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 (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 July 1, 2024.

CTI Increase Summary

Jan 2000 valueRate 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
* Assuming no change in electronic gain

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Plots of the data

Cosmic Ray Rate

Particle Background 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

Particle Background Rate

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.

Parallel CTI of FI CCDs

A new sacrificial charge correction has removed most of the sharp features in the FI CCD parallel CTI.

I-array measured parallel CTI I-array corrected parallel CTI Iarray binned parallel CTI

Serial CTI of FI CCDs

So far the serial CTI of the FI CCDs has remained unmeasurable

Parallel CTI of BI CCDs

S3 measured parallel CTI S3 corrected parallel CTI S3 binned parallel CTI

Serial CTI of BI CCDs

S3 measured serial CTI S3 corrected serial CTI

CTI data

Measured CTI data

Corrected CTI data

Binned corrected CTI data

Change log:
Last update July 2, 2024
Web page by Catherine E. Grant ( )
MIT Kavli Institute for Astrophysics and Space Research
Massachusetts Institute of Technology