July 2026 STAN

July 17, 2026

COS NEWSLETTERS

In this STAN, we introduce the new COS2035 Policies, discuss items of interest to those who are preparing Phase II proposals for Cycle 34, and provide updates on recent reference file deliveries and COS conference contributions.

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Introduction of COS2035 Policies

Over the years, several strategies have been developed to mitigate the effects of gain sag, in which the response of a given detector location decreases with accumulated exposure to light, on the COS FUV detector. These include 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. A decade ago, it became clear that these strategies alone were insufficient to support continued operation of the COS/FUV detector though 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 COS2035 Policies are part of a larger COS2035 Strategy for extending the lifetime of COS into the 2030s. This strategy is described in COS ISR 2026-04, "COS2035: Extending COS/FUV Operations Through the 2030s," by M. Rafelski et al.

New Lifetime Position (LP 11) for G140L

The COS FUV detector experiences gain sag, in which the response of a given detector location decreases with accumulated exposure to light. To mitigate the effect of gain sag on the FUV detector, COS periodically introduces new lifetime positions (LPs), shifting spectra in the cross-dispersion direction to less-used regions of the detector. Beginning in Cycle 34, COS will introduce Lifetime Position 11 (LP11) for G140L observations. All G140L cenwaves will move from LP3 to the new LP11. For details, see the COS Instrument Handbook.

As with previous lifetime-position changes, the impact on science observations is minimal. The spectral resolution and sensitivity of G140L vary only slightly with lifetime position, and users do not need to handle LP11 observations any differently than at LP3 when preparing their Phase II proposals. Performance and calibration details for LP11 will be documented in forthcoming COS Instrument Science Reports.

Specifying Lifetime Positions in APT

Since the introduction of Lifetime Positions (LPs) in 2012, we have relied on APT to assign the LP for each observing mode automatically. With the arrival of LP11, the time-dependent mapping between observing mode and LP has become too complex for the simple algorithm used by APT. Beginning in Cycle 34, all COS/FUV exposures must explicitly specify the lifetime position as an Optional Parameter in the Phase II proposal. APT will compare the user input against an internal table and either allow the observation or return an error. Each cenwave has only one allowed LP in Supported mode. For Cycle 34, the allowed LP values for each configuration are as follows:

Table 1. Lifetime Positions for Cycle 34 COS/FUV Science Observations through the PSA

GRATING	CENWAVE	SEGMENT¹	FP-POS	LIFETIME POSITION
G130M	1055, 1096, 1222	FUVA, FUVB, BOTH	1, 2, 3, 4, ALL	LP7
G130M	1291	FUVA	1, 2, 3, 4, ALL	LP7
G130M	1291	FUVB, BOTH	3, 4	LP7
G130M	1300, 1309, 1318, 1327	FUVA	1, 2, 3, 4, ALL	LP5
G130M²	1309, 1318, 1327	FUVA, FUVB, BOTH	1, 2, 3, 4, ALL	LP3
G160M	1533, 1577, 1589, 1600, 1611, 1623	FUVA, FUVB, BOTH	1, 2, 3, 4, ALL	LP10
G140L	800, 1105	FUVA	1, 2, 3, 4, ALL	LP11
G140L	1280	FUVA, FUVB, BOTH	1, 2, 3, 4, ALL	LP11

¹Default values for the SEGMENT optional parameter are in bold.

²Available-but-unsupported mode for observation of Lyα at z=0. Use of this mode must be justified in the Phase I proposal. If the proposal is accepted, the COS team will evaluate the request during Phase II preparation.

Table 2. Lifetime Positions for Cycle 34 COS/FUV Target Acquisitions through the PSA

GRATING	CENWAVE¹	SEGMENT²	FP-POS	LIFETIME POSITION
G130M	1291	FUVA, FUVB, BOTH	3	LP7
G130M	1300, 1309, 1318, 1327	FUVA	3	LP5
G130M³	1291, 1300, 1309, 1318, 1327	FUVA, FUVB, BOTH	3	LP3
G160M	1577, 1589, 1600, 1611, 1623	FUVA, FUVB, BOTH	3	LP10
G140L	1105	FUVA	3	LP11
G140L	1280	FUVA, FUVB, BOTH	3	LP11

¹ Cenwaves 800, 1055, 1096, 1222, and 1533 are not available for target acquisitions.

²Default values for the SEGMENT optional parameter are in bold.

