GNU Astronomy Utilities

Next: , Previous: , Up: Invoking astcrop   [Contents][Index] Crop options

The options can be classified into the following contexts: Input, Output and operating mode options. Options that are common to all Gnuastro program are listed in Common options and will not be repeated here.

When you are specifying the crop vertices your self (through --section, or --polygon) on relatively small regions (depending on the resolution of your images) the outputs from image and WCS mode can be approximately equivalent. However, as the crop sizes get large, the curved nature of the WCS coordinates have to be considered. For example, when using --section, the right ascension of the bottom left and top left corners will not be equal. If you only want regions within a given right ascension, use --polygon in WCS mode.

Input image parameters:


Specify the first keyword card (line number) to start finding the input image world coordinate system information. Distortions were only recently included in WCSLIB (from version 5). Therefore until now, different telescope would apply their own specific set of WCS keywords and put them into the image header along with those that WCSLIB does recognize. So now that WCSLIB recognizes most of the standard distortion parameters, they will get confused with the old ones and give completely wrong results. For example in the CANDELS-GOODS South images56.

The two --hstartwcs and --hendwcs are thus provided so when using older datasets, you can specify what region in the FITS headers you want to use to read the WCS keywords. Note that this is only relevant for reading the WCS information, basic data information like the image size are read separately. These two options will only be considered when the value to --hendwcs is larger than that of --hstartwcs. So if they are equal or --hstartwcs is larger than --hendwcs, then all the input keywords will be parsed to get the WCS information of the image.


Specify the last keyword card to read for specifying the image world coordinate system on the input images. See --hstartwcs

Crop box parameters:

-c FLT[,FLT[,...]]

The central position of the crop in the input image. The positions along each dimension must be separated by a comma (,) and fractions are also acceptable. The number of values given to this option must be the same as the dimensions of the input dataset. The width of the crop should be set with --width. The units of the coordinates are read based on the value to the --mode option, see below.

-w FLT[,FLT[,...]]

Width of the cropped region about its center. --width may take either a single value (to be used for all dimensions) or multiple values (a specific value for each dimension). If in WCS mode, value(s) given to this option will be read in the same units as the dataset’s WCS information along this dimension. The final output will have an odd number of pixels to allow easy identification of the pixel which keeps your requested coordinate (from --center or --catalog).

The --width option also accepts fractions. For example if you want the width of your crop to be 3 by 5 arcseconds along RA and Dec respectively, you can call it with: --width=3/3600,5/3600.

If you want an even sided crop, you can run Crop afterwards with --section=":*-1,:*-1" or --section=2:,2: (depending on which side you don’t need), see Crop section syntax.

-l STR

String of crop polygon vertices. Note that currently only convex polygons should be used. In the future we will make it work for all kinds of polygons. Convex polygons are polygons that do not have an internal angle more than 180 degrees. This option can be used both in the image and WCS modes, see Crop modes. The cropped image will be the size of the rectangular region that completely encompasses the polygon. By default all the pixels that are outside of the polygon will be set as blank values (see Blank pixels). However, if --outpolygon is called all pixels internal to the vertices will be set to blank.

The syntax for the polygon vertices is similar to, and simpler than, that for --section. In short, the dimensions of each coordinate are separated by a comma (,) and each vertex is separated by a colon (:). You can define as many vertices as you like. If you would like to use space characters between the dimensions and vertices to make them more human-readable, then you have to put the value to this option in double quotation marks.

For example, let’s assume you want to work on the deepest part of the WFC3/IR images of Hubble Space Telescope eXtreme Deep Field (HST-XDF). According to the webpage57 the deepest part is contained within the coordinates:

[ (53.187414,-27.779152), (53.159507,-27.759633),
  (53.134517,-27.787144), (53.161906,-27.807208) ]

They have provided mask images with only these pixels in the WFC3/IR images, but what if you also need to work on the same region in the full resolution ACS images? Also what if you want to use the CANDELS data for the shallow region? Running Crop with --polygon will easily pull out this region of the image for you irrespective of the resolution. If you have set the operating mode to WCS mode in your nearest configuration file (see Configuration files), there is no need to call --mode=wcs on the command line. You may also provide many FITS images/tiles and Crop will stitch them to produce this cropped region:

$ astcrop --mode=wcs desired-filter-image(s).fits           \
   --polygon="53.187414,-27.779152 : 53.159507,-27.759633 : \
              53.134517,-27.787144 : 53.161906,-27.807208"

In other cases, you have an image and want to define the polygon yourself (it isn’t already published like the example above). As the number of vertices increases, checking the vertex coordinates on a FITS viewer (for example SAO ds9) and typing them in one by one can be very tedious and prone to typo errors.

