Nobj is the number of photons per pixel incident on the detector from the object; Nsky is the number of photons per pixel incident on the detector from the sky; Nrn is the detector noise. See the Observer's Guide, Chapter 6, and The RGO Spectrograph Manual, Section 2.6.
Please use the AAT median seeing of 1.5" for proposals. If your project requires you to assume 1.0" seeing (or better), you must justify this in your proposal, and provide a backup program for poor seeing