November 29, 2023

About This Article

1. Slitessutils: A Python Package for Extracting and Simulating Wide-Field Slitless Spectroscopy for ACS and WFC3

R. Ryan

We present 'slitlessutils', the new STScI-supported package for analyzing wide-field slitless spectroscopy observed with all active HST instrument/detector configurations: ACS/WFC; ACS/SBC; WFC3/IR; and WFC3/UVIS.  Slitlessutils uses a forward-modeling engine to determine the mapping between a direct image and its corresponding slitless-spectroscopic images.  When coupled with detector calibrations such as the sensitivity curve, flat-field, pixel-area map, etc., these transformation coefficients enable the user to extract one-dimensional spectra from slitless observations and also to simulate a slitless-spectroscopic scene.  Slitlessutils offers two methods for extracting the spectra, which are delineated by the assortments of available HST orients.  Specifically, slitlessutils implements the LINEAR method (Ryan, Casertano, & Pirzkal 2018) for fields with multiple orients, as well as an HSTaXe extraction for fields with a single orient.  Additionally, slitlessutils provides several utility functions to prepare observed spectroscopic data for analysis (e.g., astrometric updates, background subtraction, and cosmic-ray rejection).  

Slitlessutils is written entirely in Python and is publicly available on GitHub/PyPI with documentation on ReadTheDocs. The software distribution includes worked examples that demonstrate several of the key analysis options, such as simulating a simple field, extracting an isolated calibration star, etc.  This initial public release includes basic functionality for ACS/WFC and WFC3/IR; the documentation describes the additional capabilities (incl. full ACS/SBC and WFC3/UVIS support) that are under active development and are anticipated to be available in near-term updates.  We welcome feedback from the community on the performance and code enhancements through the HST Help Desk.

2. ACS/WFC Focus-Diverse ePSF Webtool

G. Anand, N. Grogin, J. Anderson, Y. Cohen, A. Bellini

We present the newly released ACS/WFC Focus-Diverse ePSF webtool, available for public access at This tool allows easy access to focus-diverse, effective point-spread functions (ePSFs) for the Wide Field Channel on the ACS instrument. Uneven heating of the telescope’s assembly is known to cause variations in the shape of the PSF, and the empirical ePSF models provided here take this into account. The webtool is automatically updated to include the best-fit focus-diverse ePSFs for new ACS/WFC images every morning. This webtool is based on the initial work in ACS ISR 2018-08 (A. Bellini et al.), and further details can be found in ACS ISR 2023-06 (G. Anand et al.). Updates to the webtool will be advertised in the Change Log at the bottom of the webpage.

We also provide a Python module as part of the new 3.7.0 release of the acstools software package, which allows for the batch retrieval of the same focus-diverse ePSFs. A walkthrough tutorial will be provided soon in a Jupyter Notebook, which also showcases the usage of interpolation routines within the same Python module. For additional user support of this tool, please contact the HST Help Desk.

3. Dithering for ACS and WFC3 Primes and Parallels

J. Anderson and N. Grogin

The dither patterns available in APT were designed with only one instrument in mind -- the instrument that is “prime”. In ACS ISR 2023-04,  we explore how effective the prime-instrument-based “box” patterns are for observations taken in parallel. To this end, we have developed a metric to describe good and bad pixel-phase coverage. Not surprisingly, we find that a pattern that has been optimized for one detector observed in prime is often quite poor for another detector observed in parallel. We constructed some additional patterns in the form of POS-TARGs that achieve a good sub-pixel dither for both prime and parallel observations for ACS/WFC and the two WFC3 cameras. It is worth noting that on account of distortion, there are sometimes tradeoffs between achieving good pixel-phase coverage and mitigating artifacts (bad pixels, blobs, persistence, bad columns, etc.). In the process of this exploration, we discovered that the then-current ACS box dither likely got corrupted by post-SM4 changes in the SIAF files. We have since corrected those dither specifications to provide the intended sub-pixel phase sampling. In ACS ISR 2023-04, we provide 2-point and 3-point dithers in Appendix B that are good in prime/parallel instruments, in addition to the 4-point dithers. Users can group N dithers into sets of 2, 3, or 4 to achieve a good N-point dither in both prime and parallel. Details are published in ACS ISR 2023-04. For additional questions, please contact the HST Help Desk.

4. HSTaXe — ACS and WFC3 Cookbook Tutorials

B. Kuhn, D. Som, A. Pidgeon, N. Hathi, R. Ryan, R. Avila, N. Pirzkal

In the recently published ACS ISR 2023-05, we discuss a collection of six Jupyter Notebook tutorials that were released on the HSTaXe GitHub repository in Spring 2023. These ‘cookbooks’ present examples of how to preprocess data from ACS and WFC3 slitless spectroscopic modes and use the core HSTaXe routines to extract 1D spectra. The specific preprocessing procedures, described in the ISR and cookbooks, are meant to highlight three steps of the data analysis process users should consider in order to obtain optimal spectral extraction with HSTaXe. The three steps include a custom multi-component background subtraction for WFC3/IR grism data, embedding subarray data into a full-chip image, and checking that the active World Coordinate System (WCS) of dispersed images matches the corresponding direct images. We will continue to maintain the cookbooks to ensure their compatibility with any future changes to the HSTaXe software, and any issues with the installation or running of the cookbooks should be reported to the HST Help Desk. Finally, users seeking additional information and resources pertaining to slitless spectroscopic modes should visit the ACS prism/grism performance page and/or the WFC3 Grism Resources page.

5. Recent Instrument Science Reports

For a complete list with abstracts, please visit the ACS ISR webpage.

  • ACS ISR 2023-03 “Measuring the Column Dependence of Read Noise on ACS/WFC Bias Frames” (Guzman and McDonald)
  • ACS ISR 2023-04 “Dithering for ACS and WFC3 Primes and Parallels” (Anderson and Grogin)
  • ACS ISR 2023-05 “HSTaXe – ACS and WFC3 Cookbook Tutorials” (Kuhn et al.)
  • ACS ISR 2023-06 “The ACS/WFC Focus-Diverse ePSF Webtool” (Anand et al.)


HST Help Desk 

Please contact the HST Help Desk with any questions.