5.12 FUV Detector Lifetime Positions

Prolonged exposure to light causes the COS FUV detectors to become less efficient at photon-to-electron conversion, a phenomenon called "gain sag." The more a particular region of the detector has been used, the smaller the "pulse height" of the charge cloud generated by an individual photon becomes. As long as all pulse heights are above the minimum threshold needed to distinguish real photons from detector background events there is no loss in sensitivity. However, as the average pulse height at a location on the detector approaches and drops below this threshold, real photon pulses are increasingly misidentified as background and the effective throughput decreases. Since the amount of gain sag increases with the total amount of previous illumination, these effects appear first on regions of the detector that are illuminated by the bright Lyman-α airglow line, but eventually the entire spectrum becomes affected.

STScI is undertaking a number of actions to mitigate the effects of gain sag and extend the lifetime of the COS FUV XDL detector. Primarily, the position of the science spectrum on the COS FUV detectors is periodically moved to an un-sagged region, to start afresh without the Lyman-α gain-sag holes and other gain-sag artifacts.

• On July 23, 2012 the spectrum was moved from its original lifetime position (LP1) to its second lifetime position (LP2). LP2 is offset by +3.5 arcsec from LP1 in the cross-dispersion direction.
• On February 9, 2015, the spectrum was moved to the third lifetime position (LP3), which is offset by –2.5 arcsec from LP1, for all modes except the G130M 1055 and 1096 central wavelengths. These settings have wide cross-dispersion profiles that would be severely impacted by the proximity of LP3 to LP1, and so they continue to be executed at LP2. The G130M 1222 central wavelength executed at LP3, but has been operated at a higher voltage setting to minimize the impact of gain-sagged regions.
• The commissioning of the fourth lifetime position (LP4) took place on October 2, 2017, at the beginning of Cycle 25 and at the same time as the introduction of the COS 2025 policy (see Section 5.12.1 below). LP4 is located at –5.0 arcsec from LP1.

Starting in Cycle 29, COS began to use multiple lifetime positions at the same time, with the aim of extending the COS FUV lifetime to 2030 and beyond

• G130M cenwaves 1291, 1300, 1309, 1318, and 1327 were moved to LP5 (located at +5.4 arcsec from LP1).
• The two blue-mode G130M cenwaves (1055 and 1096) remain at LP2.
• G130M/1222 remains at LP4.
• G140L cenwaves were moved to LP3, but with G140L acquisition modes remaining at LP4.
• G130M BOA observations also remain at LP4.

The spectral resolving power at LP5 is approximately 20% higher than it was at LP4 and approximately the same as it was at LP3, depending on wavelength. A summary of the supported and available FUV gratings at each lifetime positions for COS FUV is provided in Table 5.12.1.

Starting with Cycle 30, G160M cenwaves were moved to a new LP6, located at +6.5 arcsec from LP1.

• Wavelength calibration exposures at LP6 incur larger overheads than those taken at previous LPs (Section 5.7.6). To minimize the impact of these overheads at LP6, G160M users may under certain conditions use fewer than 4 FP-POS or may take data at LP4. 
• Changes in LP yield minor changes in sensitivity, and resolution. The COS team will provide additional updates to users as they become available via STScI Analysis Newsletters (STANs), the COS website, and updates to the COS IHB.

Table 5.12.1 Supported and Available-but-Unsupported FUV Gratings versus Lifetime Position for COS FUV.

Starting with the commissioning of LP3, and continuing at LPs 4-6, a new spectral extraction algorithm was implemented. This TWOZONE algorithm uses the shape of a point source profile to define the region over which counts are included in the extracted spectrum and to decide when bad pixels in the profile wings compromise the accuracy of the spectral extraction. Sources that have substantial spatial extent may have significant overlap with the gain-sagged regions and may require specialized extractions that are currently not performed with CalCOS. For these reasons, observations of extended sources will not be optimally calibrated. Users should set the optional APT parameter EXTENDED=YES to flag such sources (see Section 5.9), even if the CalCOS pipeline calibration will not treat extended sources differently from point sources.

Throughput and most other calibrations at LPs 4-6 are very similar to those at the original position. See the COS website and the COS Instrument Science Reports (ISRs) for additional information about the calibration of the different lifetime positions.

Gain sag is an inevitable result of using the detector. Gain sag holes will appear at LP5, with the timing of their appearance depending on the locally accumulated signal. The restricted use of FP-POS positions at LP5 to reduce the impact of Ly α airglow is described next.

5.12.1 COS 2025 Policy

A "COS 2025" policy was developed with the goal of retaining the full science capability of COS/FUV until 2025 (which has now been extended to 2030 and beyond). It was implemented at LP4 at the start of Cycle 25, on October 2, 2017. It places restrictions on the G130M cenwaves with the goal of reducing the impact of Ly α airglow. The COS 2025 website includes tables to illustrate the policy, which also appear below, a graphical tool to examine the effects of the policy at different wavelengths, and examples of how observations for different science cases might be modified to align with the policy.

The primary restriction is that spectroscopy with the FUVB detector segment is no longer permitted at cenwaves 1300, 1309, 1318, or 1327. At cenwave 1291, FUVB spectroscopy is allowed only at FP-POS 3 and 4. Furthermore, FUV target acquisition with segment B is no longer permitted at cenwaves 1300, 1309, 1318, or 1327. Tables 1 and 2 at the COS 2025 website as well as Tables 5.12.2 and 5.12.3 below illustrate these restrictions.

Even with this policy, FUVB spectroscopy with the 1291 cenwave and FP-POS 3 or 4 will continue to place Ly α on the detector. Over time, airglow will sag the region of the detector exposed to Ly α, which will become unusable. Therefore, at other settings, the 5–6 Å range of wavelengths located in this region on the detector will be affected. Tables 3a and 3b at the COS 2025 website as well as Tables 5.12.2 and 5.12.3 below list these ranges as a function of grating, cenwave, and FP-POS.

Table 5.12.2: Supported and Available-but-Unsupported Science Modes versus Lifetime Position (LP) for COS FUV.

AbU = Available-but-Unsupported mode use only (Phase I approval needed)
♦ = Supported mode (all General Observers may use; no approval needed)
FUVA = Supported mode, but only with Segment = FUVA (all General Observers may use)
X = Not supported or available to General Observers

ALL = FP-POS ALL (i.e., 1+2+3+4)

ANY = any FP-POS value, including FP-POS ALL.

Table 5.12.3: Supported and Available Target Acquisition Modes versus Lifetime Position for COS FUV.

♦ = Supported mode (all guest observers may use)
FUVA = Supported mode, but only with Segment = FUVA (all General Observers may use)
X = Not supported or available to guest observers
Note: FP-POS = 3 is the default FP-POS for dispersed-light target acquisition.
AbU = Available-but-Unsupported mode use only (approval needed in Phase I)
FUVA, BOTH = Supported mode, but only with Segment = FUVA or Segment = BOTH (all GOs may use)

Table 5.12.4: G130M Wavelength Ranges Affected by Segment B Gain Sag.

Table 5.12.5: G160M Wavelength Ranges Affected by Segment B Gain Sag.