Sometime near the start of Cycle 22, the location of COS FUV spectra on the detector is expected to be moved from the second COS FUV Lifetime Position (LP2) to the third COS FUV Lifetime Position (LP3). This move is needed to mitigate the continuing effects of gain sag.
With the LP2 move (July 2012), spectra were shifted perpendicularly to the dispersion direction by the equivalent of +3.5", or about 41 pixels, from the original LP1 location. With the LP3 move, spectra will be shifted below the original LP1 position by about -2.5", which maximizes resolution and the detector area available for future lifetime positions. The modes with the highest cross dispersion profiles, G130M 1055 and 1096, will remain at LP2, while all other modes will be moved to LP3. Improved extraction techniques are being developed to ensure that the gain sagged regions near LP1 do not affect the spectral quality of point source observations at LP3.
While the exact change in performance is not yet known, typical spectral resolving power (λ/Δλ) for the modes that are moved to LP3 may decline by about 15%. The resolution values quoted in Section 4.4 of the HST Primer, include this estimated 15% resolution degradation for all modes; however the resolution of the G130M 1055 and 1096 settings will be unchanged.
Observations of spatially extended sources may also be slightly impacted by the proximity to the gain sagged regions near the original LP1 location and care may be needed not to confuse gain sag artifacts with real spectral features for such sources. Instrument performance is otherwise expected to be similar to that seen during Cycle 21. Updated information will be provided to the community as it becomes available.