Phase II Roadmap for Cycle 25
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HST Cycle 25 Phase II Proposal Instructions > Chapter 12: Cosmic OriginsSpectrograph (COS) > 12.1 Introduction to COS

12.1
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 at Lifetime Position 4 (LP4) will begin at the start of Cycle 25. In Cycle 25 all FUV observations will be executed by default at LP4, with the exception of the blue modes (G130M/1055 and G130M/1096) that will remain at LP2. The move to LP4 is being made in order to mitigate the effect of gain sag and hence to maximize the lifetime of the FUV detector. LP4 is located 5.0" below LP1 in the cross-dispersion direction, and 2.5" below LP3.
New for Cycle 25
With the move to LP4, new restrictions on allowed observing modes with the G130M grating are being put in place. 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 the 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:
-
for 1291 observations, only two FP-POS positions (3 and 4) will be allowed if SEGMENT=BOTH or SEGMENT=B.
-
for 1300, 1309, 1318, and 1327 observations, only SEGMENT=A will be 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 absorption and who would be adversely affected by these FP-POS and SEGMENT restrictions will be 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.
www.stsci.edu/hst/cos/cos2025/
for detailed information about the COS2025 restrictions as well as specific science use case examples, comparing Phase II created under these constraints vs. the same Phase II in Cycle 24.
Observations with the G160M and G140L gratings and with the NUV channel are unaffected by these new policies. The supported FUV settings for Cycle 25 are summarized in Table 12.1 (for science exposures) and Table 12.2, Supported settings for FUV spectroscopic target acquisitions in Cycle 25 (for dispersed-light acquisitions).
Table 12.1:  Supported settings for FUV ACCUM and TIME-
TAG
exposures in Cycle 25.
SEGMENT1
A, B, BOTH
A, B, BOTH
A, B, BOTH
G130M/12233
A, B, BOTH
G130M/12914
A, B, BOTH
A, B, BOTH
A, B, BOTH
A, B, BOTH
A, B, BOTH
A, B, BOTH
A, B, BOTH
1
Default values for the SEGMENT optional parameter are in bold.
2
These modes are executed at LP2.
3
G130M/1223 is a new cenwave created to compensate for wavelength coverage loss due to COS2025 restrictions. Please see the COS2025 website at: ww.stsci.edu/hst/cos/cos2025/.
4
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.

 
SEGMENT1
A, B, BOTH
A, B, BOTH
A, B, BOTH
A, B, BOTH
A, B, BOTH
A, B, BOTH
A, B, BOTH
1
Default values for the SEGMENT optional parameter are in bold.

At Lifetime Positions 3 (LP3) and 4 (LP4) 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 the STAN article on this topic for further details.
NUV science and target acquisition observations are not affected by the FUV move to LP4. COS target acquisitions of extended targets have always had caveats, as discussed in the 'Special Cases' section of the COS Instrument Handbook.
Table 12.3, Instrument Parameters for COS lists the permitted Instrument Configurations, Operating Modes, Apertures, Spectral Elements, and Optional Parameters for COS.
SEGMENT,
BUFFER-TIME,
EXTENDED1,
FP-POS,
FLASH
EXTENDED1,
FP-POS
1
Deprecated, see Section 3.1.4

 
 
The G130M central wavelength 1223 has been added for Cycle 25. This central wavelength is very similar to the 1222 central wavelength setting except that the spectral resolution is optimized for SEGMENT=A. Target acquisition is not enabled for this mode, but may be offered in the future. Observations using 1223 will be taken at LP4. More information about the 1223 resolution will be made available on the COS website at:
stsci.edu/hst/cos/cos2025/.

HST Cycle 25 Phase II Proposal Instructions > Chapter 12: Cosmic OriginsSpectrograph (COS) > 12.1 Introduction to COS

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