This time we have used dmtcalc to create a new column called "DTIME" in the output file evt2_hacked.fits (new or changed columns always have their names written in upper case in the output file, however they are written in the expression).

The contents of the DTIME column are set equal to

(time-TSTART)*1.0e-3

where TSTART is replaced with the value of the TSTART keyword from the header. The case of header keywords is unimportant, so

(time-tstart)*1.0e-3

would give the same result.

For this particular example DTIME can be considered to be the time since the start of the observation in kilo-seconds. In general the expressions used in these examples are just used to illustrate dmtcalc's capabilities and so should not be taken to have any real meaning for Chandra (or any other) data. More details of the allowed expressions are given in "ahelp syntax".

Here we use dmtcalc to over-write the values in the energy column to be ten percent of the PHA values. Since the PHA values are stored as integers we convert them to floating-point numbers - by use of the term "(float)" - before the division.

If we had omitted the "(float)" term, the output would still have been automatically converted to a real since we are multiplying by a real number. It would, however, have changed the ENERGY column to Real8 (ie double) format instead of keeping the Real4 (float) format.

The name of the energy column in the output file (here "evt2_hacked.fits") will be in upper case - whatever its case in the input file - since its values have been changed by dmtcalc.

Since "expression" is a required (automatic) parameter, you may be tempted to enter something like:

  unix% dmtcalc evt2.fits evt2_hacked.fits "energy=((float)pha)*0.1"

This will fail because the parameter parser interprets this as setting a (nonexistent) dmtcalc parameter named "energy" to "((float)pha)*0.1".

The contents of the NEWPHA column are set equal to the PHA column added to the integer part of

1.25e-3*(time-TSTART)

The data type of the output column is integer (Int4), since the PHA column is an integer and the time expression has been converted (also called cast) to an integer value (here of type "long").

This does the same as the previous example except that the input is now two files - evt2.obs1 and evt2.obs2. There is only one output file - evt2.combined - which contains the NEWPHA column. The row order of the output image is determined by the order of the files in the input stack: the first files contents, then the second file, etc until all the input files have been processed. The dmsort tool can be used to change the row order of the output file if required.

Here the file lc.orig (which is assume to be a lightcurve so contains a rate column) is converted to lc.smoothed in which the RATE column is equal to the smoothed - by 9 rows - value of the rate and then a dc offset (0.123) is subtracted from it. The value for the dc offset would have to be calculated prior to running the program - e.g with dmstat as shown in the following csh/tcsh example

dmstat "lc.orig[cols rate]" sigma-
set dc=`pget dmstat out_mean`
dmtcalc lc.orig lc.smoothed expression="rate=(rate:9-$dc)"

The expression can also be stored in an ASCII file and accessed as a stack. Here we use the contents of calcphase.lis to create an output file in which two new columns have created - PHASE and GPHASE - which contain phase information for the events. The contents of the stack file are:

  unix% cat calcphase.lis
  .dtime=(time-TSTART)
  GPHASE=.dtime*19.794885
  PHASE=GPHASE-(long)GPHASE

We have also included the use of a temporary variable (here .dtime) even though it is not really necessary for this particular example.

The results are only valid if the frequency of the source is 19.794885 Hz (and it's period is not changing, and you have applied the barycenter correction to the events file). For more details on using dmtcalc to add phase information to an event file see the Create a Phase-binned Spectrum thread on the CIAO website.

Here we repeat the previous example but give the expression directly - i.e. without the use of a stack file.

  unix% cat randpha.lis
  newpha= pha + (long) \
    ( 1.25e-3*(time-TSTART) + 10*(#rand(1034)-0.5) )

Here we define the expression in an external file and use the "\" character to indicate a continuation line. The contents of the NEWPHA column are set equal to the PHA column added to the integer part of

1.25e-3*(time-TSTART) + 10*(#rand(1034)-0.5)

where #rand(1034) means a random number between 0 and 1 where 1034 is used to seed the random number generator. This seeding only happens once - ie it is not re-set for each row of the table.

The result is similar to the earlier example, except that a random element of +-5 has been added onto the calculation.

Here we use the support for conditional expressions to filter out the NaN values from the PSRATIO column of srclist.wav, replacing any such values by 0.0.

This expression creates the column SCOL which contains boolean values: TRUE (1) or FALSE (0). The row entry is true if the entry shape column contains the string "cir" and false otherwise. The "~=" operator returns true if the string on the left-hand side contains the string given on the right-hand side.