COS Spectroscopy between 900 and 1150 Angstroms

During SMOV, pre-launch expectations that COS could record low-resolution spectra in the wavelength range 900-1150 Å were verified. This observing mode, using the COS G140L grating with the new central wavelength setting of 1280, is now available to the community for Cycle 18. Before using this mode, several factors must be considered.

1. The sensitivity is reasonably well determined, and the effective area ranges from about 10 cm² near 950 Å to 1000 cm² near 1150 Å. This enormous change in sensitivity will create observing problems that are discussed in more detail below.
2. The spectral resolution is poorly characterized, but estimated to be about 2000.
3. The wavelength scale is poorly calibrated, but thought to be accurate to a few tenths of an Angstrom.

While Cycle 18 observations will refine the calibration in this wave band, the huge contrast in sensitivity between 950 and 1150 Å will severely restrict target selection. Because flux from 900 to 1150 Å falls on a single detector (FUVB), the flux at 1150 Å may exceed the local bright limit even though the count rate below 1050 Å is very low. This has two effects: First, it will restrict targets to those that are quite faint near 1150 Å. Second, it will make the integration times needed to obtain good S/N below 1050 Å quite long, even for very blue objects. The problem is alleviated somewhat by using FP-POS=4. This setting moves wavelengths longer than 1137 Å off of the detector. Consequently, turning the FUVA detector off and using FP-POS=4 is the optimum approach for observing bright objects below 1100 Å with the G140L. See the COS Instrument Handbook (Sec 6.1.2) and McCandliss et al. (2010, ApJ, 709L, 183) for more information.