One of the things I have been curious about in ODI is the vignetting. While the optical design was based on a one degree field of view, the square format of ODI actually puts the corner pixels as far as 44 arcminutes from the field center. I have played a bit with trying to compare the flat field illumination with the photon statistics in order to separate the gain from the vignetting, but I wasn't happy with that approach. Last night, I got a good sequence of frames going through the best focus, and so I took the most out of focus frame and looked at the donuts - the pupil images. I took the best image from each OTA and made up a little picture that shows them placed on top of the position of that OTA. As you can see the illumination is remarkably uniform until you get to the far corners of the focal plane. The two OTAs that are in one square from the corner have centers about 28 arcminutes from the field center, and you can see that there is really minimal vignetting at that point. In the one OTA in the real corner, centered about 39 arcminutes from the field center, the vignetting increases rapidly and is about 60-70% at the outermost pixel.