Compressive spectral imagers have gained popularity recently due to their ability to sense a three-dimensional (3D) data cube with just a few two dimensional (2D) coded aperture projection snapshots. The coded apertures are realized by digital micromirror devices (DMD) which often do not match the pitch resolution of the focal plane array (FPA). This paper introduces the forward model and associated reconstruction algorithm for such mismatched spectral imagers, without the loss of spectral and spatial resolution. Simulations show the improvements in the reconstructions achieved with the proposed approach yielding up to 12 dB gain in PSNR with respect to traditional.