³Available-but-unsupported mode for observation of Lyα at z=0. Use of this mode must be justified in the Phase I proposal. If the proposal is accepted, the COS team will evaluate the request during Phase II preparation.

Note that all FUV observations through the BOA are considered available but unsupported.

Table 3. Lifetime Positions for Cycle 34 COS/FUV Science Observations through the BOA

GRATING	CENWAVE	SEGMENT¹	FP-POS	LIFETIME POSITION
G130M	1055, 1096, 1222, 1291, 1300, 1309, 1318, 1327	FUVA, FUVB, BOTH	1, 2, 3, 4, ALL	LP4
G160M	1533, 1577, 1589, 1600, 1611, 1623	FUVA, FUVB, BOTH	1, 2, 3, 4, ALL	LP4
G140L	800, 1105	FUVA	1, 2, 3, 4, ALL	LP3
G140L	1280	FUVA, FUVB, BOTH	1, 2, 3, 4, ALL	LP3

¹Default values for the SEGMENT optional parameter are in bold.

Table 4. Lifetime Positions for Cycle 34 COS/FUV Target Acquisitions through the BOA

GRATING	CENWAVE¹	SEGMENT²	FP-POS	LIFETIME POSITION
G130M	1291, 1300, 1309, 1318, 1327	FUVA, FUVB, BOTH	3	LP4
G160M	1577, 1589, 1600, 1611, 1623	FUVA, FUVB, BOTH	3	LP4
G140L	1105	FUVA	3	LP4
G140L	1280	FUVA, BOTH	3	LP4

¹ Cenwaves 800, 1055, 1096, 1222, and 1533 are not available for target acquisitions.

²Default values for the SEGMENT optional parameter are in bold.

To specify the LP within APT: Under Optional Parameters, click Add, set the Parameter Name to LIFETIME-POS, select the appropriate value, and click “OK,” as shown in Figure 1.

Best Practices for COS Phase II Submissions

Provide ETC ID numbers for each exposure, including acquisition exposures.
Confirm that the program complies with the signal-to-noise and maximum lifetime usage limits of the COS2035 policies.
Specify the Lifetime Position (LP) for each exposure as an Optional Parameter.
Verify that each exposure is safe by running the Bright Object Tool (BOT) in APT, using the GALEX catalog whenever possible. Programs submitted with unexplained BOT warnings may lead to a delay in scheduling the observations.
If a target consists of multiple sources, a target field is crowded, or a target is faint, consider executing an offset target acquisition.
Specify the buffer time for all TIME-TAG exposures. Correctly calculating the buffer time is important to ensure that no data are lost during readout. In most cases, the buffer time should be 2/3 of the value calculated by the ETC, but there are exceptions for bright targets.
Follow the target list and instrument configuration specified in the approved Phase I proposal. Changes of grating, central wavelength, or lifetime position can be requested by a minor-change request to your Contact Scientist, provided there is no change to the science goals. More substantial changes, such as target changes, more restrictive scheduling constraints, instrument changes, and anything that alters the science goals, must be requested by a major-change request to the Telescope Time Review Board (TTRB), using the link from the Program Status webpage.
Consider alternative observational approaches to achieve your science goals if observations require a string of more than 6 consecutive orbits, as such strings will execute at shared risk (i.e., it will not be eligible for repeat if impacted by observatory problems).

Delivery of New TDSTAB and FLUXTAB Files

The sensitivity of the COS FUV detector declines with time. The time-dependent sensitivity (TDS) for each mode is modeled as a piecewise linear function of time and is assumed to be a smooth function of wavelength. This TDS is characterized in a TDSTAB reference file. After the initial conversion of counts to fluxes, the fluxes are further modified by a factor that changes with time. The TDS models are derived from observations of flux standards, which are monitored every month for the FUV. The monitoring program uses the following subset of available cenwaves to track the TDS for both standard modes and blue modes: 1055, 1096, 1222, 1291, and 1327 for the G130M grating; 1533, 1577, and 1623 for the G160M grating; and 800, 1105 and 1280 for the G140L grating.

Starting in 2025, the TDS monitoring program showed systematic deviations from the model (see Figure 1) . For the FUVA detector, the decline was about 1%/yr steeper than predicted at the short wavelength end and about 3%/yr steeper at the long wavelength end. For the FUVB detector, the decline was about 1%/yr shallower than predicted at the short wavelength end and the long wavelength end. To correct these deviations, new parameters for the standard modes were derived. The model did not require a new temporal breakpoint at this time.

