Previous: Mask image, Up: SubtractSky [Contents][Index]
SubtractSky will find the sky value on a grid in an image and subtract it. The executable name is astsubtractsky with the following general template:
$ astsubtractsky [OPTION ...] Image
One line examples:
$ astsubtractsky image.fits $ astsubtractsky --hdu=0 --mhdu=1 image.fits $ astsubtractsky --nch1=2 --nch2=2 image.fits $ astsubtractsky --mask=maskforimage.fits image.fits
The only required input to SubtractSky is the input data file that currently has to be only a 2D image. But in the future it might be useful to use it for 1D or 3D data too. Any pixels in the image with a blank value will be ignored in the analysis, see Blank pixels. Alternatively a mask can be specified which indicates pixels to not be used (see --mask and --mhdu). The common options to all Gnuastro programs can be seen in Common options and input data formats are explained in Arguments.
SubtractSky uses a mesh grid to tile the image. This enables it to deal with possible gradients, see Tiling an image. The mesh grid options are common to all the programs using it, and are listed in Mesh grid options. In order to ignore some pixels during the analysis, you can specify a mask image, see Mask image for an explanation and the relevant options. For a more accurate result, a kernel file name can be specified so the image is first convolved, see Quantifying signal in a mesh, see Convolution kernel for the kernel related options. The --kernel option is not mandatory and if not specified anywhere prior to running, the original image will be used. Please run SubtractSky with the --help option to list all the recognized options, irrespective of which part of the book they are fully explained in.
(=FLT) The multiple of the standard deviation to clip from the distribution in \(\sigma\)-clipping. This is necessary to remove the effect of cosmic rays, see Sky value and Sigma clipping.
(=FLT) The tolerance of sigma clipping. If the fractional change in the standard deviation before and after \(\sigma\)-clipping is less than the value given to this option, \(\sigma\)-clipping will stop, see Sigma clipping.
A two extension FITS image ending with _smooth.fits will be created showing how the interpolated sky value is smoothed.
In the interpolation and sky checks above, include the sky standard deviation too. By default, only the sky value is shown in all the checks. However with this option, an extension will be added showing how the standard deviation on each mesh is finally found too.
Previous: Mask image, Up: SubtractSky [Contents][Index]
JavaScript license information
GNU Astronomy Utilities 0.2.28-34fb manual, October 2016.