Removing Warm ACIS Data

Get Started

Download the sample data: 45549 (ACIS Engineering data --not intended for scientific analysis)

unix% download_chandra_obsid 45549 evt2,mtl

The mission timeline file is stored in the secondary/ directory. For the rest of this thread we assume that both the event file and mtl files have been moved to be in the same directory.

Plot Focal Plane Temperature Values

The ACIS focal plane temperature (FP_TEMP) has increased over the mission's lifetime, and is no longer stable and cold for most observations. During an observation, the FP_TEMP may change by several degrees. Thermal modeling is used during scheduling to help mission planners ensure that data are collected within appropriate temperatures bounds to allow for the reliable calibration of each science investigation.

We begin by plotting the FP_TEMP vs time. FP_TEMP is stored in the mission timeline file in Kelvin. To make the plot more legible, we plot time-offset from the first record.

The resulting plot is shown in Figure 1.

% python

from pycrates import read_file
import matplotlib.pyplot as plt

mtl=read_file("acisf45549_000N001_mtl1.fits.gz[cols time,fp_temp]")
times = mtl.get_column("time").values
time_offset = (times - times[0]) / 1000.0  # sec -> ksec
fp_temp = mtl.get_column("fp_temp").values

p = plt.plot(time_offset,fp_temp)
p[0].set_marker("o")
p[0].set_markersize(1)
p[0].set_linestyle('None')

plt.xlabel("Time offset since {:.3f} [ksec]".format(times[0]))
plt.ylabel("FP_TEMP [k]")
plt.title("OBS_ID : 45549")
ll=plt.axhline(165.15)
ll.set_linestyle(":")

plt.savefig("fptemp.png")
exit()

Figure 1: Focal-plane Temperature vs Time

[Version: full-size]

Figure 1: Focal-plane Temperature vs Time

Plot of ACIS focal plane temperature (black) vs. time. The horizontal line at 165.15K is the temperature limit chosen (see text below).

Assuming our source of interest has emission and/or absorption features, lands on a front-illuminated chip and has between 500-2500 counts then we'll want to filter out data warmer than -108 C (165.15 K) for this observation (see synopsis above). For roughly the first 5 ksec the FP_TEMP value is colder than this limit of 165.15 K and events in this time window are less affected by uncertainties in spectral response associated with warm focal plane temperatures.

Choose Good Time Intervals (GTIs)

We can time filter the event file with a custom good time interval created using the dmgti tool

In this example, we choose the a temperature limit of 165.15 K (-108 C) beyond which systemic offsets due to calibraiton uncertainty in the ACIS spectral response could cause line-centroids to shift by 1-2%.

unix% pset dmgti infile=acisf45549_000N001_mtl1.fits.gz
unix% pset dmgti outfile=fp_temp.gti
unix% pset dmgti userlimit="fp_temp<165.15"
unix% dmgti
Input MTL file (acisf45549_000N001_mtl1.fits.gz): 
Output GTI file (fp_temp.gti): 
User defined limit string (fp_temp<165.15): 

The userlimit parameter is a dmmath expression that should evaluate to TRUE when the data are GOOD. In this example we want to know the times when the FP_TEMP value is less than 165.15.

Inspect GTI file

We can now use dmlist to inspect the good time intervals.

unix% dmlist fp_temp.gti blocks,subspace

--------------------------------------------------------------------------------
Dataset: fp_temp.gti
--------------------------------------------------------------------------------

     Block Name                          Type         Dimensions
--------------------------------------------------------------------------------
Block    1: PRIMARY                        Null
Block    2: FILTER                         Table         1 cols x 0        rows
Block    3: GTI                            Table         2 cols x 1        rows

--------------------------------------------------------------------------------
Data subspace for block FILTER: Components: 1 Descriptors: 1
--------------------------------------------------------------------------------

 --- Component 1 ---
   1 TIME                 Real8               TABLE GTI

                                              766773190.1659966707:766778258.1660431623
                                              766778313.3660730124:766778332.5660732985

This shows us that there are two time intervals during which the FP_TEMP was less than 165.15 K. The second time interval is very short and has to do with how events are assigned a focal plane temperature.

Apply GTI to Event File

The good time interval filter can be applied using the "@" dmfiltering datamodel syntax.

unix% dmcopy "acisf45549_000N001_evt2.fits.gz[@fp_temp.gti]" good_events.fits

We can check the output file to verify that the times have been appropriately recomputed.

unix% dmkeypar acisf45549_000N001_evt2.fits.gz ONTIME echo+
8117.9113694429
unix% dmkeypar good_events.fits ONTIME echo+
5087.5113242864

We can see that roughly 3 ksec, or almost 40% of the time has been removed from the observation using this filter criterion.

This file can now be used in spectral analysis and the correct exposure times will be used when making response files.

Users are strongly encouraged to perform their spectral analysis both with and without filtering the warm times.