The cropped box can potentially include pixels that are beyond the image range. For example, when a target in the input catalog was very near the edge of the input image. The parts of the cropped image that were not in the input image will be filled with the following two values depending on the data type of the image. In both cases, SAO DS9 will not color code those pixels.
BLANK keyword in the cropped image header.
The value assigned to it is the lowest value possible for that type, so you will probably never need it any way.
Only for the unsigned character type (
BITPIX=8 in the FITS header), the maximum value is used because it is unsigned, the smallest value is zero which is often meaningful.
You can ask for such blank regions to not be included in the output crop image using the --noblank option. In such cases, there is no guarantee that the image size of your outputs are what you asked for.
In some survey images, unfortunately they do not use the
BLANK FITS keyword.
Instead they just give all pixels outside of the survey area a value of zero.
So by default, when dealing with float or double image types, any values that are 0.0 are also regarded as blank regions.
This can be turned off with the --zeroisnotblank option.