Beginning on July 23, 2012, the default location for COS FUV spectra and FUV target acquisitions on the cross delay line micro-channel plate detectors has been shifted by +3.5" (corresponding to ~ 41 pixels or ~ 1 mm) in a direction orthogonal to the spectral dispersion. This change is transparent to observers, requiring no changes to phase 2 proposals, and will apply to all COS FUV observations performed after July 22, 2012, regardless of Cycle. At this time it is not possible to support continued routine observations at the original position that was used prior to July 23. No changes will be made to the location of the COS NUV spectra or NUV target acquisitions; these observations will continue to use the original position on that detector.
This change will cause the pointing of any observations taken in parallel with COS FUV observations to shift by 3.5" with respect to their previously planned positions. APT version 20.2.2 or later is required to correctly display the positions of these parallel observations.
The shift in position moves the FUV spectra off the region of the micro-channel plates that has suffered "gain sag" due to the charge extracted from the plates during routine observations. Our preparatory observations for this move show that COS FUV calibrations will require only modest adjustments. This change will result in a slight decrease in spectral resolution (~5-10%), but will avoid the low response areas that have resulted from the gain sag and which are now compromising the quality of the data taken at the original position.
Over the next two months special calibration programs will be undertaken to verify and remeasure the spectral resolution, line-spread functions, sensitivity, and flat fields at the new position. Changes to the sensitivity are expected to be <3% at most wavelengths. Differences in the grid-wire shadows are expected to be small and existing flat fields should continue to do a good job of removing those artifacts. Small changes to the wings of the line spread function are also predicted; new LSF models have been computed and will be made available to users once they are validated by calibration observations at the new lifetime position.
The calibrations described above will be incorporated into standard pipeline reference files as they become available and can then be retroactively applied by re-retrieving the data from the MAST archive. Information about the new calibrations will also be distributed via STAN newsletters and will be posted on the HST COS web site.
Additional information about gain sag effects on the COS FUV detector can be found in the COS Instrument and Data Handbooks, previous COS STAN articles, and also in COS ISR 2011-05