3.2 Size of a Resolution Element

Throughout this document, we assume that a resolution element (resel) spans 6 × 10 pixels on the FUV detector (in the dispersion and cross-dispersion directions, respectively) and 3 × 3 pixels on the NUV detector (Table 1.1). These values were determined before launch. Even then, it was known that the true size of a resel would vary with wavelength. In-flight data suggests that the FUV resel is somewhat larger than previously assumed (see the discussion of the line-spread function in Section 3.3), while the NUV resel is smaller. For the G130M 1055 Å and 1096 Å settings the resel is between 8 and 32 pixels. For the 1222 Å setting it is 7–13 pixels. We will continue to refine our analysis of the instrument parameters. In the mean time, keep in mind that, for the FUV, the COS Exposure Time Calculator (ETC) uses a 6-pixel resel size (dispersion) in most of its calculations in the FUV. It uses resel sizes of 8, 9, and 6 for the G130M/1055 and G130M/1096 blue modes, and G130M/1222 respectively. Users who adopt a larger or smaller resel should adjust the ETC results accordingly.