Random dither offsets applied to fibers should be constant as a function of wavelength

[schlafly]

Currently specsim applies a random fiber offset when simulating spectra. This offset is configurable in

specsim/specsim/data/config/desi.yaml

Line 189 in b702907

sigma1d: 5.22 um

At the moment, this random fiber offset is applied independently at each wavelength. get_focal_plane_optics loops over wavelength, calling get_centroid_offset, eventually calling

specsim/specsim/instrument.py

Line 730 in b702907

def offset_function(angle_x, angle_y, wlen,

and applying a different random offset to each wavelength.

I didn't immediately see a clean API change to fix this. Perhaps get_focal_plane_optics should call get_centroid_offsets out of the loop, and then offset_function should be taught how to broadcast over the wavelength axis. But at the moment I thought I'd just file an issue.

Not clear to me that this has practical impact---it seems to me like the fraction of flux will go something like (offset/FWHM)^2, so roughly 0.25% multiplicative noise at each wavelength?