STScI Analysis Newsletter (STAN)
Cosmic Origins Spectrograph (COS)
In this STAN we announce the move to Lifetime Position 4 as well as an update on the 1223 setting.
- Less Restrictive Updated NUV Detector Shutdown Limits for M-Dwarf Observations
- Move to Lifetime Position 4 (LP4) of the COS FUV Detector
- 1223 Central Wavelength No Longer Being Offered
The COS team has recalculated the NUV detector shutdown limits used for screening M-dwarfs. These limits protect the detector in the event of a flare. Using a new method for calculating these limits, which relies on the fraction of light falling into the brightest pixel and the actual shutdown criteria coded into the flight software, we are now adopting the following less restrictive values for the NUV local rate limit:
NUV imaging: 104 cts/sec/pixel
NUV spectroscopy: 180 cts/sec/pixel
These values can be used for both target acquisitions and science exposures, and apply to currently approved and future COS NUV observations of M-dwarfs. The FUV limits remain unchanged. Full information on our M-dwarf bright-object clearance procedures is available in COS ISR 2017-01.
On October 2, 2017, routine operations of the COS FUV channel were moved to a new Lifetime Position, LP4. This date was chosen to coincide with the start of Cycle 25. All Cycle 25 COS/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 was made in order to mitigate the effect of gain sag and provide the community with access to an unsagged region of the FUV detector. LP4 is located 5.0 arcseconds below LP1 in the cross-dispersion direction, and 2.5 arcseconds below LP3. The spectral resolution at LP4 has been confirmed to be ~10-15% below its value at LP3, depending on cenwave and wavelength, as expected. The move to LP4 is accompanied by a new set of restrictions on detector segment, FP-POS position, and target acquisition mode, which are designed to maximize the lifetime of the FUV detector. These restrictions are discussed in full on the COS2025 website.
During summer 2017, the COS team investigated the possibility of a new G130M central wavelength (cenwave) to be offered at LP4, known as the 1223 setting, intended to optimize FUVA resolution over the same wavelength range as 1222. This setting was advertised to the community for use in Cycle 25. However, following detailed programs to focus and characterize the spectral resolution of the 1222 and 1223 settings, it was found that 1222 has resolution on segment FUVB comparable to that of the G130M/1291 cenwave, and resolution on segment FUVA is within ~20% of that of G130M/1291. This was the resolution goal the new 1223 cenwave was trying to achieve, and so this finding removes the need for the 1223 cenwave. There is no need to offer two equivalent cenwaves, since there is significant internal overhead in maintaining the COS tools (ETC, pipeline, APT) for multiple cenwaves.
Therefore, the 1223 cenwave is no longer being offered, and all Cycle 25 COS observations using 1223 are being transferred to 1222. PIs of all relevant programs have been contacted. Information on the spectral resolution of each G130M setting is provided here: http://www.stsci.edu/hst/cos/cos2025/lp4_resolution.
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