deosc - remove ``oscillations'' from event lists
deosc options [input file]
deosc uses long options. Options may be abbreviated, and the
``='' character shown below in the option templates is optional.
A command line argument which begins with the @ character
is taken to be the name of a file which contains white-space
delimited options.
events
x, y, and time, respectively.
1.
x and y, respectively.
pixels.
/xs, which plots
to the screen. Useful other ones are /cps and /vcps, which
are, respectively, landscape and portrait color PostScript output.
Hardcopy will be left in files with extensions of .ps for PostScript,
.png for PNG, etc. Output to the screen via the /xs device
will be sent to mutiple windows. The default /xs device number
to start at is 5; this may be changed by specifying the device
number directly: 6/xs.
See Output for more information.
fitx fity
inclip
time time0
x x0
y y0
The default list is
time
x x0
y y0
The following parameters apply to centering in the coordinates given by the --xcol and --ycol parameters:
0.01.
1000.
10.
3.
The following parameters apply to centering in the coordinates given by the --sky_xcol and --sky_ycol parameters, if --sky_filt is used. See Sky Filtering.
0.01.
1000.
10.
3.
--sky_xcol
coordinate of the center of the object.
--sky_ycol
coordinate of the center of the object.
deosc removes time dependent ``oscillations'' from an input event list by modelling the motion of an object's centroid and subtracting it from the events.
The correction is determined by fitting the gross motion of events in a circular region centered around the object, as a function of time.
The input events must be in a binary table extension of a FITS file. CFITSIO is used to extract the data, so the CFITSIO extended filename syntax may be used to filter the data. The event positions may be scaled (to convert to seconds of arc, for example).
If --force_ctr is specified, the source center is provided by the --xc and --yc options.
If --force_ctr is not specified, the center is determined from
the data. If the event coordinates are in a reference frame which is
dithered (such as Chandra chip coordinates), multiple objects may
overlap in space (but not time), and it may be difficult to accurately
determine the object center. In this case, it may be possible to
select events associated with the object based on their
dither-corrected coordinates (such as Chandra sky coordinates). This
is accomplished by what is termed here sky filtering. The center
of the object in sky coordinates is determined, a region around that
center is excised, and the contained events are used to derive the
center in dithered coordinates.
In the case, as described above, that the analysis is to be performed in dithered coordinates, but those result in spatial confusion of sources, a subset of source events may be selected by centering and extracting events in dither-corrected, or sky coordinates.
To accomplish this, specify --sky_filt, and set the following parameters. --sky_force --sky_dtol, --sky_clip, --sky_iter, --sky_nsigma, --sky_pixcent, --sky_xc, --sky_yc, --sky_xcol, --sky_ycol.
These are akin to the centering parameters described in the following section. The size of the region to use as the source events is determined by the --sky_clip_r or --sky_clip_sigma parameters.
If --force_ctr is not specified, the location and scale of the object is determined by iteratively centering on the event coordinates as given by the --xcol and --ycol parameters. The initial center may be specified in one of the following methods:
If none of these methods is specified, the average event position is used.
An initial clip is performed about the initial center, with the clipping radius provided by the --iclip parameter. Events outside of a clipping radius are discarded, and the average center is redetermined.
This step is repeated until either the center hasn't changed, or the absolute change in the center is less than the tolerance specified by the --dtol option. After the initial clip, the clipping radius is a multiple of the standard deviation of the events; the --nsigma option specifies the mutiplication factor.
The radius of the region used to fit the time-varying motion is determined either from the standard deviation of the event distribution about the center, if deosc determined the center and --clip_sigma is specified, or may be given directly via the --clip_r option.
Within this region the events are divided (in time) into subsets composed of three segments, each with a length specified by --seglen. The subsets are staggered such that each overlaps the preceding by two segments. The one-dimensional differences (along each coordinate) in event positions from the determined object centroid are fit as a function of time by polynomials of maximum order 10. The actual number of orders various by subset, and is that which produces the best fit. The fit is done in a least-squares sense, so the resultant polynomials track the center of the event ``cloud'' (PSF).
The resultant fits to the events' motion in the inner segment of each subset is then used to correct the positions of all of the events which fall within that time slice. No quantitative effort is made to ensure the continuity of the fits across subsets. The overlapping of the subsets and the fitting across neighboring segments seems, empirically, to constrain the fits so that the polynomials in neighboring segments are reasonably continuous.
The correction of events which occur outside the circular analysis region may introduce errors in the positions of events whose time motion is not correlated with that of events in the analysis region, but this is unimportant if the former are used only for background measurements.
Several files are output upon successful completion:
events is written to a file with name
tag_fix.fits. It is a simple FITS file (not corresponding to
a CXC level 1 or level 2 event file). It has the following columns:
Please note that all positions reflect the application of the --scale scaling factor!
A scatter plot of the event positions and the polynomial fit to them is written to tag_pos.ext if --device is set to a hardcopy device..ext is .ps, .png, etc.
A scatter plot of the corrected event positions is written to
tag_fix.ext if --device is set to a hardcopy device. .ext
is .ps, .png, etc.
An image of the events and the polynomial fit to them is written to
tag_pos_img.ext if --device is set to a hardcopy device.
.ext is .ps, .png, etc.
An imgea of the corrected events is written to tag_fix_img.ext
if --device is set to a hardcopy device. .ext is .ps,
.png, etc.
$Revision: 1.24 $
Diab Jerius (djerius@cfa.harvard.edu)