Creating Accurate RA, Dec Coordinates for src2 Files

[CIAO 3.4 Science Threads]

Overview

Last Update: 1 Dec 2006 - reviewed for CIAO 3.4: no changes

Synopsis:

The (RA,Dec) positions of sources in src2 files are incorrect for some automatically processed (AP) data. The regions are shifted, essentially at random. The range of affected observations is unknown, so users should consider the src2 file for all observations suspect. The x and y positions in the file, however, are correct.

Purpose:

To determine if the source positions in the pipeline-produced src2 file are affected, and correct them if necessary.

Read this thread if:

having accurate (RA,Dec) source positions in the src2.fits is important to your analysis.

Proceed to the HTML or hardcopy (PDF: A4 | letter) version of the thread.

Get Started
Run celldetect
Summary
Parameter files:
- dmcoords
- celldetect
History

Get Started

Sample ObsID used: 578 (ACIS-S, 3C 295)

File types needed: evt2; src2; asol1

We need to determine if the (RA,Dec) coordinates as supplied by AP are accurate. Since we know that the (x,y) coordinates are correct, we can use them as input for dmcoords, and compare the output to the values listed in the src2.fits file.

To begin, display the RA, Dec, and POS columns:

unix% dmlist "acisf00578N002_src2.fits[cols RA,DEC,POS]" data rows=1:1
 
--------------------------------------------------------------------------------
Data for Table Block SRCLIST
--------------------------------------------------------------------------------
 
ROW  RA               DEC            POS(X,Y)                                
 
 1   212.8785247803   52.2400550842  (3896.5952380952,4385.7857142857)

Note that the POS column may be named SRC_POS instead.

Now use the given (x,y) position (which are known to be correct) to calculate what the RA and Dec should be:

unix% punlearn dmcoords
unix% dmcoords acisf00578N002_evt2.fits asolfile=pcadf052378346N002_asol1.fits \
      option=sky x=3896.5952380952 y=4385.7857142857 celfmt=deg

The conversion to RA and Dec can be obtained from the parameter file:

unix% pget dmcoords ra dec 
212.8785255685877
52.24005379720089

In this case, the (RA,Dec) produced by dmcoords matches that in the src2.fits file. There is therefore no need to produce a new src2 files for this observation. If the two results do not match, however, complete the rest of this thread.

Run celldetect

In order to generate a new source file, we need to re-run celldetect as it is run in the pipeline:

unix% punlearn celldetect
unix% pset celldetect infile=acisf00578N002_evt2.fits
unix% pset celldetect outfile=new_578_src2.fits
unix% celldetect
Input file (acisf00578N002_evt2.fits): 
Output source list (new_578_src2.fits):

The contents of the parameter file may be checked using plist celldetect.

Summary

This thread is now complete. The file new_578_src2.fits contains accurate RA and DEC coordinates for the sources.


Parameters for /home/username/cxcds_param/dmcoords.par


        infile = acisf00578N002_evt2.fits Input dataset/block specification
#
# Position of photon in different coord systems
#
       chip_id = 7                Chip ID number
         chipx = 403.6431782100918 Chip X [pixel]
         chipy = 229.5449957964382 Chip Y [pixel]
         tdetx = 4320.643178210092 TDETX [pixel]
         tdety = 1931.544995796438 TDETY [pixel]
          detx = 4280.246288025161 FPC X [pixel]
          dety = 4396.309005800286 FPC Y [pixel]
             x = 3896.5952380952  Sky X [pixel]
             y = 4385.7857142857  Sky Y [pixel]
      logicalx = 3896.5952380952  X coordinate in binned image [pixel]
      logicaly = 4385.7857142857  Y coordinate in binned image [pixel]
            ra = 212.8785255685877 RA [deg or hh:mm:ss]
           dec = 52.24005379720089 Dec [deg or dd:mm:ss]
         theta = 2.883417576772354 Off axis angle [arcmin]
           phi = 58.49679169514548 Azimuthal angle [deg]
         order = 0                Grating order
        energy = 1                Energy [keV]
    wavelength = 0                Wavelength [A]
         ra_zo = 212.8785255685877 RA of zero order
        dec_zo = 52.24005379720089 Dec of zero order
     (asolfile = none)            Input aspect solution file
       (option = )                Conversion option
#
# Override setup for observation
# All parameters here are strings so that they can
# be set blank, in which case the data file value is used
#
       (celfmt = hms)             RA and Dec format [deg or hms] (xx.xx or xx:xx:xx.x)
     (detector = )                Detector (ACIS or HRC-I or HRC-S)
      (grating = )                Grating
        (fpsys = )                FP convention
          (sim = )                SIM position (eg 0.0 0.0 -190.6)
     (displace = )                STF displacement (X,Y,Z,AX,AY,AZ)
       (ra_nom = )                Nominal pointing RA [deg or hh:mm:ss]
      (dec_nom = )                Nominal dec [deg or dd:mm:ss]
     (roll_nom = )                Nominal roll [deg]
       (ra_asp = )                Instantaneous pointing RA [deg]
      (dec_asp = )                Instantaneous pointing Dec [deg]
     (roll_asp = )                Instantaneous Aspect roll [deg]
#
      (geompar = geom)            Parameter file for Pixlib Geometry files
      (verbose = 0)               Debug Level
         (mode = ql)


Parameters for /home/username/cxcds_param/celldetect.par


#
#   celldetect parameter file
#
#
#   input  
#
        infile = acisf00578N002_evt2.fits Input file
#
#   output
#
       outfile = new_578_src2.fits    Output source list
       (expstk = )                list of exposure map files
      (regfile = none)            ASCII regions file
#
#   output options
#
       (kernel = default)         Output file format
      (clobber = no)              Overwrite exiting outputs?
#
#   output content/format options
#
       (thresh = 3)               Source threshold
    (findpeaks = yes)             Find local peaks?
     (centroid = yes)             Compute source centroids?
     (ellsigma = 3)               Size of output source ellipses (in sigmas)
     (expratio = 0)               cutoff ratio for source cell exposure variation
#
#   detect cell size parameters
#
    (fixedcell = 0)               Fixed cell size to use (0 for variable cell)
      (xoffset = INDEF)           Offset of x axis from data center
      (yoffset = INDEF)           Offset of y axis from data center
        (eband = 1.4967)          Energy band
      (eenergy = 0.8)             Encircled energy of PSF
     (psftable = ${ASCDS_CALIB}/psfsize20010416.fits -> /soft/ciao/data/psfsize20010416.fits) Table of PSF size data
     (cellfile = )                Output cell size image stack name
#
#   background parameters
#
      (bkgfile = )                Background file name
     (bkgvalue = 0)               Background count/pixel
  (bkgerrvalue = 0)               Background error
#
#   using defaults is recommended here
#
     (convolve = no)              Use convolution?
      (snrfile = )                SNR output file name (for convolution only)
#
#   run log verbosity and content
#
      (verbose = 0)               Log verbosity level
          (log = no)              Make a celldetect.log file?
#
#   mode
#
         (mode = ql)

History

01 Jun 2004	updated for CIAO 3.2: corrected screen output, include `dmcoords asolfile` parameter
12 Dec 2005	reviewed for CIAO 3.3: no changes
01 Dec 2006	reviewed for CIAO 3.4: no changes