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GratingStim - How to correctly adjust s.d. of Gaussian contrast envelope

Dear all,

I am entirely new to PsychoPy, so please forgive this naive question.

I would like to display a Gabor stimulus with a 1.5 degree Gaussian contrast envelope.
Now, it seems to me that I am somehow misunderstanding the maskParams parameter because setting this to {‘sd’ : 1.5} results in a square grating (when, without this additional parameter, the Gabor is circular, as expected). The code I am using to generate the Gabor is as follows:

tex = ‘sin’
mask = ‘gauss’
units = ‘deg’
sf = 2.4
phase = 0.5
maskSD = {‘sd’: 1.5} → results in square Gabor, while maskSD=None results in circular Gabor
size = [1.5, 1.5]

gaborStim = visual.GratingStim(win, tex=tex, mask=mask, units=units, sf=sf, phase=phase, maskParams=maskParams, ori=orient, size=size)

I would very much appreciate any help on what I am doing wrong/misunderstanding.

Thanks a lot!