Object-centered Aspect Solution and Exposure Map
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CIAO 4.0 Science Threads
OverviewLast Update: 18 Jan 2008 - new for CIAO 4.0 Synopsis: In addition to reprojecting the event data to the reference frame of the object, the sso_freeze tool can create an object-centered aspect solution file. The new aspect solution file may then be used to generate an exposure map or response file (ARF) for the data. This thread focuses on how to use the aspect solution file. The Reprojecting Coordinates of a Solar System Object has more general information on running sso_freeze. Purpose: To create an object-centered aspect solution file and build an exposure map with it. Read this thread if: you are working with any observation of a solar system object. Related Links:
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Contents
- Choosing the Ephemeris Files
- Run sso_freeze
- Updating the Event File Header
- Create an Exposure Map
- Create the Fluxed Image
- Optional: Confirm the Accuracy of the Aspect Solution
- Parameter files:
- History
- Images
Choosing the Ephemeris Files
Sample ObsID used: 3726 (ACIS-S, Jupiter)
File types needed: evt2; asol1; eph1; msk1
There are two ephemeris input files needed for the tool to run: the spacecraft ephemeris file (scephemfile) and the SSO ephemeris file (ssoephemfile). Both of these files are part of the primary data products. In ChaSeR, they are listed as the "MT (moving target) ephem" and "Orbit ephemeris" data products.
unix% ls -1 *eph1.fits jupiterf162475200N001_eph1.fits orbitf162216300N001_eph1.fits
The ephemeris files are selected based on the start time (TSTART) of the observation:
unix% dmkeypar acisf03726N003_evt2.fits TSTART echo+ 162488245.80274
Make sure that the timestamp in the filenames is less than the start time of the data. Also, if there is more than one version of a file - e.g. N001 and N002 - choose the newest (higher number) version available.
Spacecraft and SSO ephemeris files are also available from the CDA FTP site (cda.harvard.edu) if you have trouble retrieving them via ChaSeR. Login as "anonymous", using your email address as the password; the files are in the /pub/arcftp/bary/ephem directory.
Run sso_freeze
Once you have the necessary input files, you are ready to run the tool:
unix% punlearn sso_freeze unix% pset sso_freeze infile=acisf03726N003_evt2.fits unix% pset sso_freeze scephemfile=orbitf162216300N001_eph1.fits unix% pset sso_freeze ssoephemfile=jupiterf162475200N001_eph1.fits unix% pset sso_freeze asolfile=pcadf162488590N002_asol1.fits unix% pset sso_freeze ocsolfile=3726_oc_asol1.fits unix% pset sso_freeze outfile=3726_jupiter_oc.fits unix% sso_freeze Input event file or stack (acisf03726N003_evt2.fits): Input spacecraft ephemeris file (orbitf162216300N001_eph1.fits): Input solar system object file (jupiterf162475200N001_eph1.fits): Output event file name (3726_jupiter_oc.fits):
In some cases, there will be more than one asol1.fits file for an observation. All the files must be input to the asolfile parameter, either as a comma-separated list or as a stack.
The contents of the parameter file may be checked using plist sso_freeze.
unix% ds9 "3726_jupiter_oc.fits[bin=ocx,ocy]" &
Jupiter is displayed in object-centered (OC) coordinates
(ocx,ocy) in Figure 1 ![[Link to Image 1: Jupiter in OC coordinates
(ocx,ocy)]](../../imgs/imageicon.gif){kind=small}
. Notice that in the reference
frame of Jupiter, the point source at (4099,4092) becomes a
long streak. The source may be removed from the field with
the dmcopy or dmfilth tools, if desired.
Updating the Event File Header
In the new event file, the origin of the OC coordinate system is (0,0). The RA and Dec header keyword values are updated to reflect this, so that the correct reference point is used in the rest of the analysis.
An edit list file is used as input to dmhedit:
unix% cat edits.lis #add RA_NOM = 0.0 DEC_NOM = 0.0 RA_PNT = 0.0 DEC_PNT = 0.0 unix% dmhedit 3726_jupiter_oc.fits filelist=edits.lis
You can check the parameter file that was used with plist dmhedit.
Create an Exposure Map
1. Make the aspect histogram
With the OC aspect solution file we can create a binned histogram detailing the aspect history of the observation.
Data Model syntax is used in the infile parameter to rename the "OCSOL" block to "ASPSOL". The "ocra" and "ocdec" columns are also renamed on-the-fly to "ra" and "dec", respectively. The changes are required because asphist expects the block and columns to have those names.
unix% punlearn asphist unix% pset asphist infile="3726_oc_asol1.fits[OCSOL][cols time,ra=ocra,dec=ocdec,roll,dy,dz,dtheta][ASPSOL]" unix% pset asphist outfile=3726_oc_asphist.fits unix% pset asphist evtfile=3726_jupiter_oc.fits unix% asphist Aspect Solution List Files (3726_oc_asol1.fits[OCSOL][cols time,ra=ocra,dec=ocdec,roll,dy,dz,dtheta][ASPSOL]): Aspect Histogram Output File (3726_oc_asphist.fits): Event List Files (3726_jupiter_oc.fits):
If you created more than one OC aspect solution file, all the files must be input to the infile parameter, either as a list or as a stack.
