GNU Astronomy Utilities

6.3.4 Convolution kernel

All the programs that need convolution will need to be given a convolution kernel file and extension. In most cases (other than Convolve, see Convolve) the kernel file name is optional. However, the extension is necessary and must be specified either on the command-line or at least one of the configuration files (see Configuration files). Within Gnuastro, there are two ways to create a kernel image:

The two options to specify a kernel file name and its extension are shown below. These are common between all the programs that will do convolution.


The convolution kernel file name. The BITPIX (data type) value of this file can be any standard type and it does not necessarily have to be normalized. Several operations will be done on the kernel image prior to the program’s processing:

  • It will be converted to floating point type.
  • All blank pixels (see Blank pixels) will be set to zero.
  • It will be normalized so the sum of its pixels equal unity.
  • It will be flipped so the convolved image has the same orientation. This is only relevant if the kernel is not circular. See Convolution process.

The convolution kernel HDU. Although the kernel file name is optional, before running any of the programs, they need to have a value for --khdu even if the default kernel is to be used. So be sure to keep its value in at least one of the configuration files (see Configuration files). By default, the system configuration file has a value.