You can take the following steps to avoid the frustration and possible typos: Open the image with ds9 and activate its “region” mode with Edit→Region. Then define the region as a polygon with Region→Shape→Polygon. Click on the approximate center of the region you want and a small square will appear. By clicking on the vertices of the square you can shrink or expand it, clicking and dragging anywhere on the edges will enable you to define a new vertex. After the region has been nicely defined, save it as a file with Region→Save Regions. You can then select the name and address of the output file, keep the format as REG and press “OK”. In the next window, keep format as “ds9” and “Coordinate System” as “fk5”. A plain text file (let’s call it ds9.reg) is now created.

You can now convert this plain text file to Crop’s polygon format with this command (when typing on the command-line, ignore the “\” at the end of the first and second lines along with the extra spaces, these are only for nice printing):

$ v=$(awk 'NR==4' ds9.reg | sed -e's/polygon(//'        \
           -e's/\([^,]*,[^,]*\),/\1:/g' -e's/)//' )
$ astcrop --mode=wcs image.fits --polygon=$v

Keep all the regions outside the polygon and mask the inner ones with blank pixels (see Blank pixels). This is practically the inverse of the default mode of treating polygons. Note that this option only works when you have only provided one input image. If multiple images are given (in WCS mode), then the full area covered by all the images has to be shown and the polygon excluded. This can lead to a very large area if large surveys like COSMOS are used. So Crop will abort and notify you. In such cases, it is best to crop out the larger region you want, then mask the smaller region with this option.

-s STR

Section of the input image which you want to be cropped. See Crop section syntax for a complete explanation on the syntax required for this input.


The column in a catalog to read as a coordinate. The value can be either the column number (starting from 1), or a match/search in the table meta-data, see Selecting table columns. This option must be called multiple times, depending on the number of dimensions in the input dataset. If it is called more than necessary, the extra columns (later calls to this option on the command-line or configuration files) will be ignored, see Configuration file precedence.


Column selection of crop file name. The value can be either the column number (starting from 1), or a match/search in the table meta-data, see Selecting table columns. This option can be used both in Image and WCS modes, and not a mandatory. When a column is given to this option, the final crop base file name will be taken from the contents of this column. The directory will be determined by the --output option (current directory if not given) and the value to --suffix will be appended. When this column isn’t given, the row number will be used instead.

Output options:

-c INT

Box width (odd number of pixels) of region in the center of the image to check for blank values. If the value to this option is zero, no checking is done. This option is only relevant when the cropped region(s) are defined by their center (not by the vertices, see Crop modes). If any of the pixels in this central region of a crop (defined by its center) are blank, then it will not be created.

Because survey regions don’t often have a clean square or rectangle shape, some of the pixels on the sides of the survey FITS image don’t commonly have any data and are blank (see Blank pixels). So when the catalog was not generated from the input image, it often happens that the image does not have data over some of the points.

When the given center of a crop falls in such regions and this option has a non-zero, odd value, no crop will be created. Therefore with this option, you can specify a width of a small box (3 pixels is often good enough) around the central pixel of the cropped image. You can check which crops were created and which weren’t from the command-line (if --quiet was not called, see Operating mode options), or in Crop’s log file (see Crop output).

-p STR

The suffix (or post-fix) of the output files for when you want all the cropped images to have a special ending. One case where this might be helpful is when besides the science images, you want the weight images (or exposure maps, which are also distributed with survey images) of the cropped regions too. So in one run, you can set the input images to the science images and --suffix=_s.fits. In the next run you can set the weight images as input and --suffix=_w.fits.


Pixels outside of the input image that are in the crop box will not be used. By default they are filled with blank values (depending on type), see Blank pixels. This option only applies only in Image mode, see Crop modes.


In float or double images, it is common to give the value of zero to blank pixels. If the input image type is one of these two types, such pixels will also be considered as blank. You can disable this behavior with this option, see Blank pixels.

Operating mode options:


Operate in Image mode or WCS mode when the input coordinates can be both image or WCS. The value must either be img or wcs, see Crop modes for a full description.




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