Processing ACA Monitor Window Data

Background Information

This thread assumes familiarity with the basic ACA operating principles as described in the Pointing Control and Aspect Determination System chapter of the Proposers' Observatory Guide (POG). This includes the concepts of image slots and readout windows.

The key challenge in deriving a photometric light curve from ACA monitor window data is determining the background dark current. Due to ionizing radiation in the space environment, more than 1/3 of the pixels on the ACA CCD have detectable damage as manifested by an elevated dark current, as shown in Figure 1 .

The main gaussian peak of the dark current distribution, representing undamaged pixels, dominates below about 30 e-/sec. Above that value the pixels have been damaged by cosmic radiation. In aspect pipeline processing a pixel with a dark current of more than 200 e-/sec is considered "warm" because at this level centroiding can be perturbed. A 15th magnitude source produces only about 500 e-/sec, so for faint sources the impact of these warm pixels is significant. To make matters worse, many warm pixels show time dependent flickering behavior in which the pixel dark current can abruptly change by a factor of two or more. This flickering occurs on time scales of thousands of seconds, making it essentially impossible to produce a reliable dark current map using the ACA calibration mode. Instead we use on-the-fly warm pixel detection.

The basic idea of the detection algorithm is to sample pixels from the outer edge of the 8x8 pixel readout window and look for values higher than a threshold. At the edge the contamination of the dark current measurement from the monitor star is minimized. Since the readout window moves with respect to the CCD due to dither, the edge pixels end up sampling much of the CCD region used for imaging. In this way one can build up a map of warm pixels.

There are three limitations to the algorithm:

• Background pixel sampling is not complete nor uniform.

• For brighter stars even the edge pixels have significant contribution from the star light so it is not possible to get a true dark current measurement. The dark_ratio tool parameter (see below) specifies that the warm pixel threshold be no less than dark_ratio * avg_source_counts.

• The current algorithm in the monitor_photom script does not account for flickering, though this could be done with some effort.

Get Started

Sample ObsID used: 4924 (ACIS-S, Mrk 590)

File types needed: evt1; PCAD L1 ACA image data (adat71.fits)

Check the ASCDSVER keyword in the header to see which software version was used in processing the data.

unix% dmkeypar acisf04924_001N002_evt1.fits ASCDSVER echo+
7.6.7.2

As mentioned in the Overview, only monitor window data processed with an ASCDSVER of DS 7.6.0 or higher should be used in this thread.

Note that monitor window data are only available if the observer has specified target photometry (Photometry=Y) in the observation setup. This can be determined by examining the Obscat parameters, available via the Target Search Page for a specific ObsID. If you would like to request custom processing of a dataset to obtain the window monitor data, please contact the Helpdesk.

unix% grep Id `which monitor_photom`
% $Id: monitor_photom,v 1.0 2005/06/23 taldcroft Exp $

Merge the Data Files

Typically the image data are split over a number of files, so the first step is to merge the ACA image data into a single file using dmmerge:

unix% ls -1 *adat71.fits > adat71.lis
unix% cat adat71.lis 
pcadf205280136N002_adat71.fits
pcadf205280251N002_adat71.fits
pcadf205309295N002_adat71.fits
pcadf205322477N002_adat71.fits
pcadf205351521N002_adat71.fits
pcadf205361476N002_adat71.fits

unix% dmmerge infile=@adat71.lis outfile=pcad_adat71.fits \
              outBlock=ACADATA 

The ACA files are input to the tool as a stack; see ahelp stack for more information.

Run the monitor_photom script

The main processing steps of the monitor_photom script are:

• Read the image data file.

• Median filter image data in time on a pixel-by-pixel basis to remove cosmic rays.

• Search for "warm" pixels that have a dark current well outside the normal distribution. These pixels can significantly affect photometry as they dither in and out of the 8x8 pixel image readout window.

• Subtract the background from each image readout. For warm pixels the detected value for that pixel is used, while for all others the median background dark current reported by the ACA is used.

• Produce lightcurves in counts, counts/sec, and mags.

This script has a parameter file associated with it:

unix% plist monitor_photom

Parameters for monitor_photom.par

        infile =                  ACA image data file
       outfile =                  Output light curve
   (dark_ratio = 0.005)           Dark ratio
(min_dark_limit = 80.0)            Minimum warm pixel dark current
(min_dark_meas = 10)              Minimum warm pixel measurements
(max_dither_motion = 10)              Maximum possible dither motion (pixels)
         (mode = ql)              

The parameters dark_ratio, min_dark_limit, and min_dark_meas affect the way in which warm pixels are detected. The default values in the supplied parameter file are a good starting place, but it is often helpful to adjust the the dark_ratio and/or min_dark_limit to obtain better results.

