Introduction

The Instrument Configurations and Operating Modes described in this chapter are used to specify exposures on the Visit and Exposure Specifications. More complete descriptions of Instrument Configurations, Modes, Apertures, Spectral Elements, etc. are available in the COS Instrument Handbook.

COS FUV operations continue to evolve with Lifetime Position (LP) usage. Beginning in Cycle 33, LP7 becomes the default for G130M spectroscopy using cenwaves 1055, 1096, 1222, and 1291, while cenwaves >1291 remain at LP5. Similarly, LP10 becomes the new default for all G160M cenwaves, replacing LP6. As with G160M observations at LP6, G130M observations at LP7 will incur increased wavelength calibration overheads due to the use of SPLIT Wavecals. In contrast, G160M observations at LP10 will no longer require SPLIT Wavecals, thereby reducing overhead.

Proposers planning to use G130M cenwaves 1055, 1096, 1222, or 1291 in Cycle 33 are strongly encouraged to consult the Cycle 33 COS Instrument Handbook, especially the section discussing SPLIT Wavecal overheads at LP7.

For context, G130M/1055 and 1096 were previously at LP2, and G130M/1222 at LP4, but all now move to LP7. G130M TIME-TAG/ACCUM science observations with cenwaves 1300–1327 remain at LP5. Lyman-alpha observations with G130M remain available at LP3 if justified in the Phase I proposal, with associated spectroscopic acquisitions at LP5. G140L FUV TIME-TAG/ACCUM science observations continue at LP3, while G140L spectroscopic acquisitions remain at LP4. Defaults for the SEGMENT and FP-POS optional parameters for all cenwaves are unchanged.

Since Cycle 26, two new COS/FUV central wavelengths (cenwaves), G140L/800 and G160M/1533, are being offered for TIME-TAG and ACCUM exposures. These modes are not offered for dispersed-light acquisitions. The G140L/800 mode places a broad range of wavelengths (800–1950 Å) on Segment A of the COS FUV detector, with a small astigmatic height at low wavelengths. The G160M/1533 mode extends the G160M coverage by 44 Å towards shorter wavelengths to overlap with the longest wavelengths of the G130M/1222 mode, allowing high signal-to-noise coverage of the full FUV bandpass with only two settings without placing Lyman-alpha on the detector. A more detailed motivation for these cenwaves is given in the COS Instrument Handbook. 

New from Cycle 25 to 32

Restrictions on allowed observing modes with the G130M grating, which were put in place beginning in Cycle 25 at LP4 also apply at LP5. These restrictions, commonly referred to as "COS2025 restrictions", are designed to reduce the impact of geocoronal Lyman-alpha emission on FUV detector segment B, since this emission is a principal agent of gain sag. The restrictions affect the choice of SEGMENT and FP-POS available for certain G130M central wavelength settings (cenwaves), and apply to both science exposures and dispersed-light acquisitions. The policies can be summarized as follows:

• Observations with cenwave 1291, 1300, 1309, 1318, and 1327 have restrictions on Optional Parameters FP-POS and detector SEGMENT at LP4 and LP5: 
• -  for 1291 observations, only two FP-POS positions (3 and 4) are allowed if SEGMENT=BOTH or SEGMENT=B.
• -  for 1300, 1309, 1318, and 1327 observations, only SEGMENT=A is allowed, but all four FP-POS positions can (and should) be used.
• Users of the 1291, 1300, 1309, 1318, and 1327 settings whose science case involves the study of Galactic Lyman-alpha and who would be adversely affected by these FP-POS and SEGMENT restrictions are allowed to request their observations be taken at LP3. At LP3, all four FP-POS and both SEGMENTS can be used for each of these five central wavelengths, as has been the case in the past. Such requests will be reviewed by the COS team.
• All COS users are urged to check the COS2025 website at: http://www.stsci.edu/hst/instrumentation/cos/proposing/cos2025-policies for detailed information about the COS2025 restrictions.

Observations with the G160M and G140L gratings and with the NUV channel are unaffected by the COS2025 policies. The supported FUV settings for Cycle 30 and beyond are summarized in Table 11.1 (for science exposures) and Table 11.2 (for dispersed-light acquisitions).

New from Cycle 33 Onward

To extend the lifetime of the FUV detector, starting in Cycle 33 COS operates at 4 different lifetime positions simultaneously. The default lifetime positions for each available central wavelength can be summarized as follows (see also Table 11.1 Supported settings for FUV ACCUM and TIME-TAG exposures):

• Observations with G130M central wavelengths < 1300 are taken at LP7.
• Observations with G130M central wavelength ≥ 1300 continue to be taken at LP5.
• Science observations with G140L central wavelengths 800, 1105, 1230, and 1280 are taken at LP3, target acquisitions are taken at LP4.
• Observations with G160M are taken at LP10.  
• All COS users are urged to check the COS2025 website at: http://www.stsci.edu/hst/instrumentation/cos/proposing/cos2025-policies for detailed information about the COS2025 restrictions.

G130M science observations with central wavelengths 1291, 1300, 1309, 1318, and 1327 are AVAILABLE at LP3 with approval from the COS team and if justified in the Phase I proposal, but LP3 is not AVAILABLE for spectroscopic acquisitions.

Due to low throughput, the NUV G285M grating is now AVAILABLE but unsupported, and can be requested with approval by the COS team.

Table 11.1: Supported settings for FUV ACCUM and TIME- TAG exposures in Cycle 33 and beyond

¹ Default values for the SEGMENT optional parameter are in bold.
² G130M/1291 observations using SEGMENT=A  can use all FP-POS settings (1, 2, 3, 4, ALL), but G130M/1291 observations using SEGMENT=B or SEGMENT=BOTH are restricted to FP-POS=3, 4.

Table 11.2: Supported settings for FUV spectroscopic target acquisitions in Cycle 33 and beyond

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

At all active Lifetime Positions of the COS/FUV detector, observations of extended objects and observations using the Bright Object Aperture (BOA) might not be properly calibrated due to the extraction algorithm used ("two-zone").  In these two cases the standard pipeline calibration will not always produce the flux accuracy achieved for point sources observed with the Primary Science Aperture (PSA), and it will be the responsibility of the user to evaluate the adequacy of the spectral extraction and, if necessary, undertake any customized extractions. See COS ISR 2015-03 for further details.

Use of the BOA at Lifetime Position 7 (LP7) and 10 (LP10) is not permitted so cannot be used with  grating/wavelength combinations that use LP7 or LP10; those need to request use of LP4. 

NUV science and target acquisition observations are not affected by the FUV lifetime position moves. COS target acquisitions of extended targets have always had caveats, as discussed in the 'Special Cases' section of the COS Instrument Handbook.

Table 11.3: Instrument Parameters for COS lists the permitted Instrument Configurations, Operating Modes, Apertures, Spectral Elements, and Optional Parameters for COS.

Table 11.3: Instrument Parameters for COS

¹ Deprecated, see Target Extent

Related Links

11.2 COS Target Acquisition Modes
11.3 COS Observing Modes
11.4 COS Restricted Engineering Modes
11.5 COS Reference Information

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