You can check the parameter file that was used with plist asphist.
2. Calculate the Instrument Map
In this example we choose to create a monoenergetic distribution of source photons of 1.0 keV. The ACIS dead area correction is not applied.
This observation of Jupiter falls on chip ACIS-S3 (ccd_id=7). You may need to change the detsubsys parameter value for your observation.
Note that it is not necessary for the instrument map to be congruent with the exposure map; the instrument map should describe the chip with full resolution. We will use the default values for the detector region (pixelgrid parameter), creating a 1024 x 1024 pixel image that covers the whole chip.
unix% punlearn mkinstmap unix% pset mkinstmap obsfile="3726_jupiter_oc.fits[EVENTS]" unix% pset mkinstmap outfile=3726_oc_instmap.fits unix% pset mkinstmap maskfile=acisf03726_000N003_msk1.fits unix% pset mkinstmap detsubsys=ACIS-S3 unix% pset mkinstmap pixelgrid="1:1024:#1024,1:1024:#1024" unix% pset mkinstmap pbkfile=NONE dafile=NONE unix% pset mkinstmap spectrumfile=NONE unix% pset mkinstmap monoenergy=1.0 unix% mkinstmap Output File Name (3726_oc_instmap.fits): Energy Spectrum File (see docs) (NONE): Energy for mono-chromatic map [keV] (0.1:10) (1): Pixel grid specification x0:x1:#nx,y0:y1:#ny (1:1024:#1024,1:1024:#1024): Name of fits file + extension with obs info (3726_jupiter_oc.fits[EVENTS]): Detector Name (ACIS-S3): Grating for zeroth order ARF (NONE|LETG|HETG) (NONE): NONE, or name of ACIS window mask file (acisf03726_000N003_msk1.fits): NONE, or the name of the parameter block file (NONE):
Including the maskfile parameter is particularly important if you are interested in having an accurate exposure map at the very edge of a CCD, subarray or window. For more information, see the dictionary entry on mask files.
You can check the parameter file that was used with plist mkinstmap.
3. Calculate the Exposure Map
Now we use mkexpmap and the aspect information stored in the histogram to project the instrument map onto the sky.
The xygrid parameter is set to produce a 512 x 512 pixel exposure map, centered at (4096,4096).
unix% punlearn mkexpmap unix% pset mkexpmap instmapfile=3726_oc_instmap.fits unix% pset mkexpmap outfile=3726_oc_expmap.fits unix% pset mkexpmap xygrid="2048:6144:#512,2048:6144:#512" unix% pset mkexpmap asphistfile=3726_oc_asphist.fits unix% pset mkexpmap useavgaspect=no unix% pset mkexpmap normalize=no unix% mkexpmap Aspect Histogram File (3726_oc_asphist.fits): Output File Name (3726_oc_expmap.fits): Name of Instrument Map (3726_oc_instmap.fits): grid specification syntax x0:x1:#nx,x0:x1:ny (2048:6144:#512,2048:6144:#512): Use Average Aspect Pointing (no): Exposure map limits: 0.000000e+00, 1.701507e+07 Writing exposure map to 3726_oc_expmap.fits
The exposure map may be displayed in
ds9 .
You can check the mkexpmap parameter file that was used with plist mkexpmap.
Since we set the normalize parameter = no, the exposure map has units of [cm2*s*counts/photon]. This allows us to simply divide the image by the exposure map to derive an image in units of flux [photons/cm2/s/pixel]. If the setting had been left as yes (the default), the units of the exposure map would be [cm2*counts/photon]. Please see the help file for mkexpmap for more details on this.
Create the Fluxed Image
The exposure map is in units of [cm2*s*counts/photon] since it was created by projecting the instrument map (in [cm2*counts/photon]) onto the tangent plane of the observation. To create an image in units of [photon/cm2/s/pixel], we simply need to divide by the exposure map. This is done by the tool dmimgcalc.
Data Model syntax is used to create a virtual image for the infile parameter which has the same binning used in making the exposure map.
unix% punlearn dmimgcalc unix% pset dmimgcalc infile="3726_jupiter_oc.fits[bin ocx=2048:6144:#512,ocy=2048:6144:#512]" unix% pset dmimgcalc infile2=3726_oc_expmap.fits unix% pset dmimgcalc outfile=3726_oc_norm.fits unix% pset dmimgcalc operation=div unix% dmimgcalc Input file #1 (3726_jupiter_oc.fits[bin ocx=2048:6144:#512,ocy=2048:6144:#512]): Input file #2 (3726_oc_expmap.fits): output file (3726_oc_norm.fits): arithmetic operation (div): warning: CONTENT has 1 different values. warning: DETNAM has different value...Merged...
