By grouping channels in appropriately large numbers, the combined signal-to-noise
of groups will jump into the Gaussian regime. There are two ways to do this: the
FTOOL grppha, or the RGS pipeline. grppha can group
channels using an algorithm which bins up consecutive channels until a count rate
threshold is reached. This method conserves the resolution in emission lines above
the threshold while improving statistics in the continuum.
To rebin with grppha in the Command Window, type:
> Please enter PHA filename P0134520301R1S001SRSPEC1001.FIT > Please enter output filename P0134520301R1S001SRSPEC1001.bin.FIT > GRPPHA group min 30 > GRPPHA exit
The disadvantage of using grppha is that, although channel errors are propagated through the binning process correctly, the errors column in the original spectrum product is not strictly accurate. The problem arises because there is no good way to treat the errors within channels containing no counts. To allow statistical fitting, these channels are arbitrarily given an error value of unity, which is subsequently propagated through the binning. Consequently, the errors are overestimated in the resulting spectra.
The other approach, which involves calling the RGS pipeline, bins the data during
spectral extraction. The following rebins the pipeline spectrum by a factor 3.
Note that the entry stage is much later in the rgsproc chain of processes,
so this takes far less time than when run with an earlier entry stage.
To rebin with rgsproc in the GUI:
To rebin with rgsproc in the Command Window, type:
One disadvantage of this approach is that you can only choose integer binning
of the original channel size. To change the sampling of the events, the pipeline
must be run from the second stage (``angles'') or earlier.
To change the event sampling with rgsproc in the GUI:
To change the event sampling with rgsproc in the Command Window, type:
where the parameters are as defined previously, and
The disadvantage of using rgsproc, as opposed to grppha, is that the binning is linear across the dispersion direction. Velocity resolution is lost in the lines, so the accuracy of redshift determinations will be degraded, transition edges will be smoothed, and neighboring lines will become blended.