About This Article
In this STAN, we discuss items of interest to those who are preparing Phase II proposals for Cycle 32, provide updates on recent instrument calibration efforts, and share information on the new Reduced Gyro Mode for Hubble.
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.
- If a target consists of multiple sources, a target field is crowded, or a target is faint, consider executing an offset target acquisition.
- 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.
- Follow the target list and instrument configuration specified in the approved Phase I proposal. Changes of grating, central wavelength, or lifetime position 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, more restrictive scheduling constraints, 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.
- Consider alternative observational approaches to achieve your science goals if observations require a string of more than 6 consecutive orbits, as such strings will execute at shared risk (i.e., it will not be eligible for repeat if impacted by observatory problems).
Observing Bright Objects with COS: Updated Guidance for Phase II Proposals
Recent findings and internal reviews have highlighted significant impacts on the lifetime of the COS instrument when observing bright objects with exposures longer than necessary for a maximum achievable signal-to-noise ratio (SNR). The maximum achievable SNR of an observation is limited by the fixed pattern noise on the detector, which is a function of grating and the number of FP-POS obtained. Observations with ETC SNRs over this limit generally do not yield improvements in usable signal and thus reduce the life of the detector without adding to the science return of the program. Observations should limit exposure times within an orbit such that they do not exceed the maximum achievable SNR for a given combination of grating and number of employed FP-POS, provided as the 50th percentile measurements listed in Table 1 of COS ISR 2023-11, with exceptions for specific science cases as justified in Phase Is.
Following the prescribed guidelines in Phase IIs helps preserve the longevity of the COS instrument while continuing to achieve high-quality scientific results. The COS team will also review programs for exposure time efficiency and request revisions if there are not clear justifications in the Phase I proposal for the exposure times chosen. For any questions or further assistance, please contact your Contact Scientist.
Observing with COS in Reduced Gyro Mode (RGM): Brief Guidance for Observers
Operating HST in Reduced Gyro Mode (RGM) impacts scheduling efficiency and flexibility, reducing the available field of regard to approximately 40-50% of the sky at any given time. This decrease primarily affects the ability to observe transient events, schedule long sequences of COS observations, and coordinate observations with specific timing or orientation constraints. Observers are advised to avoid scheduling constraints unless essential for their science goals as they can result in a significantly delayed program execution. Observers are reminded that increasing the visit schedulability percentage (i.e., shortening the visibility) in Phase IIs makes visits less efficient in terms of how much can be accomplished in each orbit, but easier to schedule.
For detailed information on RGM and its implications, please refer to our webpage on Reduced Gyro Mode Tips and Resources and the HST Reduced Gyro Mode Primer
Delivery of New Near-UV (NUV) Time-Dependent Sensitivity (TDS) File
A new Near-UV (NUV) Time-Dependent Sensitivity (TDS) reference file has been delivered, updating all modes except G285M. This update incorporates wavelength dependence to better suit NUV flux calibrations and uses data from the NUV Spectroscopic Sensitivity Monitor up to August 2023 and a supplemental monitor (PID 17395). The TDSTAB format was refined from 50-Angstrom to 10-Angstrom bins for increased precision, with cubic fits applied to G225M and G230L, and a linear fit to G185M. Testing included comparing TDS observations from PID 17319 and NUV Target Acquisition observations. Unlike previous updates for the FUV, this new NUV TDS file does not require an updated FLUXTAB. All observers with NUV COS data taken in 2010 or later should redownload their observations to take advantage of the TDS update.
The residuals of the original TDSTAB (black circles) increase over time since first light. A wavelength dependence was implemented to account for the sensitivity loss (blue circles), bringing
the measurements back within spec.
COS Poster at the 244th Meeting of the American Astronomical Society
A COS poster was presented at the AAS meeting in Madison, Wisconsin, and is available on the AAS website . We invite all COS users to review it to learn about the latest developments for COS. Additionally, all COS posters are archived on the COS poster page.
Abstract: After 15 years of operations, the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope remains scientifically productive and continues to provide a high-quality spectroscopic view of the ultraviolet Universe through the far-ultraviolet (FUV) and near-ultraviolet (NUV) channels. The FUV channel is currently operating at five different lifetime positions (LPs), with each LP being used for a subset of observing modes. Here we present updates on the current status and performance of COS and summarize recent calibration and user support work of interest to all COS users. First, we report on new geometric and walk corrections which will be made available soon with corresponding updated calibration reference files. Second, we provide updated time-dependent sensitivity corrections for both the FUV and NUV based on regular monitoring. For the FUV, we implement a steeper slope from 2023 onward to correct for the high solar activity. For the NUV, we implement a wavelength-dependence for all available and supported gratings and will improve flux calibration by 70% for the medium resolution. Third, we show that the dark rates continue to be atypically high, as expected due to the peaking solar activity in 2024, but are largely unchanged from the last update to the dark rates.