## GNU Astronomy Utilities

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#### 7.3.5.2 MakeCatalog general settings

Some of the columns require particular settings (for example the zero point magnitude for measuring magnitudes), the options in this section can be used for such configurations.

-z FLT
--zeropoint=FLT

The zero point magnitude for the input image, see Flux Brightness and magnitude.

-E
--skysubtracted

If the image has already been sky subtracted by another program, then you need to notify MakeCatalog through this option. Note that this is only relevant when the Signal to noise ratio is to be calculated.

-T FLT
--threshold=FLT

For all the columns, only consider pixels that are above a given relative threshold. Symbolizing the value of this option as $$T$$, the Sky for a pixel at $$(i,j)$$ with $$\mu_{ij}$$ and its Standard deviation with $$\sigma_{ij}$$, that pixel will only be used if its value ($$B_{ij}$$) satisfies this condition: $$B_{ij}>\mu_{ij}+{T}\sigma_{ij}$$. The only calculations that will not be affected are is the average river values (--riverave), since they are used as a reference. A commented row will be added in the header of the output catalog that will print the given value, since this is a very important issue, it starts with **IMPORTANT**.

NoiseChisel will detect very diffuse signal which is useful in most cases where the aggregate properties of the detections are desired, since there is signal there (with the desired certainty). However, in some cases, only the properties of the peaks of the objects/clumps are desired, for example in attempting to separate stars from galaxies, the peaks are the major target and the diffuse regions only act to complicate the separation. With this option, MakeCatalog will simply ignore any pixel below the relative threshold.

This option is not mandatory, so if it isn’t given (after reading the command-line and all configuration files, see Configuration files), MakeCatalog will still operate. However, if it has a value in any lower-level configuration file and you want to ignore that value for this particular run or in a higher-level configuration file, then set it to NaN, for example --threshold=nan. Gnuastro uses the C library’s strtod function to read floats, which is not case-sensitive in reading NaN values. But to be consistent, it is good practice to only use nan.

--nsigmag=FLT

The median standard deviation (from the standard deviation image) will be multiplied by the value to this option and its magnitude will be reported in the comments of the output catalog. This value is a per-pixel value, not per object/clump and is not found over an area or aperture, like the common $$5\sigma$$ values that are commonly reported as a measure of depth or the upper-limit measurements (see Quantifying measurement limits).

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