Loving ISIS - Confessions of a Former XSPEC User
Grouping the Data
ISIS has the ability to dynamically group a spectrum during an
analysis session. You're not just rebinning the plots, you're
rebinning the data. This is a very powerful ability. With great
power, however, comes great responsibility. ISIS expects you
to rebin your data during the analysis session; therefore, it ignores
any default binning in the FITS file header (unless you set
Isis_Use_PHA_Grouping=1)!
The function I use most often for rebinning the data is:
group([ids];min_sn=#,min_chan=#,bounds=#,unit=string);
This is a very versatile function that will take multiple data
sets on a common energy/wavelength grid and group them identically
using multiple criteria, such as minimum signal-to-noise and minimum
number of channels, each as a function of energy or wavelength bounds.
Furthermore, it allows you to do fancier things such as identically
group multiple data sets, but use only a subset these data sets in the
signal-to-noise calculation.
For the purposes of this example, we employ a simple call to this function, e.g.,
group(pca_id;min_sn=5.5,bounds=3,unit="kev")
In this instance, we have grouped the PCA data to a minimum
signal-to-noise of 5.5 (roughly 30 counts per channel in the regions
of the spectrum with low background), for channels 3 keV and above.
This grouping can be changed at any point during the analysis if
one desires. This is a very powerful feature of ISIS, and
eliminates the need to create multiple copies of a PHA data file, or
to exit the data session, run grppha, and then restart the
analysis.
In the above you will notice that function options were passed via
qualifiers,
which, following a semi-colon, have the form of name=value. A
number of ISIS functions use qualifiers to pass values. There are
several advantages to qualifiers: they are order-independent, any
undefined qualifiers are simply ignored, and they typically have
defined default values so only those that are to be changed need to be
entered. Throughout this example you will see a number of commands
(both ISIS intrinsics and script-defined functions) that take
qualifiers as their inputs.
ISIS provides several other intrinsic functions for rebinning data, among which are:
group_data(id, factor); % Group data by number of channels
rebin_data(id, # or array); % Group data by minimum counts (or using array)
These functions start at short wavelength and follow a wavelength
ascending order. Additionally, with the use of the intrinsic
function set_rebin_error_hook, they allow you to control
how error propagation is handled. (ISIS pretty much gives you full
control over errors and statistics - but that is a topic for a
later page.)
Ignoring Data
The function that I use most often is:
notice_values([ids],lo1,hi1,lo2,hi2,...;unit=string);
This function only notices data between the ranges lo1...hi1,
lo2...hi2, etc., with all other data being ignored. The units of these
bounds are specified by the unit qualifier. As for the
group command, this function can operate simultaneously on
multiple datasets that share a common energy/wavelength grid. Here,
for example, we notice only the 3-22 keV PCA data via the command:
notice_values(pca_id,3.,22.;unit="kev");
ISIS has a number of other intrinsic functions for ignoring/noticing data, among which are:
ignore(id); % Ignore dataset id
notice(id,a,b); % Notice wavelength range [a,b] in dataset id
notice_en(id,a,b); % Notice energy range [a,b] in dataset id
xnotice(id,a,b); % *Only* notice wavelength range [a,b] in dataset id
xnotice_en(id,a,b); % *Only* notice energy range [a,b] in dataset id
exclude(id); % Don't change the ignored/noticed channels, but
% exclude the whole data set until reinstated with:
include(id); % Reinstate the data with prior excluded channels
