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July 6, 2021

July 2021 STAN

In this STAN we announce new COS walkthrough notebooks and provide information for preparing Cycle 29 Phase II proposals.

First Batch of COS Walkthrough Notebooks Released

The COS team has built a suite of eight Jupyter Notebooks to walk new and returning COS data users through common data analysis procedures. The procedures explained in the Notebooks include basics such as downloading, plotting and binning a spectrum, as well as more complex processes, like convolving line spread functions and altering spectral extraction boxes. They have been written for a broad audience, from professional astronomers to undergraduate students. The Notebooks are hosted by the STScI GitHub. The COS team plans to release Notebooks on new topics and implement improvements to the existing tutorials. More information on the Notebooks and detailed installation instructions can be found on the STScI website.


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Multiple Lifetime Positions in Cycle 29

Every few years, the location where spectra fall on the COS FUV detector is changed. This is one strategy to reduce the effects of gain sag, where the ability of a given location on the detector to record photons diminishes with repeated exposure to light. With past changes in lifetime position (LP), nearly all FUV settings have moved from the previous LP to the new one. Beginning in Cycle 29, nearly every LP will host a subset of the settings. Figure 1 schematically illustrates the locations of each LP on the COS FUV detector segments and also serves as a guide to the COS2025 policies discussed below.

  • Cenwaves G130M/1055 and G130M/1096, the blue modes, will continue to be observed at LP2.
  • G140L cenwaves will move back to LP3.
  • G130M/1222 and all G160M cenwaves will remain at LP4.
  • G130M cenwaves 1291 and up will move to a new LP5.

This change will be mostly transparent to users, although the spectral resolution and sensitivity vary slightly with LP. The characteristics of lifetime positions through LP4 are discussed in existing documentation, while the characteristics of LP5 will be covered in forthcoming instrument science reports.

schematic of FUV detector segments A and B with colorful lines designating where the different FUV modes will fall on the detector
Figure 1.  A portrayal of where available modes will fall on each segment of the COS FUV detector in Cycle 29. For simplicity, the widths of the regions do not represent the actual cross-dispersion widths of each mode. Annotations also serve as a guide to the COS2025 policies discussed below.

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Reminder of the COS2025 Policies

Users preparing Cycle 29 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 Ly α emission. They were introduced when COS FUV spectroscopy moved to Lifetime Position 4 (LP4) in 2017 and will continue when the affected cenwaves move to LP5 at the beginning of Cycle 29. Under COS2025, there are 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 of the zero-redshift Ly α wavelength range can be performed at LP3 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.


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Best Practices for COS 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.


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New Dark Rates and Throughputs Adopted for ETC v29.2

The dark rates for the COS FUV detector (FUVA and FUVB) and the COS NUV detector are monitored regularly. The dark rate of the FUV detector, primarily segment A, experiences occasional changes from its nominal baseline. See COS ISR 2019-11 for discussion of the variable, spatially structured component of the FUVA dark rate and recent efforts to more accurately account for it. 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.

The FUV dark rates adopted by ETC version 29.2 have changed since the previous version. The new dark rates are 2.01E-6 counts/sec/pixel (up by 1%) and 1.70E-6 counts/sec/pixel (down by 11%) for FUVA and FUVB, respectively. The dark rates for Spectroscopic Target Acquisition are now 3.13E-6 counts/sec/pixel (up by 4%) and 2.83E-6 counts/sec/pixel (down by 7%) for FUVA and FUVB, respectively.

The NUV dark rate adopted by ETC version 29.2 has been raised to 1.20E-3 counts/sec/pixel (up by 6%).

Additionally, the ETC has been updated with the latest throughputs for all COS modes, incorporating changes in sensitivity with time. PIs are strongly encouraged to use the latest version of the ETC for determining exposure times for use in their Phase II proposals.


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