Fix incorrect shift in PSF when calculating atmospheric transmission

Laszlo Dobos reported incorrect shift of atmospheric transmission.

Dear @kiyoyabe , I've found this bug while debugging an issue with SNR in my updated version of the ETC. This causes a shift in the atmospheric transmission curve with respect to the sky continuum which isn't a big issue but I saw numerical differences everywhere until it got fixed. The hard-coded value of 7.5 is only correct when SP_PSF_LEN=16 but it's set to 64 by default. By the way I don't think using such a wide PSF is worth it, at least not at normal resolution.