About This Article
1. Phase I Proposal Preparation Tips
J. Green, A. O'Connor
The Phase I Cycle 32 deadline is March 26! Please keep in mind the following points about WFC3 when preparing your proposal submission:
- NEW: If your observations force more than 6 consecutive orbits, that string will execute at shared risk (i.e., it will not be eligible for repeat if impacted by observatory problems). Observers should consider alternate strategies, as there is significant risk for any observation lasting more than a few orbits.
- Phase 1 proposals should be as explicit as possible about requirements and restrictions; anything needed will require special justification that should be available to the panels. For example, Special Requirements such as timing and orientation constraints MUST be specified in phase 1 (see HST New and Important Features). We encourage submitted proposals to minimize scheduling constraints (see HST Proposal Implementation and Execution).
- Please include all the intended filters in your observing proposal (even pre-imaging for exoplanetary observations) - adding or swapping them requires some justification later on.
- Please remember to search for and address all duplications with previous sources. If your science targets involve variability, noting that in the duplication section will expedite the justification process. For previously-observed exoplanet targets, see WFC3's TrExoLiSTS.
- When using the ETC, include your estimated postflash values to obtain the correct S/N values for your observations.
- Late-breaking news items are announced on the Cycle 32 Call for Proposals coverpage. The most recent announcement includes reminders of the special requirements and information for proposers requiring more than 6 consecutive or considering HST-JWST joint programs.
2. WFC3/IR Time-Dependent Sensitivity
M. Marinelli, V. Bajaj, K. Huynh, and D. Som
Recent analysis over a 15-year baseline of calibration data indicates cumulative photometric sensitivity losses for the WFC3/IR detector of about ~1-2% since installation in 2009 (see Table 1 below). A new IMPHTTAB reference file, with updated inverse sensitivities, should be delivered in 2024. In the interim, we provide correction factors users can apply to manually correct their photometry.
The photometric sensitivity losses range between 0.06% and 0.120% per year, depending on filter. In an upcoming ISR (Marinelli et al., in prep), we will provide corrections for relative photometry across multiple epochs. Table 1 (below) lists the recommended sensitivity corrections for F098M and the wide band filters; for narrowband and the remaining medium filters, use the nearest wavelength solution available.
The changing photometric sensitivity was evaluated using staring mode photometry of globular clusters (see ISR 2022-07 for previously published results), scanning mode photometry of an open cluster (see ISR 2021-05 for previously published results), and grism observations of four CALSPEC standard white dwarf stars (see Bohlin & Deustua, 2019 for methodology). An upcoming ISR (Som et al., in prep) will present updated results for the open cluster scanning calibration program as well as grism observations of CALSPEC standards. Below, Figure 1 shows the recommended photometric corrections as a function of wavelength/filter. Staring mode observations of CALSPEC standards were used for testing and validation.
| Filter | Recommended Correction Factor (% / year) | Sensitivity Loss Since 2009 (%) |
|---|---|---|
| F098M | 0.120 +/- 0.003 | ~1.68 |
| F105W | ||
| F110W | ||
| F125W | 0.075 +/- 0.006 | ~1.05 |
| F140W | 0.060 +/- 0.005 | ~0.84 |
| F160W |
Table 1: Recommended correction factors for the time-dependent sensitivity losses of WFC3/IR. To manually correct your relative photometry (in counts or count rate) we suggest the following steps:
- Calculate ΔT = MJD - 55008.0 (ie. observation date - initial date).
- Multiply ΔT by the recommended correction factor for the appropriate filter (for narrowband and medium filters, use the nearest wavelength solution available) to yield the amount of missing flux, as a percentage.
- Multiply either the imaging data or the existing, measured photometry (for existing photometry) by the percentage of missing flux to yield the actual amount of missing flux, in flux units (counts or count rate).
- Add missing flux back in.
Figure 1: Recommended photometric corrections (percentage per year) are displayed as colorful, horizontal dashed lines spanning the pivot wavelengths of the corresponding filters. Measurements of sensitivity loss rates from previous studies are represented as gray points with error bars (for multiple measurements in the same filter, the pivot wavelengths are slightly offset horizontally to ensure readability).
3. Updates to DrizzlePac's Readthedocs
M. Revalski, J. Mack, S. Goldman, R. Avila, G. Anand, B. Kuhn
DrizzlePac is STScI's primary tool for aligning and combining HST images, and is documented through the DrizzlePac website, handbook, notebooks, and readthedocs. The DrizzlePac team has restructured the readthedocs webpages to more clearly describe DrizzlePac's functionality and common workflows. In addition, extensive efforts have been made to separate the documentation into two main categories. The first contains materials that are most relevant to users who want to reprocess their data, while the second describes the functionality of the MAST archive pipeline that generates single and multi-visit mosaics. Specifically, the main navigation bar has been reorganized, and the landing pages now contain a description of the information detailed in each section. Users are encouraged to submit their questions and feedback through the HST Help Desk.
4. Updated Configuration & Sensitivity Files for HSTaXe
M. Alam, B. Kuhn, A. Pagul, D. Som, A. Calamida, S. Baggett
WFC3/UVIS G280 spectra extracted with HSTaXe require the use of configuration files, which include as input the absolute sensitivity calibration of the detector. Configuration files (version 2.0) downloaded prior to March 6, 2024 can result in spurious spectral features (e.g., a bump around ~2100 A in the extracted spectra of GRW+70 5824; see below and Figure 3 of Rothberg, Pirzkal, & Baggett 2011) that are inconsistent with the expected SED for a given source. This discrepancy is due to these configuration files pointing to older detector sensitivities that were derived during TV3 (thermal vacuum testing). Updated configuration files pointing to the most recent flux calibrations for the G280 grism (Pirzkal 2020) are now available on Box and the WFC3 G280 Calibration page and provide improved extracted spectra. We therefore recommend HSTaXe users to use the latest configuration files (version 2.5) moving forward. Spectral extraction cookbooks for HSTaXe can be found here. We will release a set of updated sensitivity files implementing the most recent CALSPEC SEDs in the near future. 
Figure 2: Left: WFC3/UVIS G280 spectra for GRW+70 5824 extracted using the TV3 sensitivities (blue) and the updated 2021 sensitivities (orange), compared to the CALSPEC SED (green). Right: Zoom in to ~1900-2500 Å.
The complete WFC3 ISR archive is available here. Additional information about WFC3 calibration, performance, data analysis, software tools, and more can be found online.
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