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Last modified: 1 April 2024


Bugs: Data Model

Table of Contents


Filtering Data

ASCII Kernel

Binning & Rebinning Images



    Linear transforms have an extra 0.5 bin shift applied.

    When creating a file with a Linear WCS, the CXCDM adjusts the transform parameters to force a half bin offset. While mathematically consistent with the values input, the result may be confusing especially when the transform is well known, eg °C to °F.

    World Coord Transforms for Columns in Table Block simple2.dat
    ColNo Name
    2: tempF = +32.90 [degree F] +1.80 * (tempC -0.50)

    which is mathematically correct, but more commonly written as just

       tempF = +32 + 1.8 * tempC

    TNULL raw header keywords are not copied to the output file  (19 Aug 2011)

    DM support for FITS TC* keywords

    There are three issues with the generic use of TC* keywords in FITS files read and written by DM tools.

    The first issue: TC*n[A-Z]. The DM function that composes this keyword does not strip off the 'P' from the before adding number and letter to the end.

    Second issue: TCTY* keys are not recognized and therefore are not processed.

    Third issue: DM is not always retaining the T*NAM information when it stores information on the DM descriptor, resulting in output keywords T*TYP instead.

    EXTNAME is forced to be same a HDUNAME on output

    When a FITS file is copied, the EXTNAME is forced on output to be the same as the HDUNAME. Typical Chandra/CIAO files require these to be the same; however, data from other missions and projects may not have the same requirement.

    Updating file in place

    Various CIAO tools including dmappend, dmhedit, dmreadpar, acis_clear_status_bits, and dmgti attempt to modify a files contents in place. In addition users of crates may attempt to write scripts that try to open a file with read+write access.

    Files may not be writeable for several reasons. The normal UNIX file permissions may not allow a particular user to modify a file.

    File that are gziped are never writeable, regardless of whether the file itself has file-write permission.

    Standard in and standard out, accessed by the "-" (no quotes) file name are not writeable.

    Individual blocks of data that have filters on the data are not writeable. So


    is not writeable becuase it is actually filtering the data,


    is writeable since the filter is applied to select which block in the file.

Filtering Data

    Incorrect results when fitering an image using exclude syntax with full option.

    This bug is triggered when filtering an image with the [exclude ] synatx with a region together with the [opt full] directive to retain the original image size.

    unix% dmcopy "img.fits[exclude sky=region(ciao.reg)][opt full]" filt_img.fits

    The pixels outside a box that bounds the region are also filtered out (ie set to 0).


    In many cases users can easily invert the logic in their region files to avoid needing to use the [exclude ] directive. This can be done any time the region only contains included shapes. For example:

    unix% cat ciao.reg
    unix% cat ciao.reg | awk ' BEGIN {print "field()"} {print "-"$0}' > exclude_ciao.reg
    unix% cat exclude_ciao.reg

    This file can then be used without needing to use the [exclude ] synatx

    unix% dmcopy "img.fits[sky=region(exclude_ciao.reg)][opt full]" filt_img.fits

    This has the advantage of also generally being much faster.

    Region filtering images without specifing axis name(s).

    dm region filtering with unnamed axes may fail for complex regions, eg

    dmlist my_img.fits[circle(10,10,10)-box(0,0,30,30)]

    The work around is to explicilty use the axis name, eg

    dmlist my_img.fits[sky=circle(10,10,10)-box(0,0,30,30)]

    Filtering on some WCS columns produces incorrect results  (08 Oct 2012)

    When filtering on WCS columns, the range is taken by converting range of the parent columns and using those as the limits of the WCS columns. When the transform is highly non-linear, eg the TAN-P transform used to go from DETX,DETY to THETA,PHI, this can leads to incorrect limits and incorrect filters. Users who want to filter on WCS columns should give explict ranges and not rely on the computed min/maxes.

    bad%  dmcopy "evt.fits[theta=:1]" 
    good% dmcopy "evt.fits[theta=0:1]"

    Creating a vector on-the-fly when region filtering

    When region-filtering images, you can create a vector on the fly from any two axes by using a filter like "(#1,#3)=circle(...)". Although the image is filtered correctly with a temporary vector, the region filter isn't recorded in the subspace. Hence, tools that use the filtered file don't know that pixels outside the filter region are invalid. As a result, dmstat reports no nulls in the filtered image (unless you explicitly tell the DM to set pixels outside the filter to null by using "opt null=...").

