The first and perhaps simplest alternative is to continue to flat-field images using current dark-sky technique, and then construct a photometric correction map and apply that as a secondary flat-field correction. The correction map is a measure of the photometric response variations across the focal plane produced by measuring a bright star or group of stars at finely spaced intervals around the array. The brightness of the star would be measured at each position, and a smooth surface would then be fit to the observed brightness variation pattern. This can be thought of as a "super" ROUND map such as that shown in Figure 1. Figure 2 shows such a map that was constructed for a NICMOS 3 equipped camera on the Steward Observatory 1.55m telescope using a 10x10 sample of observations of a single bright star.
Figure: Photometric response correction map constructed for a NICMOS 3 camera on the Steward Observatory 1.55m telescope. A bright star was images in a 10x10 grid pattern on the focal plane, and a smooth 2-d surface was fit to the resulting matrix of photometry to create this image.