Now run the script with the merged pcad_adat71.fits file as input:

unix% monitor_photom infile=pcad_adat71.fits outfile=monitor_lc.fits
Filtering image data (cosmic ray removal)...
  Image 23624 of 23624

Average counts  (e-) = 2137.996582
Warm dark limit (e-) = 80.000000

Stacking dark current data...
  Image 23624 of 23624

Warm pixel at CCD (row,col) =  -18,10     Dark current (e-) = 85
Warm pixel at CCD (row,col) =  -17,11     Dark current (e-) = 325
Warm pixel at CCD (row,col) =  -11,11     Dark current (e-) = 615
Warm pixel at CCD (row,col) =  -15,12     Dark current (e-) = 105
Warm pixel at CCD (row,col) =   -8,16     Dark current (e-) = 115
Warm pixel at CCD (row,col) =  -10,17     Dark current (e-) = 105
Warm pixel at CCD (row,col) =  -17,19     Dark current (e-) = 170
Warm pixel at CCD (row,col) =  -14,20     Dark current (e-) = 130
Warm pixel at CCD (row,col) =  -13,20     Dark current (e-) = 175
Warm pixel at CCD (row,col) =   -9,20     Dark current (e-) = 110
Warm pixel at CCD (row,col) =   -7,20     Dark current (e-) = 350

Enter return to exit the program: 

The script does cosmic ray removal, reports on the average image counts (in e- for the 1.696 second integration) and the minimum dark current for warm pixels, and then makes a dark current stack to detect warm pixels. Finally, a plot of the lightcurve in ACA magnitudes (m_ACA)  is created.

The lightcurve is written to the specified FITS file (monitor_lc.fits). It contains columns with the time, counts, count rate, magnitude, and background-subtracted image. The magnitude is defined as

m_ACA = 10.32 - 2.5 * log10(cnt_rate / 5263.0)

An approximate formula relating B and V magnitude to m_ACA is given in the POG:

m_ACA = V + 0.426 - 1.06*(B-V) + 0.617*(B-V)^2 - 0.307*(B-V)^3

This is based on the typical spectral energy distribution of bright main sequence stars.

Examine the Results

A series of ChIPS commands are used as a first step in examining the results from the script:

unix% chips

Welcome to ChIPS, version CIAO 3.4
Copyright (C) 1999-2003, Smithsonian Astrophysical Observatory

chips> lc = readfile("monitor_lc.fits")
chips> chips.symbolstyle = _chips->none;
chips> chips.curvestyle  = _chips->simpleline;
chips> () = curve((lc.time - lc.time[0])/1000 , lc.mag )
chips> ylabel 'Image magnitude (m_{ACA})'

chips> split 1 2
chips> d 2
chips> () = curve((lc.time - lc.time[0])/1000 , lc.cnt_rate )
chips> xlabel 'Time from Obs start (ksec)'
chips> ylabel 'Image count rate (e-/sec)'
chips> print postfile monitor_lc.ps

chips> d 1,2 limit x 20 25
chips> print postfile monitor_lc_zoom.ps

The plot results are shown in Figure 4  (monitor_lc.ps).

There is an obvious jump around 17 ksec into the observation which is due to a limitation of the ACA flight software. Because the source in a monitor window may not be bright enough for the ACA to independently track, the ACA relies on the motion of another "designated track star" to move the monitor window in lock step. However, if this designated track star is lost by the ACA, even momentarily, then the ACA stops moving the monitor window and leaves it fixed at its last position on the CCD. Instead of the monitor window tracking the motion of the source as Chandra dithers, the source now dithers within the fixed window, possibly going completely outside the readout window. This can be explicitly seen via manipulation of the image data file (pcad_adat71.fits):

chips> clear

chips> dat = readfile("pcad_adat71.fits")
chips> () = curve((dat.time - dat.time[0])/1000 , dat.img_row0 )
chips> ylabel 'Readout window row'

chips> split 1 2
chips> d 2
chips> () = curve((dat.time - dat.time[0])/1000 , dat.img_col0 )
chips> xlabel 'Time from Obs start (ksec)'
chips> ylabel 'Readout window column'

chips> d 1,2 limit x 12 22
chips> print postfile img_row_col.ps
chips> exit

These commands result in Figure 5 .

The impact to the photometry due to not tracking the source can be seen in Figure 6  (monitor_lc_zoom.ps). A quasi-periodic signal in the magnitude of the lightcurve is clearly visible.

Advanced Support

If the monitor photometry data indicate scientifically interesting results worthy of detailed study, users may wish to contact aspect_help@cfa.harvard.edu for further advice on possible techniques to reduce the systematic errors.