# Chroma

As part of this analysis, we have added the capability to simulate wavelength-dependent surface brightness profiles to the open source galaxy simulation package GalSim, which is being used in the GREAT3 shape measurement data challenge. The new chromatic API makes it convenient to simulate both the effects of chromatic PSFs and chromatic galaxies (also referred to as galaxies with color gradients).

As an example, a simulation of a bulge+disk galaxy convolved with an atmospheric PSF and observed in the r-band might look like:

For Euclid, the PSF is chromatic primarily due to the diffraction limit. If this were the only contribution to the PSF, then the chromatic effect would be ${\scriptsize \mathrm{FWHM} \propto \lambda}$. When combined with telescope jitter and the modulation transfer function from the CCD, however, the Euclid PSF will actually scale approximately like ${\scriptsize \mathrm{FWHM} \propto \lambda^{0.6}}$. This effect is also convenient to implement in GalSim: