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March 2017 STAN

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STScI Analysis Newsletter (STAN)

Cosmic Origins Spectrograph (COS)

March 2017

In this STAN we provide updates on COS to users preparing a program for the Cycle 25 HST Phase 1 deadline.


Moving the COS FUV channel to Lifetime Position 4 in July 2017

In July 2017 the position of the spectrum on the COS FUV detector will be moved from Lifetime Position 3 (LP3) to Lifetime Position (LP4). LP4 is located at -5.0 arcseconds in the cross-dispersion direction relative to the original Lifetime Position (LP1) and -2.5 arcseconds below the current position, LP3. The move is being made to alleviate the effect of gain sag, which leads to localized gain losses, and will prolong the lifetime of the FUV detector. The exact date of the move will be announced shortly. The move will be transparent to users but is expected to result in a loss of spectral resolution of ~10-15% from the current value at LP3. The ACQ/PEAKXD FUV spectroscopic target acquisition (TA) sequence at LP4 will be modified from that used at LP3 and before, and so PIs writing Cycle 25 phase Is will use this modified TA strategy. In case any changes are needed to pre-Cycle 25 programs that have not yet executed at the time of the LP4 move, STScI will work with the affected PIs to accommodate the new TA strategy. More information on the modified ACQ/PEAKXD sequence can be found in Section 8.5 of the COS Instrument Handbook.

Redistribution of COS G130M CENWAVEs

In order to continue mitigating the effects of gain sag caused by airglow emission in the FUVB detector when observing with the G130M grating, STScI reserves the right to switch the G130M central wavelength (CENWAVE) setting of any program to another G130M CENWAVE. This is to ensure that the G130M usage in Cycle 25 is such that a single CENWAVE is not unreasonably causing the detector to sag. Users who require a specific G130M CENWAVE must justify it in the phase I. PIs of successful Cycle 25 GO programs may be contacted about their choice of G130M CENWAVE after the submission of their Phase II programs, when the COS team assesses the distribution of requested G130M CENWAVEs in approved programs. This is described in Section 4.4.4 of the HST Primer.

New Bright Object Screening Limits for M-Dwarf Observations

The COS and STIS teams have been investigating the potential for UV flares from M-dwarfs, which have a small, but non-negligible, chance of causing damage to the UV detectors via irregular and unpredictable outbursts. Brightness variations of 8 optical magnitudes have been observed in some active M-dwarfs. We have developed new bright object protection limits that account for the possibility that an M-dwarf may flare during a COS or STIS observation. These limits and a description of the procedures necessary to apply them are described in COS ISR 2017-01.

All COS observations of M-dwarfs are subject to these new guidelines (including accepted but not-yet-executed Cycle 24 GO programs). PIs wishing to obtain future UV observations of M-dwarfs should use these guidelines to determine whether (1) their planned acquisition exposures are safe; (2) their planned science exposures are safe; (3) whether a switch from COS to STIS would be appropriate for their program, or (4) whether a target switch is necessary. COS contact scientists are in contact with PIs of accepted Cycle 24 programs observing M-dwarfs to determine the best course of action for each case.

New COS background levels adopted in ETC v25.1

The dark rate for both COS FUV detectors (FUVA and FUVB) is monitored closely. The dark rate in both FUVA and FUVB has been stable with a low scatter since mid-2015, following the solar maximum, as shown in:

As a result of the lower levels, the FUV dark rates adopted by the COS Spectroscopy ETC in version 25.1 have been lowered to 1.78x10-6 (FUVA) and 1.68x10-6 (FUVB) counts/s/pixel. For faint targets, these lower levels reduce the exposure time needed to reach a given S/N level. For spectroscopic acquisition ETC calculations, the COS dark rates have been changed to 3.08x10-6 (FUVA) and 3.17x10-6 (FUVB) cts/sec/pixel, and for the NUV detector the dark rate adopted in the ETC is 8.23x10-4 cts/sec/pixel. PIs are strongly encouraged to redo any exposure time calculations included in a Cycle 25 COS GO proposal with the latest version of the ETC.

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