The messages are related to how the tool merges the header information in the input files. The merging_rules ahelp file explains the rules and how they affect the output file header.
The units of 3726_oc_norm.fits
are [photon/cm2/s/pixel].
You can check the parameter file that was used with plist dmimgcalc.
Optional: Confirm the Accuracy of the Aspect Solution
This section is an optional exercise which demonstrates that the OC event file and aspect solutions are consistent.
1. OC (RA,DEC) vs target (RA,Dec)
To visualize the motion of the aspect solution in OC coordinates, we modify the ocra column of the OC aspect solution so that -180 < ocra < 180, instead of 0 < ocra < 360:
unix% dmtcalc 3726_oc_asol1.fits 3726_oc180_asol1.fits \ expression="if(ocra>180)then(ocra=ocra-360.0)"
Next, create images of the aspect solution in (ra,dec) and (ocra,ocdec). The dmstat tool is used to determine the limits for the binning.
unix% dmstat 3726_oc180_asol1.fits"[cols ocra]" | grep @
min: -0.029364271985 @: 3501
max: 0.014399458266 @: 120865
unix% dmstat 3726_oc180_asol1.fits"[cols ocdec]" | grep @
min: -0.013777941173 @: 115846
max: -0.0016925932556 @: 904
unix% dmcopy "3726_oc180_asol1.fits[bin ocra=-.03:.02:.0001,ocdec=-.02:0.0:.0001]" \
3726_oc_radec.img
unix% dmstat 3726_oc180_asol1.fits"[cols ra]" | grep @
min: 132.93315954 @: 80945
max: 132.93902143 @: 4997
unix% dmstat 3726_oc180_asol1.fits"[cols dec]" | grep @
min: 18.512378687 @: 104360
max: 18.517959801 @: 114119
unix% dmcopy "3726_oc180_asol1.fits[bin ra=132.933:132.940:.00001,dec=18.512:18.518:.00001]" \
3726_targ_radec.img
Figure 4 compares the images in
ds9 :
unix% ds9 3726_targ_radec.img 3726_oc_radec.img &
The OC (RA,Dec) coordinates drift over time (left ds9 frame), representing the increase in distance between the OC origin (Jupiter's position) and Chandra pointing as Jupiter moves across the sky. In the right ds9 frame there is no drift, as the pointing does not change with respect to the telescope.
2. Event data vs OC aspect solution
The sso_track script is used to create a set of 10 regions representing the current position of Jupiter:
unix% sso_track 3726_oc_asol1.fits 10 sso.reg ocsol, nreg are: 3726_oc_asol1.fits 10 nrows is 120865 dt is 13429.4444 ssora,dec are: 132.9658671282 18.5195749695 ssora,dec are: 132.9609329984 18.5202451938 ssora,dec are: 132.9560468016 18.5209561862 ssora,dec are: 132.9512045626 18.5217051671 ssora,dec are: 132.9464037802 18.5224893790 ssora,dec are: 132.9416409073 18.5233065438 ssora,dec are: 132.9369141861 18.5241544081 ssora,dec are: 132.9322207554 18.5250312062 ssora,dec are: 132.9275594288 18.5259351345 ssora,dec are: 132.9229282045 18.5268647788
A comparison between Jupiter's motion and the OC aspect
solution is shown in Figure
5 :
unix% ds9 3726_jupiter_oc.fits -region sso.reg 3726_oc_radec.img &
The event data is in sky coordinates (left ds9 frame) and RA decreases to the right along the x-axis. The circular regions (loaded from sso.reg) represent the current position of Jupiter, with the most recent position in green. These data show Jupiter moving higher in Dec (up) and lower in RA (right) over time.
The OC aspect solution is in (RA,Dec) coordinates (right ds9 frame) and RA increases to the right along the x-axis. The leftmost edge of this data have the earliest timetags, so the dither pattern is moving from left to right over time. The distance to the Chandra pointing vector (RA_NOM, DEC_NOM) as seen from the position of Jupiter moves lower in Dec (down) and higher in RA (right) over time.
Therefore, the event image and OC aspect image are consistent: from the fixed EQPOS frame, Jupiter moves higher in Dec and lower in RA. Looking back from Jupiter, the fixed dither pattern appears to be moving in the opposite direction: lower in Dec and higher in RA.
