## GNU Astronomy Utilities

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#### 7.2.2.2 Detection options

Detection is the process of separating the pixels in the image into two groups: 1) Signal and 2) Noise. Through the parameters below, you can customize the detection process in NoiseChisel. Recall that you can always see the full list of Gnuastro’s options with the --help (see Getting help), or --printparams (or -P) to see their values (see Operating mode options).

-r FLT
--mirrordist=FLT

Maximum distance (as a multiple of error) to estimate the difference between the input and mirror distributions in finding the mode, see Appendix C of Akhlaghi and Ichikawa 2015, also see Quantifying signal in a tile.

-Q FLT
--modmedqdiff=FLT

The maximum acceptable distance between the mode and median, see Quantifying signal in a tile. The quantile threshold will be found on tiles that satisfy this mode and median difference.

-t FLT
--qthresh=FLT

The quantile threshold to apply to the convolved image. The detection process begins with applying a quantile threshold to each of the tiles in the small tessellation. The quantile is only calculated for tiles that don’t have any significant signal within them, see Quantifying signal in a tile. Interpolation is then used to give a value to the unsuccessful tiles and it is finally smoothed.

The quantile value is a floating point value between 0 and 1. Assume that we have sorted the $$N$$ data elements of a distribution (the pixels in each mesh on the convolved image). The quantile ($$q$$) of this distribution is the value of the element with an index of (the nearest integer to) $$q\times{N}$$ in the sorted data set. After thresholding is complete, we will have a binary (two valued) image. The pixels above the threshold are known as foreground pixels (have a value of 1) while those which lie below the threshold are known as background (have a value of 0).

--qthreshtilequant=FLT

Only keep tiles which have a q-thresh value above the given quantile of the all successful tiles. Hence, when given a value of 1, this option will be ignored. When there is more than one channel (and --workoverch is not called), quantile calculation and application will be done on each channel independently.

This option is useful when a large and diffuse (almost flat within each tile) signal exists with very small regions of Sky. The flatness of the profile will cause it to successfully pass the tests of Quantifying signal in a tile. As a result, without this option the flat and diffuse signal will be interpreted as sky. In such cases, you can see the status of the tiles with the --checkqthresh option (first image extension is enough) and select a quantile through this option to ignore the measured values in the higher-valued tiles.

This option can also be useful when there are large bright objects in the image with large flat wings which can also pass the tests and give outlier values. When there is a sky gradient over the image (mainly due to post-processing issues like bad flat fielding), this option must be set to 1 so it is completely ignored and the sky gradient is accurately measured and subtracted.

To get an estimate of the measured qthresh distribution, you can run the following commands. The first will create the qthresh check image with one value/pixel per tile (see Processing options). Open the image in a FITS viewer and inspect it. The second command below will print the basic information about the distribution of values and the third will print the value at the 0.4 quantile. Recall that Gnuastro’s Statistics program ignores blank values (in this case: tiles with significant signal), see Statistics.

$astnoisechisel image.fits --checkqthresh --oneelempertile$ aststatistics image_qthresh.fits
\$ aststatistics image_qthresh.fits --quantile=0.4

--smoothwidth=INT

Width of flat kernel used to smooth the interpolated quantile thresholds, see --qthresh for more.

--checkqthresh

Check the quantile threshold values on the mesh grid. A file suffixed with _qthresh.fits will be created showing each step. With this option, NoiseChisel will abort as soon as quantile estimation has been completed, allowing you to inspect the steps leading to the final quantile threshold, this can be disabled with --continueaftercheck. By default the output will have the same pixel size as the input, but with the --oneelempertile option, only one pixel will be used for each tile (see Processing options).

-e INT
--erode=INT

The number of erosions to apply to the binary thresholded image. Erosion is simply the process of flipping (from 1 to 0) any of the foreground pixels that neighbor a background pixel. In a 2D image, there are two kinds of neighbors, 4-connected and 8-connected neighbors. You can specify which type of neighbors should be used for erosion with the --erodengb option, see below.

Erosion has the effect of shrinking the foreground pixels. To put it another way, it expands the holes. This is a founding principle in NoiseChisel: it exploits the fact that with very low thresholds, the holes in the very low surface brightness regions of an image will be smaller than regions that have no signal. Therefore by expanding those holes, we are able to separate the regions harboring signal.

--erodengb=INT

The type of neighborhood (structuring element) used in erosion, see --erode for an explanation on erosion. Only two integer values are acceptable: 4 or 8. In 4-connectivity, the neighbors of a pixel are defined as the four pixels on the top, bottom, right and left of a pixel that share an edge with it. The 8-connected neighbors on the other hand include the 4-connected neighbors along with the other 4 pixels that share a corner with this pixel. See Figure 6 (a) and (b) in Akhlaghi and Ichikawa (2015) for a demonstration.