The new TDS parameters were used to update the sensitivities at the reference time at lifetime positions (LP) 1, 2, 3, 4, 5, 6, 7, and 10 so the corresponding FLUXTAB files were also updated. A new FUV TDSTAB and new FUV FLUXTABs for LP1, LP2, LP3, LP4, LP5, LP6, LP7, and LP10 were delivered the week of 2026 July 13. Users who have obtained FUV spectra since March 1st, 2023 are encouraged to retrieve their newly recalibrated data from the HST archive.

Delivery of New GSAGTAB and SPOTTAB Files

A new gain sag (GSAGTAB) reference file became active on May 27, 2026. This update expands the gain sagged region affecting LP5 and LP6 FUVB exposures due to Lyman alpha airglow from G130M/1291 observations, and also adds a small number of additional columns on the extreme blue wavelength end (see Figure 3). However, since these gain sag holes are relatively small, the end result is typically 2 small wavelength gaps of ~ 0.5 Angstroms each for G130M/1291 LP5 exposures and 1 similarly small wavelength gap for LP6 exposures, assuming the recommended number of FP-POS are used (see Figure 4). This update mostly affects LP5 and LP6 data using segment FUVB and taken after ~ November 2025. The update is also the first application of a new gain sag flagging method (see COS Instrument Handbook Section 5.12.2), in which pixels are flagged only when the integrated flux loss along a column exceeds a ~3% threshold, rather than when individual pixels fall below modal gain = 3.

A new spot position table (SPOTTAB) was delivered on June 26, 2026. This SPOTTAB adds a handful of new hot spots that have appeared since July 1, 2023, and also updates active time frames of previous hot spots. While most of the hot spots do not affect science data, a small number of the active spots do overlap with LP4 on FUVA and LP5 and LP6 on FUVB. The SPOTTAB update successfully removes the effects of these hot spots, as illustrated in Figure 5. Users with COS FUV data taken after July 1, 2023 are encouraged to download new data products from MAST that have the effects of these hot spots removed.

COS Presentations at Recent Meetings

COS posters were presented at the June AAS meeting in Pasadena, California and the July SPIE meeting in Copenhagen, Denmark. We invite all COS users to review them to find out about the latest developments for COS.

HST’s Cosmic Origins Spectrograph: The Latest on Performance and Calibration

The Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) is in its seventeenth year of operations and continues to be scientifically productive at unveiling the far-ultraviolet (FUV) and near-ultraviolet (NUV) universe. Here we present updates on the current status and performance of the instrument, ongoing efforts in user support, and recent calibration work of interest to all current and future COS users. In support of these, we provide updated time-dependent sensitivity corrections based on regular monitoring, discuss gain map measurements, and report on the latest updates to the dark rates. To prolong the life of the COS FUV channel and mitigate gain sag, science data are taken on different areas of the detector, known as lifetime positions (LPs). We discuss which LPs are currently available for use and provide information on ongoing efforts for commissioning future LPs. We also detail the latest set of policies designed to retain full science capabilities of COS/FUV beyond 2035. Overall, these developments are opening the door to groundbreaking science with both COS UV channels for years to come.

LP-Infinity: overcoming limits to the number of lifetime positions in the COS FUV channel

Since the Cosmic Origins Spectrograph (COS) was installed on the Hubble Space Telescope (HST) in 2009, thousands of spectra have been collected with the Far Ultraviolet (FUV) channel. Due to the gain sag inherent in the cross delay line detector, regular adjustments to the positions of the spectra have been made, with the seventh and eighth becoming available to Guest Observers in October 2025. Recent improvements to models of the FUV channel, including more extensive optical modeling and better prediction and accommodation of detector gain sag, have allowed us to identify previously excluded locations on the detector for future use, thus requiring additional Lifetime Positions (LPs) to be defined. Since the onboard flight software limited the number of Lifetime Positions to eight, we developed a new method for implementing additional LPs to avoid dealing with the complications of reassigning previously used LPs. This novel strategy, dubbed LP-Infinity, allows an effectively unlimited number of LPs that is now constrained only by the available detector real estate, allowing COS to continue obtaining high-quality FUV spectra well into the 2030s. We discuss the conception, development and implementation of LP-Infinity, its initial use in Cycle 33, and the plans for future LPs starting in 2027.

All COS posters are archived on the COS poster page.