Dear list;

I am honestly not certain whether this, or the SciPy list, is the appropriate place to post this; please let me know if I got it wrong.

I am convolving a 1D data set containing a relatively narrow peak, with a relatively narrow Gaussian kernel, in order to emulate the effect of atmospheric seeing on astrophysical observations.

I have a 1D data array 45 pixels long, and a Gaussian kernel, and run np.convolve(data, kernel, mode='same') on the two arrays, the resulting array's peak is shifted relative to the origin. I have attached a plot to illustrate.

The original data is shown in blue. When I convolve it with a symmetric kernel (black), I get an offset resulting peak (magenta). If I flip the kernel -- even though it is perfectly symmetric -- the resulting curve is offset in the opposite direction (yellow). However, if I offset the kernel so it is centered exactly one pixel below the central value, the output array gets centered correct (red), even if I flip the (now no longer symmetric) kernel.

This is using Numpy 1.11.3, python 2.7.13, on Anaconda 4.3.0 64-bit on Ubuntu 16.10

Using astropy.convolution, reproduces the correct red curve, so I can use that for now, but it seems to me this is either a bug or, if it is indeed the intended behavior, a word of caution would be merited in the docstring.


Emil Rivera-Thorsen

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