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For a project I am undertaking, I will need to calculate the derivative of a given surface brightness profile which is convolved with a pixel response function (as well as PSF etc.)

For various reasons, but mainly for consistency, I wish to do this using the guts of the GALSIM code. However, since in this case the `flux' defined as the sum of the non-parametric model no longer has a physical meaning in terms of the image itself (it will always be considered as noise-free in this case), there are certain situations where I would like to be able to define the interpolated image without a flux normalisation.

The code does not seem to care if the 'flux' is negative, but I am coming across certain situations where the 'flux' is within machine precision of zero, and thus the assertion ``dabs(flux-flux_tot) <= dabs(flux_tot)'' fails.

My question is therefore: Can one specify a non-parametric model to interpolate over without specifying a flux normalisation value?

Christopher Duncan
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There is currently no way to do this using the galsim.InterpolatedImage() class; you could open an issue to make this feature request at the GalSim repository on GitHub.

There is a way to do this using the guts of GalSim; an example is illustrated in the lensing power spectrum functionality if you are willing to dig into the source code (lensing_ps.py -- just do a search for SBInterpolatedImage to find the relevant code bits). The basic idea is that instead of using galsim.InterpolatedImage() you use the associated C++ class, galsim._galsim.SBInterpolatedImage(), which is accessible in python. The SBInterpolatedImage can be initialized with an image and choices of interpolants in real- and Fourier-space, as shown in the examples in lensing_ps.py, and then queried using the xValue() method to get the value interpolated to some position.

This trick was necessary in lensing_ps.py because we were interpolating shear fields which tend to have a mean of zero, so we ran into the same problem you did. Use of the SBInterpolatedImage class is not generally recommended for GalSim users (we recommend using only the python classes) but it is definitely a way around your problem for now.

rmandelb
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