GNU Astronomy Utilities
7.2.1 NoiseChisel changes after publication
Before using NoiseChisel it is strongly recommended to read
Akhlaghi and Ichikawa  to
gain a good understanding of what it does and how each parameter influences
the output. Thanks to that paper, there is no need to go into the details
of the major processing steps. Hence we can just dive into the details of
running NoiseChisel in Invoking NoiseChisel.
However, the paper cannot undergo any further updates, but NoiseChisel will
evolve: better algorithms or steps will be found, thus options will be
added or removed. So this book is the final and definitive guide. For a
more detailed list of changes in each release, please follow the
NEWS file. The NEWS file is in the released Gnuastro tarball
(see Release tarball). You can also view the most recent NEWS
To make the transition form the paper to this book easier (and encourage
reading the paper), below you can see the major changes since the paper was
published. First, the options that have been removed are discussed,
followed by those that have been added.
- --dilate: In the paper, true detections were dilated for a final
dig into the noise. However, simple 8-connected dilation can produce boxy
results which are not realistic and could miss diffuse flux. The final dig
into the noise is now done by “grow”ing the true detections, similar to
how true clumps were grown, see the description of --detgrowquant
below and in Detection options for more on the new alternative.
- --widekernel: NoiseChisel uses the difference between the mode and
median to identify if a tile should be used for estimating the quantile
thresholds (see Quantifying signal in a tile). Until now, NoiseChisel
would convolve an image once and estimate the proper tiles for quantile
estimations on the convolved image. The same convolved image would later be
used for quantile estimation. A larger kernel does increase the skewness
(and thus difference between the mode and median), however, it disfigures
the shapes/morphology of the objects.
This new --widekernel option (and a corresponding --wkhdu
option to specify its HDU) option are added to solve such cases. When its
given, the input will be convolved with both the sharp (given through the
--kernel option) and wide kernels. The mode and median are
calculated on the dataset that is convolved with the wider kernel, then the
quantiles are estimated on the image convolved with the sharper kernel.
- --noerodequant: to specify a quantile threshold where erosion
will not apply. This is useful to detect sharper point-like sources that
will be missed due to too much erosion. To completely ignore this features
give this option a value of 1 (only the largest valued pixel in the input
will not be eroded).
- --qthreshtilequant: to manually remove the measured qthresh from
some tiles. This feature helps in detecting large and extended diffuse
(almost flat) signal when necessary, see Detection options.
- --detgrowquant: is used to grow the final true detections until a
given quantile in the same way that clumps are grown during segmentation
(compare columns 2 and 3 in Figure 10 of the paper). It replaces the old
--dilate option in the paper and older versions of
Gnuastro. Dilation is a blind growth method which causes objects to be boxy
or diamond shaped when too many layers are added. However, with the growth
method that is defined now, we can follow the signal into the noise with
any shape. The appropriate quantile depends on your dataset’s correlated
noise properties and how cleanly it was Sky subtracted. The new
--detgrowmaxholesize can also be used to specify the maximum hole
size to fill as part of this growth, see the description in Detection options for more details.
This new growth process can be much more successful in detecting diffuse
flux around true detections compared to dilation and give more realistic
results, but it can also increase the NoiseChisel run time (depending on
the given value and input size).
- --cleangrowndet: A process to further clean/remove the possibility
of false detections, see the descriptions under this option in
Read in other formats.
GNU Astronomy Utilities 0.5 manual, December 2017.