If a Gaussian PSF is selected, its FWHM is given by seeing . The delta psftype simply means that no PSF convolution is applied to the model. TinyTim automatically performs the proper centering. WFPC2 undersampling has been a major constraint on the way models are generated. First, a central core oversampled by a factor of 5 is generated.
Bilinear interpolation is performed on the logarithmic intensity values to minimize errors.
This is essentially the same procedure as the one used by Krist.