---
name: "astsubtractsky"
section: "1"
platform: "linux"
package: "gnuastro"
version: "0.2.33-1"
release: "Debian 9 (Stretch)"
platforms: ["linux"]
url: "https://new.carta.tech/astsubtractsky"
---

# astsubtractsky(1)

subtract the sky value in an image

## NAME

SubtractSky - subtract the sky value in an image

## SYNOPSIS

**astsubtractsky** \[*OPTION*...\] *ASTRdata*

## DESCRIPTION

SubtractSky is part of GNU Astronomy Utilities 0.2.33-8a3b. SubtractSky Finds the sky value over a grid on the input and subtracts it from the image to give a clear and uniform output.

For more information, please run any of the following commands. They will respectively show you the \`Invoking SubtractSky' subsection, the complete \`SubtractSky' section, or the full GNU Astronomy Utilities manual. In particular the first contains a very complete explanation of each option.

-

  info astsubtractsky

-

  info SubtractSky

-

  info gnuastro

If you couldn't find your answer in the manual, you can get direct help from experienced Gnuastro users and developers. For more information, please run:

-

  info help-gnuastro

SubtractSky options:

-

  Input:

- **-h**, **--hdu**=*STR*

  Extension name or number of input data.

- **-H**, **--mhdu**=*STR*

  Mask image header name.

- **-k**, **--kernel**=*STR*

  Kernel image file name for convolution.

- **-M**, **--mask**=*STR*

  Mask image file name.

- **-U**, **--khdu**=*STR*

  Kernel image header name for convolution.

-

  Output:

- **--checkconvolution**

  Store convolved image in \`\_conv.fits' file.

- **--checksky**

  Store final sky and its STD in \`\_sky.fits' file.

- **--checkskystd**

  Include sky standard deviation in all checks too.

- **-D**, **--dontdelete**

  Don't delete output if it exists.

- **-K**, **--keepinputdir**

  Do not remove input's dir info for output.

- **-o**, **--output**=*STR*

  Output file or directory name.

-

  Mesh grid:

- **-a**, **--nch1**=*INT*

  Number of channels along first FITS axis.

- **-b**, **--nch2**=*INT*

  Number of channels along second FITS axis.

- **--checkmesh**

  Store mesh IDs in \`\_mesh.fits' file.

- **-d**, **--mirrordist**=*FLT*

  Distance beyond mirror point. Multiple of std.

- **--fullconvolution**

  Ignore channels in imageconvolution.

- **--fullinterpolation**

  Ignore channels in interpolation.

- **--fullsmooth**

  Ignore channels in smoothing.

- **--interponlyblank**

  Only interpolate over the blank pixels.

- **-L**, **--lastmeshfrac**=*INT*

  Fraction of last mesh area to add new.

- **--meshbasedcheck**

  Each mesh in one pixel in mesh check images.

- **-n**, **--numnearest**=*INT*

  Number of nearest neighbors to interpolate.

- **-Q**, **--minmodeq**=*FLT*

  Minimum acceptable quantile for the mode.

- **-s**, **--meshsize**=*INT*

  Size of each mesh (tile) in the grid.

- **-T**, **--smoothwidth**=*INT*

  Width of smoothing kernel (odd number).

-

  Statistics:

**-t**, **--sigcliptolerance**=*FLT* Difference in STD tolerance to halt iteration.

- **-u**, **--sigclipmultip**=*FLT*

  Multiple of standard deviation in sigma-clipping.

-

  Operating modes:

- \-?, **--help**

  give this help list

- **--cite**

  BibTeX citation for SubtractSky.

- **--nolog**

  No log file for programs which make one.

- **-N**, **--numthreads**=*INT*

  Number of CPU threads to use.

- **--onlydirconf**

  Only read current directory configuration file.

- **--onlyversion**=*STR*

  Only run if the program version is this.

- **-P**, **--printparams**

  Print parameter values to be used and abort.

- **-q**, **--quiet**

  Only report errors, remain quiet about steps.

- **--setusrconf**

  Set default values for this user and abort.

- **-S**, **--setdirconf**

  Set default values for this directory and abort.

- **--usage**

  give a short usage message

- **-V**, **--version**

  print program version

Mandatory or optional arguments to long options are also mandatory or optional for any corresponding short options.

GNU Astronomy Utilities home page: http://www.gnu.org/software/gnuastro/

## AUTHOR

Written by Mohammad Akhlaghi

## REPORTING BUGS

Report bugs to bug-gnuastro@gnu.org.

## COPYRIGHT

Copyright © 2015-2016, Free Software Foundation, Inc. License GPLv3+: GNU General public license version 3 or later.\
 This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.

## SEE ALSO

The full documentation for **SubtractSky** is maintained as a Texinfo manual.  If the **info** and **SubtractSky** programs are properly installed at your site, the command

-

  **info SubtractSky**

should give you access to the complete manual.