2.4 The G285M Grating is Available but Unsupported

The COS/NUV G285M grating lost sensitivity at a rate of about 10% per year after the 2009 installation of COS. By 2018, its sensitivity had fallen to less than 15% of its original value. Due to a lack of use and the loss of sensitivity, the COS team elected in 2018 to discontinue monitoring its sensitivity and the stability of its wavelength solution. Exposure time estimates, flux calibration, and wavelength calibration for this grating may not be reliable.

COS/NUV observations with the G285M grating are therefore available but unsupported. Alternative COS gratings are described in Section 13.3. Users interested in medium-resolution spectroscopic coverage of the wavelength region from 2500 to 3200 Å are encouraged to use STIS instead.

For a setting that is available but unsupported, proposers should only consider use if their science cannot be performed with supported instrument parameters, and only if the technical requirement and scientific justifications are particularly compelling. STScI adopts a policy of shared risk with the observer for the use of unsupported capabilities. Requests to repeat failed observations taken with unsupported capabilities will not be honored if the failure is related to the use of the unsupported capability. In addition, user support from STScI for observation planning, as well as data reduction and analysis, with unsupported capabilities will be limited and provided at a low priority. Users taking data with unsupported capabilities should be prepared to shoulder the increased burden of the planning, calibration, reduction and analysis. Available but unsupported mode capabilities should only be implemented in the Phase II APT when observers have been specifically granted permission to do so, and some options accessible in the APT software may be prohibited for a variety of technical or policy reasons. Finally, calibrations for unsupported capabilities will not be provided by STScI. Either users must determine that they can create calibration files from data in the HST Archive or they must obtain calibrations as part of their observations. The STScI pipeline will not calibrate data taken in unsupported modes but will deliver uncalibrated FITS files (or in some cases partially calibrated FITS files) to the observer and the HST Archive.

During the Phase II proposal submission process, STScI must also formally approve your use of an available but unsupported mode. This allows STScI to evaluate your request and ensures that no problems associated with your request have come to light since the submission of your Phase I proposal.

The increased burden of calibrating data taken using available but unsupported modes also makes the use of such data for archival research significantly more difficult. As a result, requests for use of unsupported modes which do not adequately address the above four points, or which will result in only marginal improvements in the quality of the data obtained, may be denied by STScI, even if the request was properly included in your Phase I proposal.