Generate a photometric lightcurve from ACA monitor data
monitor_photom infile output [dark_ratio] [min_dark_limit]
[min_dark_meas] [max_dither_motion]
The Aspect Camera Assembly (ACA) is capable of providing
simultaneous optical monitoring of a target during an
observation. The monitor_photom script is used to generate a
photometric lightcurve for a Chandra target which was observed
using an ACA monitor window.
The basic idea of the detection algorithm is to sample pixels
from the outer edge of the 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.
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.
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"
parameter specifies that the warm pixel threshold be no less
than dark_ratio * avg_source_counts.
-
The current algorithm does not account for flickering.
This script is used in the Processing ACA Monitor Window Data thread.
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.
monitor_photom pcad_adat71.fits monitor_lc.fits
monitor_photom is run with all the default parameter
values. The following screen output is generated:
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:
ACA image data file
The PCAD Level 1 ACA image data files (pcad...adat71.fits)
are a "supporting" data product. The "7" in "adat71"
refers to image slot 7, which is the slot where monitor
window data always appear.
The monitor window data are available beginning with
standard data processing (SDP) version DS 7.6.0. Check
the ASCDSVER keyword in the event file header to see which
software version was used in processing the data.
Typically the image data are split over a number of
files. Use dmmerge to merge them into a single file for
input to monitor_photom:
unix% ls -1 *adat71.fits > adat71.lis
unix% dmmerge infile=@adat71.lis outfile=pcad_adat71.fits \
outBlock=ACADATA
Output lightcurve
The lightcurve 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 Aspect chapter of the Proposers' Observatory Guide.
Dark ratio
This parameter affects the way in which warm pixels are
detected.
Minimum warm pixel dark current
This parameter affects the way in which warm pixels are
detected.
Minimum warm pixel measurements
This parameter affects the way in which warm pixels are
detected.
Maximum possible dither motion (pixels)
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