2.3 Central Wavelength Settings Added in Cycle 26

Starting in Cycle 26, two new central wavelengths (cenwaves) were made available for the COS FUV channel. These are cenwave 800, for the G140L grating, and cenwave 1533, for the G160M grating.

The use of cenwave 800 was explored by McCandliss et al. in calibration program 12501, and their group published a preliminary characterization of the mode (Redwine et al. 2016, PASP, 128, 105006). This cenwave places a broad range of FUV wavelengths (800–1950 Å) on Segment A of the detector. Its coverage starts and ends about 300 Å blueward of the G140L/1105 cenwave, and it lacks the gap between segments that affects the G140L/1280 cenwave. It is optimized to reduce the astigmatic height of the spectrum in the region below ~1100 Å, allowing for decreased detector noise and correspondingly better S/N at these wavelengths compared to that obtained on Segment B with the 1280 cenwave.

Cenwave 1533 extends coverage at the short-wavelength end of G160M by 44 Å to overlap with the longest wavelengths covered by Segment A of cenwave 1222. This allows a broad range of wavelengths to be covered by just two M mode settings without placing Ly α on the detector, avoiding a key contributor to gain sag.