Introduction

This chapter provides information for specifying visit and exposure parameters within APT.  More complete descriptions of instrument configurations, modes, apertures, spectral elements, etc., are available in the COS Instrument Handbook.

COS2035 Policies

The COS FUV detector experiences gain sag, by which the response of a given detector location decreases with accumulated exposure to light.  Over the years, several strategies have been developed to mitigate this effect, 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.  A decade ago, it became clear that these strategies alone were insufficient 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.

Change in overheads for G130M and G160M

To counter the effects of gain sag and preserve sensitivity, COS periodically introduces new lifetime positions (LPs), shifting spectra in the cross-dispersion direction to less-used regions of the FUV detector.  In Cycle 33, LP7 became the default lifetime position for G130M spectroscopy using cenwaves 1055, 1096, 1222, and 1291, while cenwaves > 1291 remained at LP5. Similarly, LP10 became the new default for all G160M cenwaves, replacing LP6. Because G130M observations at LP7 require the use of SPLIT wavecals, they will incur greater wavelength-calibration overheads than before.  G160M observations at LP10 no longer require SPLIT wavecals, thereby reducing overhead.

To minimize the overheads associated with G130M observations, a new policy allows the use of fewer FP-POS settings for short (<1 orbit) observations.  See Section 9.5.1 of the COS Instrument Handbook for details.

New Lifetime Position (LP11) for G140L

In Cycle 34, COS will introduce Lifetime Position 11 (LP11) for G140L observations. All G140L cenwaves will move from LP3 to the new LP11.  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, LPs have been assigned automatically for each FUV observing mode.  With the arrival in Cycle 34 of LP11, the cycle-dependent mapping between observing mode and LP has become too complex to automate.  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 then allow the observation or return an error message stating the allowed LP.  Each cenwave has only one allowed LP in Supported mode. For Cycle 34, the allowed LP values for each configuration are as follows:

Table 11.1. Lifetime Positions for Cycle 34 COS/FUV Science Observations through the PSA

¹ Default 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 11.2. Lifetime Positions for Cycle 34 COS/FUV Target Acquisitions through the PSA

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

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

³ Available-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 11.3. Lifetime Positions for Cycle 34 COS/FUV Science Observations through the BOA

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

Table 11.4. Lifetime Positions for Cycle 34 COS/FUV Target Acquisitions through the BOA

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

² Default 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 below.

Figure 11.1.  Setting the LP for an exposure in APT.

Target Acquisitions

Because they are more precise and often faster, ACQ/IMAGE acquisitions are preferred over dispersed-light acquisitions.  When dispersed-light acquisitions are required, they must be performed using FP-POS = 3.

COS target acquisitions of extended targets have caveats, as discussed in Section 8.10 of the COS Instrument Handbook.

Bright Object Aperture (BOA)

Spectroscopic observations with the BOA are AVAILABLE and should be performed only if the technical requirements and scientific justification are particularly compelling.  Please see Section 5.13 of the COS Instrument Handbook.  Allowed lifetime positions for the BOA are listed in Table 11.3 (science) and Table 11.4 (acquisition).

FUV observations of extended objects and observations using the BOA may not be properly calibrated due to use of the TWOZONE extraction algorithm.  In these cases, the standard pipeline calibration may not produce the flux accuracy achieved for point sources observed with the PSA, and it will be the responsibility of the user to evaluate the adequacy of the spectral extraction and, if necessary, perform a customized extraction. See COS ISR 2015-03 for further details.

NUV Considerations

For a list of the spectral elements and central wavelengths available for NUV observations, see Table 11.7: COS Spectral Elements and Central Wavelengths.

NUV science and target-acquisition observations are not affected by FUV lifetime position moves.  Users need not (and cannot) specify the lifetime position for NUV exposures.

COS Instrument Parameters

Table 11.5 lists the permitted Instrument Configurations, Operating Modes, Apertures, Spectral Elements, and Optional Parameters for COS.

Table 11.5: Instrument Parameters for COS

¹ Deprecated, see Target Extent

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