A.1 COS Lifetime Positions

To mitigate the effect of gain sag and provide the community with access to a fresh (unsagged) region of the FUV detector, the location of the COS FUV spectrum is periodically moved to a new part of the detector. The moves to Lifetime Position 2 (LP2), Lifetime Position 3 (LP3), and Lifetime Position 4 (LP4) occurred on July 23, 2012, February 9, 2015, and October 2, 2017, respectively. For Cycle 29, G130M cenwaves 1291 and longer are at a new LP5, while other G130M cenwaves remain at their previous LPs: LP4 for 1222 and LP2 for 1055 and 1096. G140L cenwaves have returned to LP3, and G160M cenwaves remain at LP4.

For Cycle 30, G160M exposures that are longer than approximately half an orbit will use a new LP6, while shorter G160M exposures will remain at LP4 (if requested). There will be several notable changes for users at LP6, such as increased overheads and a slight reduction in resolution compared to LP4. The COS team will provide additional updates to users as they become available.

The Lifetime Positions are located on the detector in the following way: LP1 is considered to be at the center. LP2 is located at +3.5" in the cross-dispersion direction, LP3 is located at –2.5" in the cross-dispersion direction, and LP4 is located at –5.0" in the cross-dispersion direction, or 2.5" below LP3. LP5 is located at +5.4" in the cross-dispersion direction, or 1.9" above LP2. Figure A.1 shows the positions of the different Lifetime Positions on top of a modal gain map showing the gain sag of the detector as of March 2021.

The LP4 move was accompanied by a new set of restrictions on detector segment usage, FP-POS selection, and target acquisition settings. These changes, which will remain in effect at subsequent LPs, are together known as the COS2025 policy, described at http://www.stsci.edu/hst/instrumentation/cos/proposing/cos2025-policies. The main culprit of gain sag in the COS/FUV detector is Ly α geocoronal emission at 1216 Å that fills the COS 2.5" aperture whenever the G130M grating is used with the 1291/1300/1309/1318/1327 cenwaves on Segment B. In order to mitigate this gain sag, the COS2025 policy minimizes the number of locations on the detector where the geocoronal Ly α emission falls by not supporting the Segment B G130M 1300/1309/1318/1327 cenwaves and reducing the number of FP-POS for 1291 from four to two. In this way the geocoronal Ly α emission falls on fewer locations on the detector, and each LP can be used until the continuum level reaches a low modal gain.

Each change in Lifetime Position has resulted in a change in the spectral resolution, and small changes in the core of the line profile and the wings of the line spread function (LSF). The spectral resolution at different Lifetime Positions has been characterized (COS ISR 2013-07, COS ISR 2017-06, and COS ISR 2018-07), and they find a decrease of 5–10% for LP2 and LP3 below LP1, and another 10–15% below LP3 for LP4. The resolution at subsequent LPs will be discussed in forthcoming ISRs. The resolution tabulated in Table 1.1 corresponds to LP4 for the G130M FUV cenwaves. The resolution as a function of Lifetime Position is shown in Figure 1.1. The shapes of the line profiles are well represented in the LSF models available to the community at http://www.stsci.edu/hst/instrumentation/cos/performance/spectral-resolution.

The flux and wavelength calibrations are performed in a similar way for all Lifetime Positions. Calibration observations are executed in preparation for lifetime moves, and updates to calibration reference files are made. These observations include measurements of the spatial and spectral resolution, verification of the wavelength scales, verification of the FUV BOA operations if available, and flux and flat field calibration. In addition, LP3 and beyond make use of profile calibration files for the two-zone extraction described in Section 3.2.1, while LP1 and LP2 use a boxcar extraction.

The calibration pipeline automatically uses the correct calibrations methods based on the Lifetime Position specified in the header keyword LIFE_ADJ, which also enables CRDS to determine the most up to date reference files for that Lifetime Position. LIFE_ADJ can have values of 0, 1, 2, 3, 4, 5, –1, –11, and –999. A value of 0 corresponds to pre-launch data. Values of 1 through 5 correspond to those Lifetime Positions, and as new Lifetime Positions are added they will be designated 6 and higher. A value of –1 is specified if the aperture position is at a non-standard location. A value of –11 is only used in reference files and indicates that the file should be applied to all data with LIFE_ADJ=1 and –1. Lastly, a value of –999 is used for dark exposures, where there is no defined LP.

The varying Lifetime Positions are just for the FUV, and NUV spectroscopic observations remain at LP1 of the NUV detector.

Figure A.1: Location of the Lifetime Positions on Segment B.

The color map shows the modal gain of the FUVB segment as of March 2021. The allowed COS2025 Ly alpha airglow holes are indicated, along with an example of continuum gain sag. Note that the COS2025 policy, which took effect with the move to LP4 and will remain in effect for subsequent LPs, results in fewer gain sag holes due to the limited number of locations where Ly alpha can be placed on the detector. The cyan box indicates the location on the detector where LP5 will be placed.