Ivo C. Busko (Space Telescope Science Institute)
Hubble Space Telescope NICMOS images often show quadrants with a slight mismatch in DC offset levels after the calibration process. The cause of these residual offsets is still not fully understood, but the images can be empirically fixed by adding arbitrary DC constants to three arbitrarily chosen quadrants in order to bring them to the same DC level of the fourth, ``reference'' quadrant.
Thus the basic problem in fixing these images is to find the set of three constants that will ``fix'' the image. But the definition of what is a ``fixed'' image is arbitrary, and a number of solutions have been proposed. They are all based on some sort of background estimation in each quadrant, using fits to the background histogram, medians, modes, etc. The problem with these procedures is that they are sensitive to image structure and particularly to low-frequency background gradients. And no matter what the procedure is, the quality of the solution must be judged by eye.
The procedure described here avoids these pitfalls by directly building the solution that mostly pleases the human eye. When looking into an image with quadrant DC offsets, the visual system is mostly responsive to the edges between quadrants. The visual system is actually very sensitive to the amount of Fourier power in the highest frequencies associated with the edges. Thus, the eye will pick up as the best among several ``fixed'' images, the one that has the minimum Fourier power at the quadrant edges.
Based on that idea, a simple algorithm can be implemented that minimizes the Fourier power at the quadrant edges as a function of the three sought DC offsets. Since the algorithm is based only on edge information, it is very robust against image structure and background gradients, and very fast.
The algorithm, which depends only on a minimal set of parameters, was implemented as a C function, that can be incorporated into any software. It was used as the core of an IRAF task that automatically removes the quadrant DC offsets of any number of NICMOS files.