2.1 COS 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 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 temporarily eliminate 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" 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" 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 move to 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 2.1.1). LP4 is located at –5.0" from LP1. As shown in Table 5.1, the spectral resolution at LP4 is approximately 10–20% lower than it was at LP3, depending on wavelength.

Starting with the move to LP3, and continuing at LP4, 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 LP4 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 are appearing at LP4, with the timing of their appearance depending on the locally accumulated signal. The use of multiple FP-POS positions, which is now required for COS FUV observations (see Section 5.8.2), distributes the high geocoronal Lyman-α flux more uniformly over the detector, and thus will significantly extend the utility of LP4.

In order to continue mitigating the effects of gain sag in the FUVB detector segment caused by airglow emission, when observing with the G130M grating STScI reserves the right to switch the central wavelength setting of any program to another G130M central wavelength setting (cenwave). This is to ensure that future G130M usage is such that a single cenwave is not unreasonably causing the detector to sag. Users that require a specific G130M cenwave must justify it in the Phase I proposal.

2.1.1 New COS 2025 Policy

A new COS 2025 policy has been developed with the goal of retaining the full science capability of COS/FUV until 2025. It was implemented at the start of Cycle 25, on October 2, 2017. It places restrictions on the G130M cenwaves allowed at LP4 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 new 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 2.1 and 2.2 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 2.3 and 2.4 below list these ranges as a function of grating, cenwave, and FP-POS.

Table 2.1: Supported and Available Science Modes versus Lifetime Position for COS FUV.

Lifetime
Position

FP-POS

2

ANY

3

ANY

4

1

4

2

4

3

4

4

4

ALL

G130M/1055

+

X

X

X

X

X

X

G130M/1096

+

X

X

X

X

X

X

G130M/1222

X

X

+

+

+

+

+

G130M/1291

X

X

FUVA

FUVA

+

+

FUVA

G130M/1300

X

AVAIL

FUVA

FUVA

FUVA

FUVA

FUVA

G130M/1309

X

AVAIL

FUVA

FUVA

FUVA

FUVA

FUVA

G130M/1318

X

AVAIL

FUVA

FUVA

FUVA

FUVA

FUVA

G130M/1327

X

AVAIL

FUVA

FUVA

FUVA

FUVA

FUVA

G160M/ALL

X

X

+

+

+

+

+

G140L/ALL

X

X

+

+

+

+

+

AVAIL = Available mode use only (approval needed)
+ = Supported mode (all guest observers may use; no approval needed)
FUVA = Supported mode, but only with Segment = FUVA (all guest observers may use)
X = Not supported or available to guest observers

Table 2.2: Supported and Available Target Acquisition Modes versus Lifetime Position for COS FUV.

Lifetime Position

FP-POS

2

3

3

3

4

3

G130M/1055

X

X

X

G130M/1096

X

X

X

G130M/1222

X

X

X

G130M/1291

X

X

+

G130M/1300

X

X

FUVA

G130M/1309

X

X

FUVA

G130M/1318

X

X

FUVA

G130M/1327

X

X

FUVA

G160M/1533

X

X

X

G160M/OTHERS

X

X

+

G140L/800

X

X

X

G140L/OTHERS

X

X

+

+ = Supported mode (all guest observers may use)
FUVA = Supported mode, but only with Segment = FUVA (all guest 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.

Table 2.3: G130M Wavelength Ranges Affected by Segment B Gain Sag.

Cenwave

FP-POS

λmin (Å)

λmax (Å)

1222

1

1152.8

1157.8

1222

2

1150.3

1155.3

1222

3

1147.8

1152.8

1222

4

1145.4

1150.3

1291

3

1214.4

1219.4

1291

4

1212.0

1217.0


Table 2.4: G160M Wavelength Ranges Affected by Segment B Gain Sag.

Cenwave

FP-POS

λmin (Å)

λmax (Å)

15331

1

1446.7

1452.8

15331

2

1443.7

1449.8

15331

3

1440.6

1446.7

15331

4

1437.6

1443.7

1577

1

1490.7

1496.8

1577

2

1487.7

1493.8

1577

3

1484.6

1490.7

1577

4

1481.6

1487.7

1589

1

1502.4

1508.5

1589

2

1499.3

1505.4

1589

3

1496.3

1502.4

1589

4

1493.2

1499.3

1600

1

1513.8

1519.9

1600

2

1510.7

1516.8

1600

3

1507.6

1513.8

1600

4

1504.6

1510.7

1611

1

1525.6

1531.7

1611

2

1522.6

1528.7

1611

3

1519.5

1525.6

1611

4

1516.4

1522.6

1623

1

1537.7

1543.8

1623

2

1534.6

1540.7

1623

3

1531.6

1537.7

1623

4

1528.5

1534.6

1 Ranges for this new cenwave are estimates until calibration is complete.