--noerodequant

Pure erosion is going to carve off sharp and small objects completely out of the detected regions. This option can be used to avoid missing such sharp and small objects (which have significant pixels, but not over a large area). All pixels with a value larger than the significance level specified by this option will not be eroded during the erosion step above. However, they will undergo the erosion and dilation of the opening step below.

Like the --qthresh option, the significance level is determined using the quantile (a value between 0 and 1). Just as a reminder, in the normal distribution, $$1\sigma$$, $$1.5\sigma$$, and $$2\sigma$$ are approximately on the 0.84, 0.93, and 0.98 quantiles.

-p INT
--opening=INT

Depth of opening to be applied to the eroded binary image. Opening is a composite operation. When opening a binary image with a depth of $$n$$, $$n$$ erosions (explained in --erode) are followed by $$n$$ dilations. Simply put, dilation is the inverse of erosion. When dilating an image any background pixel is flipped (from 0 to 1) to become a foreground pixel. Dilation has the effect of fattening the foreground. Note that in NoiseChisel, the erosion which is part of opening is independent of the initial erosion that is done on the thresholded image (explained in --erode). The structuring element for the opening can be specified with the --openingngb option. Opening has the effect of removing the thin foreground connections (mostly noise) between separate foreground ‘islands’ (detections) thereby completely isolating them. Once opening is complete, we have initial detections.

--openingngb=INT

-s FLT,FLT
--sigmaclip=FLT,FLT

The $$\sigma$$-clipping parameters, see Sigma clipping. This option takes two values which are separated by a comma (,). Each value can either be written as a single number or as a fraction of two numbers (for example 3,1/10). The first value to this option is the multiple of $$\sigma$$ that will be clipped ($$\alpha$$ in that section). The second value is the exit criteria. If it is less than 1, then it is interpreted as tolerance and if it is larger than one it is assumed to be the fixed number of iterations. Hence, in the latter case the value must be an integer.

-B FLT
--minskyfrac=FLT

Minimum fraction (value between 0 and 1) of Sky (undetected) areas in a tile. Only tiles with a fraction of undetected pixels (Sky) larger than this value will be used to estimate the Sky value. NoiseChisel uses this option value twice to estimate the Sky value: after initial detections and in the end when false detections have been removed.

Because of the PSF and their intrinsic amorphous properties, astronomical objects (except cosmic rays) never have a clear cutoff and commonly sink into the noise very slowly. Even below the very low thresholds used by NoiseChisel. So when a large fraction of the area of one mesh is covered by detections, it is very plausible that their faint wings are present in the undetected regions (hence causing a bias in any measurement). To get an accurate measurement of the above parameters over the tessellation, tiles that harbor too many detected regions should be excluded. The used tiles are visible in the respective --check option of the given step.

--checkdetsky

Check the initial approximation of the sky value and its standard deviation in a FITS file ending with _detsky.fits. With this option, NoiseChisel will abort as soon as the sky value used for defining pseudo-detections is complete. This allows you to inspect the steps leading to the final quantile threshold, this behavior can be disabled with --continueaftercheck. By default the output will have the same pixel size as the input, but with the --oneelempertile option, only one pixel will be used for each tile (see Processing options).

-R FLT
--dthresh=FLT

The detection threshold: a multiple of the initial sky standard deviation added with the initial sky approximation (which you can inspect with --checkdetsky). This flux threshold is applied to the initially undetected regions on the unconvolved image. The background pixels that are completely engulfed in a 4-connected foreground region are converted to background (holes are filled) and one opening (depth of 1) is applied over both the initially detected and undetected regions. The Signal to noise ratio of the resulting ‘psudo-detections’ are used to identify true vs. false detections. See Section 3.1.5 and Figure 7 in Akhlaghi and Ichikawa (2015) for a very complete explanation.

-m INT
--snminarea=INT

The minimum area to calculate the Signal to noise ratio on the psudo-detections of both the initially detected and undetected regions. When the area in a psudo-detection is too small, the Signal to noise ratio measurements will not be accurate and their distribution will be heavily skewed to the positive. So it is best to ignore any psudo-detection that is smaller than this area. Use --detsnhistnbins to check if this value is reasonable or not.

--checksn

Save the S/N values of the pseudo-detections and dilated detections into three files ending with _detsn_sky.XXX, _detsn_det.XXX, and _detsn_dilated.XXX. The .XXX is determined from the --tableformat option (see Input/Output options, for example .txt or .fits). You can use these to inspect the S/N values and their distribution (in combination with the --checkdetection option to see where the pseudo-detections are). You can use Gnuastro’s Statistics to make a histogram of the distribution or any other analysis you would like for better understanding of the distribution (for example through a histogram).