    Applying a bit-filter expression to an integer column does not work, nor does it cause an error.

    Using one column from a vector column in region filter

    Trying to use one column that is part of a vector column in a region expression can lead to a crash

    % dmlist hrcf04482N003_evt2.fits"[(tg_lam,tg_d)=field()]" counts
    # 24359: Received error signal SIGSEGV-segmentation violation.
    # 24359: An invalid memory reference was made.
    # 24359: segmentation fault: DMLIST (1) is:

    Where tg_d is part of the rd = (tg_r,tg_d) vector column.

    The only work-around is to create a temporary file that dismantels the vector column by removing the other column

    % dmcopy hrcf04482N003_evt2.fits"[cols -tg_r]" tmp_evt
    % dmcopy tmp_evt"[(tg_lam,tg_d)=field()]" counts

    Filtering on array columns is undefined.

    Filter on an array column is not supported. The tool will run; however, the results are unpredictable.

ASCII Kernel

    Error when reading SIMPLE text format with 0 rows.

    SIMPLE ASCII files cannot have 0 rows. If there are 0 rows, then the column header definition row is incorrectly read. For example:

    unix% echo "#foo" > bar
    unix% dmlist bar cols
    Columns for Table Block bar
    ColNo  Name                 Unit        Type             Range
       1   foo                               String[4]                  
    unix% dmlist bar data
    Data for Table Block bar
    ROW    foo
         1 #foo
    unix% dmlist bar counts

    The SIMPLE ASCII file should have 0 rows and we see from the data option that the column definition row has been read in as data.

    Virtual columns are not supported.  (11 Dec 2007)

    In the DM, you can normally do

    unix% dmlist evt.fits"[cols ra,dec]"  data

    even though RA and Dec are just coordinate systems defined on the X and Y columns in the file; the DM applies the transform on the fly. This doesn't work yet for ASCII files.

    DTF-FIXED keyword comments may be truncated.

    DTF-FIXED header lines may be up to 1024 characters long. However, if the keyword is longer than the FITS standard, the comment is truncated.

    unix% input.txt output.dtf'[opt kernel=text/dtf-fixed]'

    In input.txt:

    TTYPE14 = 'Class' / LV Class Exo: M = missile [B = tactical ballistic  
    missile (except Redstone) apo=80:200]
    R = research rocket O = orbital LV V = RTV Y = Exo weather rocket X =  
    Big test rocket D= Deep space launch

    In output.dtf:

    TTYPE14 = "Class " / LV Class Exo: M = missile [B = tactical  
    ballistic missile (except Redstone) apo

    Long column descriptions

    ASCII files may contain keyword and column descriptions longer than 80 characters. This can cause random failures in the DM (unable to open columns, wrong data type, etc).

Binning & Rebinning Images

    Rebinning an image with different values for the two axes causes the coordinate information to be lost

    For example:

    unix% dmcopy acis.img"[bin x=::5,y=::6]" acis5x6.img

    Using the same value for both axes works correctly:

    unix% dmcopy acis.img"[bin (x,y)=::5]" acis5.img

    Running a DM tool on an image where one of the axes has been filtered results in an error.  (17 Jul 2009)

    unix% dmextract
    Input event file (ccd3.sky4.fits[y=3767:][bin sky=annulus(3786,3767,0:380:4)]):
    Enter output file name (rprof.fits):
    # dmextract (CIAO 4.0 Beta 2): WARNING: Input file, "ccd3.sky4.fits[y=3767:]", 
    has no rows in it.
    Bus error

    Images without a physical coordinate system

    For images without a physical coordinate system the DM will internally create one. This is not written to the output file which may lead to errors if the image size changes due to spatial filtering as the output file then has a different physical WCS compared to the input file.