Shown below are some images of the low-D slits for ESI, taken in the Instrument Lab on Mauna Kea by Bill Mason. (Click on an image to resize it.) In many images a straight-edge has been placed perpendicular to the slits. You can clearly see that the slit mask does not lie in a plane, and in fact suffers a discontinuity where the slit itself lies. Note also the discontinuity in the reflection of the top edge of the straight-edge in the second image.

This is undoubtedly the illumination problem we have seen on guider images when the low-D slits are in the beam. It is very difficult to get both edges of the slit in view at the same contrast/brightness level, and it is therefore nearly impossible to quickly and accurately place a target on the slit jaws. Workarounds by some observers have helped the problem, and flat-fielding may also help, but the bottom line is that this is primarily a mechanical problem.

Not shown here, but reported by Bill, are that the underlying support structure in the low-D slit holders also suffer from some sort of discontinuity. So even tying the slits to their support structure more closely (e.g. with double-sided tape) would not fully alleviate the problem.

What is indicated here is that the holders be remilled to provide an accurate surface for defining the slit surface. The slits must then be tied closely to this surface, probably with double-sided tape.