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Hubble Space Telescope
Incorrect Resolution Element Used by COS ETC for new G130M Modes

The resolving power of the two new COS/FUV G130M modes (cenwave=1055 and cenwave=1096) decreases with increasing wavelength, from R ~ 3000 at 950 Å to R ~ 1000 at 1150 Å. The resolving power for the new cenwaves is much lower than that for the other G130M cenwaves because the new cenwaves are out of focus. The new cenwaves have a dispersion of ~ 0.01 Å/pixel, similar to the dispersion of the other higher resolution modes. This implies that at 950 Å one resolution element is around 30 pixels, while it is about 115 pixels at 1150 Å. Since the ETC uses a 6-pixel resolution element that is 5 and 19 times smaller, respectively, it will overestimate the time required to achieve a certain S/N per 30-pixel or 115-pixel resolution element, while underestimating the S/N obtained for a given time. To calculate the correct exposure time per real resolution element, divide the exposure time returned by the ETC by 5 or 19 (for calculations at 950 or 1150 Å respectively).

To calculate the correct S/N per real resolution element, multiply the S/N returned by the ETC by 2.2 or 4.4 (for calculations at 950 or 1150 Å, respectively).