With this option, NoiseChisel will abort as soon as the tables are created. This allows you to inspect the steps leading to the final quantile threshold, this behavior (to abort NoiseChisel) can be disabled with --continueaftercheck.

--minnumfalse=INT

The minimum number of ‘pseudo-detections’ over the undetected regions to identify a Signal-to-Noise ratio threshold. The Signal to noise ratio (S/N) of false pseudo-detections in each tile is found using the quantile of the S/N distribution of the psudo-detections over the undetected pixels in each mesh. If the number of S/N measurements is not large enough, the quantile will not be accurate (can have large scatter). For example if you set --snquant=0.99 (or the top 1 percent), then it is best to have at least 100 S/N measurements.

-c FLT
--snquant=FLT

The quantile of the Signal to noise ratio distribution of the psudo-detections in each mesh to use for filling the large mesh grid. Note that this is only calculated for the large mesh grids that satisfy the minimum fraction of undetected pixels (value of --minbfrac) and minimum number of psudo-detections (value of --minnumfalse).

-d FLT
--detgrowquant=FLT

Quantile limit to “grow” the final detections. As discussed in the previous options, after applying the initial quantile threshold, layers of pixels are carved off the objects to identify true signal. With this step you can return those low surface brightness layers that were carved off back to the detections. To disable growth, set the value of this option to 1.

The process is as follows: after the true detections are found, all the non-detected pixels above this quantile will be put in a list and used to “grow” the true detections (seeds of the growth). Like all quantile thresholds, this threshold is defined and applied to the convolved dataset. Afterwards, the dataset is dilated once (with minimum connectivity) to connect very thin regions on the boundary: imagine building a dam at the point rivers spill into an open sea/ocean. Finally, all holes are filled. In the geography metaphor, holes can be seen as the closed (by the dams) rivers and lakes, so this process is like turning the water in all such rivers and lakes into soil. See --detgrowmaxholesize for configuring the hole filling.

--detgrowmaxholesize=INT

The maximum hole size to fill during the final expansion of the true detections as described in --detgrowquant. This is necessary when the input contains many smaller objects and can be used to avoid marking blank sky regions as detections.

For example multiple galaxies can be positioned such that they surround an empty region of sky. If all the holes are filled, the Sky region in between them will be taken as a detection which is not desired. To avoid such cases, the integer given to this option must be smaller than the hole between such objects. However, we should caution that unless the “hole” is very large, the combined faint wings of the galaxies might actually be present in between them, so be very careful in not filling such holes.

On the other hand, if you have a very large (and extended) galaxy, the diffuse wings of the galaxy may create very large holes over the detections. In such cases, a large enough value to this option will cause all such holes to be detected as part of the large galaxy and thus help in detecting it to extremely low surface brightness limits. Therefore, especially when large and extended objects are present in the image, it is recommended to give this option (very) large values. For one real-world example, see Detecting large extended targets.

--cleangrowndet

After dilation, if the signal-to-noise ratio of a detection is less than the derived pseudo-detection S/N limit, that detection will be discarded. In an ideal/clean noise, a true detection’s S/N should be larger than its constituent pseudo-detections because its area is larger and it also covers more signal. However, on a false detections (especially at lower --snquant values), the increase in size can cause a decrease in S/N below that threshold.

This will improve purity and not change completeness (a true detection will not be discarded). Because a true detection has flux in its vicinity and dilation will catch more of that flux and increase the S/N. So on a true detection, the final S/N cannot be less than pseudo-detections.

However, in many real images bad processing creates artifacts that cannot be accurately removed by the Sky subtraction. In such cases, this option will decrease the completeness (will artificially discard true detections). So this feature is not default and should to be explicitly called when you know the noise is clean.

--checkdetection

Every step of the detection process will be added as an extension to a file with the suffix _det.fits. Going through each would just be a repeat of the explanations above and also of those in Akhlaghi and Ichikawa (2015). The extension label should be sufficient to recognize which step you are observing. Viewing all the steps can be the best guide in choosing the best set of parameters. With this option, NoiseChisel will abort as soon as a snapshot of all the detection process is saved. This behavior can be disabled with --continueaftercheck.

--checksky

Check the derivation of the final sky and its standard deviation values on the mesh grid. With this option, NoiseChisel will abort as soon as the sky value is estimated over the image (on each tile). This behavior can be disabled with --continueaftercheck. By default the output will have the same pixel size as the input, but with the --oneelempertile option, only one pixel will be used for each tile (see Processing options).

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