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 Lyα airglow line, but eventually the entire spectrum becomes affected.

Over the years, STScI has developed several strategies to mitigate gain sag on the COS FUV detectors, including moving to different lifetime positions, managing the high voltage to extract a smaller amount of charge, and redistributing cenwave usage so that Lyα does not produce new gain-sag holes.  This section describes the use of detector lifetime positions.

Table 5.12.1 Summary of LP Moves

Although the spectral resolution and sensitivity vary slightly with lifetime position, these changes are mostly transparent to users. LP7, however, carries increased overheads compared to earlier LPs, which are described in Section 5.7.6.  Users should consult the COS Instrument Science Reports and STScI Analysis Newsletters (STANs) for performance updates at LP7, LP10, and LP11.

Table 5.12.2 Supported and Available-but-Unsupported FUV Gratings versus Lifetime Position for COS FUV

Starting with the commissioning of LP3, 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 current Lifetime Positions 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.

5.12.1 COS2035 Policy

A decade ago, it became clear that additional measures were required to support continued operation of the COS/FUV detector through the mid-2020’s. To address this concern, in 2017 STScI developed the COS2025 Policies to limit the use of some G130M cenwaves.  To continue offering the astronomical community high-sensitivity ultraviolet spectroscopy, STScI is now launching the COS2035 Policies, whose implementation will help to preserve the full science capability of COS beyond 2035.  This initiative retains the previously established policies from COS2025 and expands them to include additional requirements.  Please see the COS2035 Policies webpage for details.

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 11.1 and 11.2 of the HST Phase II Proposal Instructions as well as Tables 5.12.3 and 5.12.4 below illustrate these restrictions.

Even with this policy, FUVB spectroscopy with the 1291 cenwave and FP-POS 3 or 4 will continue to place Lyα on the detector. Over time, airglow will sag the region of the detector exposed to Lyα, 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 5.12.5 and 5.12.6 below list these ranges as a function of grating, cenwave, and FP-POS.

Table 5.12.3: 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 5.12.4: 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 5.12.5: G130M Wavelength Ranges Affected by Segment B Gain Sag

Table 5.12.6: G160M Wavelength Ranges Affected by Segment B Gain Sag

Note:  the gain-sagged region extends approximately 6 Å, centered on the given wavelength.

5.12.2 New Gain-Sag Flagging Method

STScI recently reassessed the previously established method for flagging gain-sagged pixels during calibration. In the past, pixels were flagged for exclusion during pipeline processing whenever their modal gain fell below 3, which is equivalent to a roughly 5% count loss. This method, however, does not account for the two-dimensional structure of the projected spectra.  Flagging an inner-contour pixel (in the 80% flux contour) causes CalCOS to reject the entire detector column, regardless of how much (or little) the pixel contributes to the total counts in a column. This treatment was identified as being overly aggressive, particularly when gain sag is due to continuum observations, which affect only a few pixels in a column, rather than airglow lines, which fill the aperture and thus impact many pixels along a column. As a result, in Cycle 32 the COS team modified the treatment of sagged pixels to flag only those with modal gain less than 3 and only if the total-count loss along a column exceeds the maximum achievable S/N level for that combination of grating and central wavelength.  This refined gain-sag flagging approach will extend the viability of the lifetime positions currently in use.