We are reducing commissioning data on reddened stars. These are roughly 30 minute exposures. We want to add many together to get very high signal to noise, beyond the values discussed in the commissioning report (S/N >100) and we want to do it for the entire spectral region covered by the echelle, to address the scientific problem of the diffuse interstellar band. These were chosen for commissioning specifically to test the high S/N regime on all spatial scales (1 to 500 pixels). After some early reductions, we placed a new slit in the focal plane, 1.6 arcsec x 3.0 arcsec, so the flats are wider than the stellar spectra. This went in on May 6, 1999, compliments of Shu-i and Jeff Sundwall. We will be using it for the above science program on May 29-June 1 (2nd halves) and will report back on how it works, in comparison with the square slit. In the process of this work, interference effects that I had not anticipated and have not seen before are showing up. Julie Thorburn has evidently been successful in removing these using the one-dimensional flats I mentioned earlier. However, there is evidence of an extreme sensitivity to the angle of the dipping mirror used for flat fields. Whatever the cause of these fringes, there is likely to be an optimal procedure for removing them that involves a specific order of flats and stellar spectra. We will experiement with these possible procedures at the end of May and get back to this group with results. DON APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 7 in the apo35-echelle archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-echelle/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-echelle@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO