ACIS CTI Plots
OUTLINE
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An exmpale of detrended CTI measurments from the Front illuminated CCD3 Node 0. The top plot is all data points without
any temperature dependency correction. The middle plot is data with the focal temperature equal or colder than -119.7 C,
and the integration time longer than 7,000 sec. The bottom plot is all data with the temperature correction.
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What Are Plotted On This Page?
Charge Transfer In-efficiencies (CTIs) of ACIS CCDs are graudually increasing with time, and it is essential to know
how the degradation progresses. Since CTI are strongly depending on focal temperature, to see the degradation, it is better
to remove the temperature dependency of CTI observations.
MIT/ACIS team suggested that there is a (close to) linear relation between a focal plane temperature and CTI, and
the CTI temperature dependency can be easily corrected (MIT ACIS Memo #204).
On this web page, we show CTI time dependent evolution for
- Un-corrected cases with an integration time > 2000 sec (plotted in red)
- The focal temperature ≤ -119.7 C cases with an integration time > 2000 sec
(plotted in light green)
- The focal temperature ≤ -119.7 C with an integration time > 7000 sec
(plotted in blue)
- MIT/ACIS temperature dependency corrected cases with an integration time > 2000 sec
(plotted in light blue)
- Newly computed temperature dependency corrected cases with an integration time > 2000 sec
(plotted in fuchia)
The third case is our cleanest estimation, but we lose many data points. The fourth/fifth cases have the largest numbers of
data points, but bringing in errors from temperature corrections.
Each table shows plots of time (Day of Mission) vs CTIs computed using Al K alpha, Mn K alpha, and Ti K alpha peaks for CCD0 to
CCD9; each CCD has four plots of four quadrants. There are also plots of
average of ACIS-I (CCD0, 1, 2, and 3), average of ACIS-S without back side CCDs
(CCD 4, 6, 8, and 9), and average of Back Side CCDs.
Under the "Fitting Results", CTI change rate per day of each quadrant, and those of
averaged cases are listed.
The second half shows similar tables for detrended CTIs. Detrending was done to remove effects of drop amp.
Since backside CCD data are used to compute the detrend factor, detrendings were done only for the front side CCDs.
How CTIs Are Computed?
CTIs were computed taking the following steps:
- An acis evt1 data for CTI observation is extracted from Database
- Comparing CTI data with focal plane temperature along observation time,
data are extracted for a specific time period corresponding to
each temperature segment. The temperature ranges are quantized like:
-119.87, -117.0, -119.54, -119.38, -119.22.... Temperature is in C.
- For each extracted data,
- If the integration time is more than 600 sec, compute CTI using
MTA CTI computing tool. The data are listed in ASCII data
The columns are start time, quad 0, quad 1, quad 2, quad3, obsid, stop time,
integration time in sec, and focal plane temperature. If 99999 appears, the
MTA tool could not compute the value.
- If the integration time is more than 1000 sec and the observation
was done before Jan 2003, the results are used for plotting.
- If the integration time is more than 2000 sec and the observation
was done from Jan 2003, the results are used for plotting.
- Note that the tables say the integration time is lager than 2000 sec, but
this reflects current selection of the integration time. Plotted data
older than Jan 2003 could have less than 2,000 sec integration time.
This crates un-adjusted basic plots (red).
- If the focal plane temperature is colder than or equal to -119.7 C, the data
are used to create a plot without any adjustment (light green).
- This data are further refined by selecting only data with the integration
time longer than 7,000 seconds (blue).
- If the focal plan temperature is warmer than -119.7 C. A correction is done
for CTI. The correction factor is computed as follows:
- Fit a straight line with time as an independent variable to the
data with the focal plane temperature ≤ -119.7 C and
the integration time longer than 7000 sec.
- Use the fitted values to remove a time dependent CTI evolution.
- Fit a straight line onto this data, but this time use the
focal plane temperature as an independent variable. Set the correction
at -119.7 C to zero.
- The resulted The correction factors are in CTI chage per degree.
- Using these correction factors, adjust all CTIs with focal temperature
warmer than -119.7 C (fuchsia). Anything data colder
than -119.7 C are used without the correction.
You may want to read
A Step by Step Example of Correcting Temperature Dependence of CTI
for a better understanding.
- Similar plots with MIT/ACIS team corrections are also plotted (light blue).
The correction factor for front side CCD is 0.036, and the backside CCD is 0.045.
- To detrace drop amp effect, detrend correction factors are computed
- A corresponding acis stat fits file is extracted from database.
- Selecte out drop amp data for CCD7, and compute an average of the drop amp
- The average is devided by 680 to normalize the correction factor. the value 680
was computed by MIT/ACIS team (08/18/2000).
- The correction factor is added to the CTI to remove drop amp effect.
All data, except for CCD5 and CCD7, are detrended and plotted above.
Memo
If you have any questions about this page, please contact swolk@head.cfa.harvard.edu
Last Update: 5/24/2013
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