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Last modified: December 2009

AHELP for CIAO 4.2


Context: dm


CIAO Data Model: syntax for filtering and binning files


The CIAO Data Model (DM) is a versatile interface used by CIAO to examine and manipulate standard format datafiles (e.g. FITS, ASCII). The DM enables powerful filtering and binning of datafiles. This document is an introduction to the DM syntax used by the CIAO tools.

Table of Contents

  • 1. DM Syntax and Virtual Files
  • 2. Virtual Columns
  • 3. Renaming and Reordering Columns

Related help files contain information and examples illustrating the capabilities of the DM. A list of these files can also be obtain from the CIAO command line with "about dm" or "ahelp -k dm".

  • ahelp dmascii: using text files in CIAO
  • ahelp dmbinning: creating images from event files/tables
  • ahelp dmfiltering: table and image filtering
  • ahelp dmregions: the CIAO region-filtering syntax
  • ahelp dmopt: controlling internal DM options (setting NULL characters, providing more memory to a tool)
  • ahelp coords: a discussion of the Chandra coordinate systems

Detailed technical information is available from the Introduction to the Data Model memo

1. DM Syntax and Virtual Files

The Data Model offers an easy and powerful means of filtering data. The filtered file can be directly input to a tool without writing it to disk first; this is known as a "virtual file." The virtual file, which can also be referred to as a subspace, is simply a means of defining a subset of interest in the dataset.

The basic syntax of a virtual file is:


filename: the input filename. All CIAO tools accept FITS file input, and many also accept ASCII files. Some tools only work on event files, while others require an input image. Refer to the individual tool help files for any restrictions.

[block]: the extension of the file to use, e.g. the name of the image or table. For FITS files, the block corresponds to an HDU and may be identified by name ("[EVENTS]") or number ("[2]"). If the block is not specified, the first "interesting" block is used (e.g. [EVENTS] for an event file). To view the blocks in a file, use "dmlist file.fits blocks".

[filter]: the filter to apply to the data. It indicates, for instance, which time period, energy range, or spatial region to use (e.g. "[time=1522012:1522320,1522400:1522600]"). Refer to "ahelp dmfiltering" for a full discussion of filtering.

[binning]: the binning specification for creating an image from an event file (e.g "[bin x=10:100:1,y=1:100:1]"). Refer to "ahelp dmbinning" for a full discussion of binning.

[columns]: the names of the columns to include ("[cols time,energy,]") or exclude ("[cols -phas]"). The syntax "[cols !phas]" may also be used, but the "!" symbol needs to be written as "\!" in the Unix shell, making the "-" syntax more convenient.

[option]: advanced options for the DM, such as specifying what the NULL character should be or how much memory to allow a tool to use. Refer to "ahelp dmopt" for a list of the available options.

[rename]: the name for the block in the output file. The default behavior is for the output to have the same block name, unless a file is binned to create an image; in that case, "_IMAGE" is added to the block name. (For information on renaming columns, refer to a later section in this file.)

2. Virtual Columns

A file may contain virtual columns whose values are calculated by applying a mathematical transform to an existing column. Virtual columns - such as EQPOS(RA,DEC) - do not physically exist in the event file; they are defined by the WCS information attached to another column, e.g. SKY.

The transformation is listed in the output of "dmlist evt2.fits cols":

1:    EQPOS(RA ) = (+278.3860) +TAN[(-0.000136667)* (sky(x)-(+4096.50))]
           (DEC)   (-10.5899 )      (+0.000136667)  (   (y) (+4096.50)) 

For most applications, these columns may be used the same as non-virtual columns in the file. It is possible to list, filter, and bin on virtual columns.

However, filtering and binning do not work reliably on virtual columns derived from non-monotonic coordinate transforms (e.g. MSC(THETA,PHI), or EQPOS near the poles; see "ahelp coords" for more information on these coordinate systems).

3. Renaming and Reordering Columns

It is possible to rename a column or change the order of the columns within a file. Note that certain CIAO tools require particular column names (e.g. time, energy), but none of the tools make assumptions about the order of the columns within a file.

To rename a column, run dmcopy with the column syntax "newname=oldname". Multiple columns may be renamed in the same command.

dmcopy "pi.fits[cols rate=count_rate]" pi_rate.fits
dmcopy "pi.fits[cols rate=count_rate, rate_err=count_rate_err,*]" \ 

The "count_rate" column in pi.fits is renamed to "rate" in pi_rate.fits. With the first command, "rate" will be the only column in the output file. The "*" operator indicates that all other columns should be copied unchanged to the output file.

The columns will appear in the output file in the order in which they are specified. So in the renaming case, "rate" will be the first column in the output. The "cols" syntax can be used to reorder columns without modifying them as well:

dmcopy "pi.fits[cols energy,time,pi,count_rate, *]" reorder.fits

Note that for a vector column sky(x,y),

"evt.fits[cols x,y]"

will retain the information that (x,y) is a vector column called "sky". Any of the following, however, will separate the vector components and lose the vector-dependent coordinate systems like RA and Dec:

[cols x]
[cols y,x]
[cols x,pha,y]

Example 1

acisf01843N002_evt2.fits[EVENTS][cols #1,#2,#3]
acisf01843N002_evt2.fits[cols time,ccd_id,node_id]

Select three columns of the EVENTS block by number or by name.

Example 2


Select rows 1-4 from a FITS file.

Example 3

dmlist "evt.fits[events][pha=30:200,time=10:20,50:60]" data

Use the tool dmlist to print specific data values from the file to the screen. A filter is applied to the "events" block in the file evt.fits. The filter selects rows in the table for which the value of the pha column is >= 30 and < 20, and for which the time is either >= 10 and < 20 or >= 50 and < 60. Both the the pha and time filters must be satisfied for a row to pass the filter.

Example 4

acisf01843N002_evt2.fits[EVENTS][bin x=3200:4800:4,y=3200:4800:4]

Bin an event file into an image with this input to the tool dmcopy.

Example 5

acisf01843N002_evt2.fits[EVENTS][bin pi=1:1024:1]

Use this specification as input to the tool dmextract to bin an event file into a PI spectrum.

Example 6

dmcopy "evt.fits[cols -status]" evt_new.fits

Removed the status column from evt.fits.


See the bugs page for the Data Model library on the CIAO website for an up-to-date listing of known bugs.

See Also

coords, level, pileup, times
autoname, ciao, ciaorc, parameter, stack, subspace
dmascii, dmbinning, dmfiltering, dmopt, dmregions
pixlib, stackio
dmcoords, dmcopy, dmextract, dmlist, dmstat

Last modified: December 2009