Parameters for /home/username/cxcds_param/sso_freeze.par
infile = acisf03726N003_evt2.fits Input event file or stack
scephemfile = orbitf162216300N001_eph1.fits Input spacecraft ephemeris file
ssoephemfile = jupiterf162475200N001_eph1.fits Input solar system object file
outfile = 3726_jupiter_oc.fits Output event file name
(asolfile = pcadf162488590N002_asol1.fits) Input aspect solution file
(ocsolfile = 3726_oc_asol1.fits) Output asol in OC coordinates
(logfile = STDOUT) debug log file ( STDOUT | stdout | <filename>)
(scale = 0) Image pixel scale (km/pixel, 0=angular coords)
(lookuptab = ${ASCDS_CALIB}/dmmerge_header_lookup.txt -> /soft/ciao/data/dmmerge_header_lookup.txt) lookup table
(clobber = no) Overwrite existing output dataset with same name?
(verbose = 0) debug level (0-5)
(mode = ql)
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Parameters for /home/username/cxcds_param/dmhedit.par
infile = 3726_jupiter_oc.fits Input dataset/block specification
filelist = edits.lis Edit list file name
operation = Operation
key = Keyword
value = Value for keyword
(datatype = indef) Keyword data type
(unit = ) Unit for keyword
(comment = ) Comment for keyword
(verbose = 0) Verbosity Level(0-5)
(mode = ql)
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Parameters for /home/username/cxcds_param/asphist.par
infile = 3726_oc_asol1.fits[OCSOL][cols time,ra=ocra,dec=ocdec,roll,dy,dz,dtheta][ASPSOL] Aspect Solution List Files
outfile = 3726_oc_asphist.fits Aspect Histogram Output File
evtfile = 3726_jupiter_oc.fits Event List Files
dtffile = Live Time Correction List Files for HRC
(geompar = geom) Parameter file for Pixlib Geometry files
(res_xy = 0.5) Aspect Resolution x and y in arcsec
(res_roll = 600.) Aspect Resolution roll in arcsec
(max_bin = 10000.) Maximal number of bins
(clobber = no) Clobber output
(verbose = 0) Verbose
(mode = ql)
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Parameters for /home/username/cxcds_param/mkinstmap.par
outfile = 3726_oc_instmap.fits Output File Name
spectrumfile = NONE Energy Spectrum File (see docs)
monoenergy = 1 Energy for mono-chromatic map [keV]
pixelgrid = 1:1024:#1024,1:1024:#1024 Pixel grid specification x0:x1:#nx,y0:y1:#ny
obsfile = 3726_jupiter_oc.fits[EVENTS] Name of fits file + extension with obs info
detsubsys = ACIS-S3 Detector Name
grating = NONE Grating for zeroth order ARF
maskfile = acisf03726_000N003_msk1.fits NONE, or name of ACIS window mask file
pbkfile = NONE NONE, or the name of the parameter block file
(mirror = HRMA) Mirror Name
(dafile = NONE) NONE, CALDB, or name of ACIS dead-area calibration file
(ardlibparfile = ardlib.par) name of ardlib parameter file
(geompar = geom) Parameter file for Pixlib Geometry files
(verbose = 0) Verbosity
(clobber = no) Overwrite existing files?
(mode = ql) Enter mode for parameter file.
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Parameters for /home/username/cxcds_param/mkexpmap.par
asphistfile = 3726_oc_asphist.fits Aspect Histogram File
outfile = 3726_oc_expmap.fits Output File Name
instmapfile = 3726_oc_instmap.fits Name of Instrument Map
#
xygrid = 2048:6144:#512,2048:6144:#512 grid specification syntax x0:x1:#nx,x0:x1:ny
useavgaspect = no Use Average Aspect Pointing
#---------------------------------------------------------------------------
# Aspect Histogram Parameters
# If UseAvgAspect is set to yes, then only the average pointing derived from
# the many pointings in the aspect histogram will be used.
#---------------------------------------------------------------------------
#asphistfile,f,a,"../data/aciss_asphist.fits",,,"Aspect Histogram File"
(normalize = no) Normalize exposure map by exposure time
#
(geompar = geom) Parameter file for Pixlib Geometry files
(verbose = 0) Verbosity
(clobber = no) Overwrite existing files?
(mode = ql) Enter mode for parameter file.
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Parameters for /home/username/cxcds_param/dmimgcalc.par
# parameter file for dmimgcalc
infile = 3726_jupiter_oc.fits[bin ocx=2048:6144:#512,ocy=2048:6144:#512] Input file #1
infile2 = 3726_oc_expmap.fits Input file #2
outfile = 3726_oc_norm.fits output file
operation = div arithmetic operation
(weight = 1) weight for first image
(weight2 = 1) weight for second image
(lookupTab = ${ASCDS_CALIB}/dmmerge_header_lookup.txt -> /soft/ciao/data/dmmerge_header_lookup.txt) lookup table
(clobber = no) delete old output
(verbose = 0) output verbosity
(mode = ql)
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History
| 18 Jan 2008 | new for CIAO 4.0 |
