GNU Astronomy Utilities

6.1.1 ImageCrop modes

In order to be as comprehensive as possible, ImageCrop has two major modes of operation listed below.

Image

The image mode uses the pixel coordinates. Depending on your command line options, this mode consists of three sub-modes. In image mode, only one image may be input.

• Catalog (multiple crops). Coordinates are read from a text file. The --xcol and --ycol columns in the catalog are interpreted as the center of a square crop box whose width is specified with the --iwidth option in pixels. Since the given pixel has to be on the center, the width has to be an odd number, so if you give an even number for the width, it will be added by one. If a catalog file name is provided (with --imagemode activated of course) this mode will be used.
• Center (one crop). The box center is given on the command-line with the --xc and --yc parameters. The image width is similar to above.
• Section (one crop). You can specify the section of pixels along each axis in the image which you want to be cropped with the --section option. See Crop section syntax for a full explanation on the syntax of specifying the desired region.

The latter two cases will only have one crop box. In both cases, ImageCrop will go into the image mode, irrespective of calling --wcsmode or the default mode. In the first two cases, since you specify a central pixel, the crop box will be a square with an odd number of pixels on the side, so your desired pixel sits right in the center, see Blank pixels on how to disable this for cases when the box exceeds the image size.

WCS

The Right ascension (RA) and Declination (Dec) of the objects in a catalog is used to define the central position of each postage stamp. In this mode, the width (--wwidth) is read in units of arc seconds and multiple images (tiles in a survey) can be input. If the objects are closer to the edge of the image than half the required width, other tiles (if they are present in the input files) are used to fill the empty space. The square output cropped box will have an odd number of pixels on the side.

In this mode, the input images do not necessarily have to be the same size, each individual tile can even be smaller than the final crop. In any case, any part of any of the input images which overlaps with the desired region will be used in the crop. Note that if there is an over lap, the pixels from the last input image read are going to be used. The input images all just have to be aligned with the celestial coordinates, see the caution note below.

Similar to the image mode, there are two sub-modes:

• Catalog (multiple crops). Similar to catalog mode in image mode. The RA and Dec column should be specified in the catalog (--racol and --deccol).
• Center (one crop). You can specify the center of only one crop box (no matter how many input images there are) with the options --ra and --dec. If it exists in the input images, it will be cropped similar to the catalog mode. If automatic output is triggered (you don’t specify a file name for --output) and several of the input images are used to stitch and crop the region around the central point, the name of the first input will be used in automatic output, see Automatic output.

CAUTION: In WCS mode, the image has to be aligned with the celestial coordinates, such that the first FITS axis is parallel (opposite direction) to the Right Ascension (RA) while the second FITS axis is parallel to the declination. If these conditions aren’t met for an image, ImageCrop will warn you and abort. You have to use other tools to transform the image to the correct directions.

In short, if you don’t specify a catalog, you have to specify box coordinates manually on the command-line. When you do specify a catalog, ImageCrop has to be in one of the two major modes (--imgmode or --wcsmode). Note that the single crop box parameters specified in the sub-modes will not be written to or read from the configuration file, they have to be specified on each execution.

Read in other formats.

GNU Astronomy Utilities 0.2.28-34fb manual, October 2016.