July 2019 STAN
New COS FUV Dark Rates Adopted in ETC v27.2
The dark rates for both the COS FUV detectors (FUVA and FUVB) and the COS NUV detector are monitored regularly by taking dark exposures every two weeks. As a result of higher levels observed in the FUV exposures starting towards the end of 2018, the FUV dark rates adopted by the COS Spectroscopy ETC in version 27.2 have been raised by about 45% for FUVA and 21% for FUVB. The new dark rates are 2.43E-6 counts/sec/pixel (FUVA) and 1.93E-6 counts/sec/pixel (FUVB). For faint targets, these will result in longer exposure times needed to reach a given signal-to-noise ratio. The dark rates for Spectroscopic Target Acquisition have been raised to 3.52E-6 counts/sec/pixel (FUVA) and 3.10E-6 counts/sec/pixel (FUVB). The levels in the NUV dark exposures have remained steady, and the dark rate adopted in the ETC continues to be 8.77E-4 counts/sec/pixel.
PIs are strongly encouraged to use the latest version of the ETC for determining exposure times for use in their Phase II proposals.
Further details about the dark rate monitor, including links to the latest plots of the dark rates against time may be found at the COS Monitoring page.
Reminder of COS2025 Policies Designed to Extend Lifetime of COS/FUV Detector
Users preparing Cycle 27 Phase II submissions are reminded that the COS2025 policies are still in effect. These policies consist of restrictions on the choice of detector segment and FP-POS positions for the G130M observing modes. The policies are designed to maximize the FUV detector lifetime by minimizing the exposure of the FUVB detector to geocoronal Lyman-alpha emission. They were introduced when the COS/FUV detector was moved to Lifetime Position 4 (LP4) on October 2, 2017. Under COS2025, there are now four G130M central wavelengths (cenwaves) that can be used with both detector segments on: 1055, 1096, 1222, and 1291. For the other G130M cenwaves (1300, 1309, 1318, 1327) only segment FUVA can be on. Observations requiring the Ly alpha wavelength range can be performed at Lifetime Position 3 and need to be justified in the Phase I. Observations with the G140L grating and with the COS/NUV channel are unaffected. While no restrictions are placed on G160M, some small wavelength ranges are now degraded due to gain sag. Detailed information about the changes is available at the COS2025 policies page.
Best Practices for COS Phase II Submissions
In this item we remind Cycle 27 COS users of best practices for preparing their Phase II submissions. Submitting a safe, technically sound Phase II file that adheres to all our policies will reduce the risk of scheduling delays. Users are reminded to:
- Provide ETC ID numbers for all exposures, including acquisition exposures.
- Verify that each exposure is safe by running the Bright Object Tool (BOT) in APT, using the GALEX catalog whenever possible. Programs submitted with unexplained BOT warnings may lead to a delay in scheduling the observations.
- Specify the buffer time for all TIME-TAG exposures. Correctly calculating the buffer time is important to ensure that no data are lost during readout. In most cases the buffer time should be 2/3 of the value calculated by the ETC, but there are exceptions for bright targets. Full details are given here.
- Follow the target list and instrument configuration specified in the approved Phase I proposal. Changes of grating or central wavelength can be requested by a minor change request to your Contact Scientist, provided there is no change to the science goals. More substantial changes (such as target changes, instrument changes, and anything that alters the science goals) need to be requested by a major change request to the Telescope Time Review Board (TTRB), using the link from the Program Status webpage.