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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:

• MakeProfiles: You can use MakeProfiles to create a parametric (based on a radial function) kernel, see MakeProfiles. By default MakeProfiles will make the Gaussian and Moffat profiles in a separate file so you can feed it into any of the programs.
• ConvertType: You can write your own desired kernel into a text file table and convert it to a FITS file with ConvertType, see ConvertType. Just be careful that the kernel has to have an odd number of pixels along its two axes, see Convolution process. All the programs that do convolution will normalize the kernel internally, so if you choose this option, you do not have to worry about normalizing the kernel. Only within Convolve, there is an option to disable normalization, see Invoking Convolve.

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.

-k FITS

--kernel=FITS

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.

-U STR

--khdu=STR

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.