GHRS Instrument Science Report 033 Preliminary Analysis of Best Focus for GHRS 08/90 Ronald L. Gilliland Abstract excerpted from the report: There may well be reason to desire different focus settings of HST to maximize the scientific returns of the various instruments. It is the purpose of this note to address the question of which focus setting would be serve GHRS science goals. The GHRS functions only in the UV, shortward of 3200A, its primary aperture is a 0.25 arcsec square. (We have argued in earlier notes that observations through the small aperture will in general be preferred to those through the large aperture coupled with deconvolution analysis.) The optimal focus for GHRS is to first approximation simply that which will put the most UV light down a 0.25 arcsec square aperture. We will also address secondary issues: Throughput of the larger 2.0 arcsec aperture, degradation of resolving power through the LSA for different focus settings, target acquisition sensitivity, etc.