |
| BI Serial | (8.27 ± 0.43) x 10-5 | (1.50 ± 0.41) x 10-7 / year |
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.
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.
A new sacrificial charge correction has removed most of the sharp features in the FI CCD parallel CTI.
So far the serial CTI of the FI CCDs has remained unmeasurable
