About this Article

Russell Ryan (rryan[at]stsci.edu) and the Slitlessutils team

Published June 16, 2026

Wide-Field Slitless Spectroscopy (WFSS) observations from Hubble remain a key capability for low-resolution spectroscopy of all sources in a field. The community has long relied on aXe to reduce this data. We are pleased to share that we have released version 2.0 of the Slitlessutils package to replace aXe in most applications. The new software is designed to handle WFSS images obtained by Hubble’s Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3).

Slitlessutils is written entirely in Python and uses most of the same, or similar, calibration files as aXe. Slitlessutils employs a full-forward-model that transforms the direct image and segmentation map into predicted WFSS images, capturing the wavelength-dependent flux distribution of every source.

The outputs of this forward model are cached and used by Slitlessutils to simulate WFSS images, extract one-dimensional spectra, create simple regions files highlighting the pixels for each source and spectral order, and group regions (i.e., regions whose spectral traces overlap in a collection of WFSS images). The simulation module includes a simplified noise model of each detector, but can also produce noiseless images, which are effectively equivalent to the contamination images generated by aXe.

For the extraction, Slitlessutils offers two modes: simple fixed aperture extraction and the linear-reconstruction methods developed by Ryan, Casertano, and Pirzkal (2018). The regions and grouping modules are useful to inspect the anticipated contamination, astrometric alignments, and other data introspection needs.

Additionally, Slitlessutils provides methods for several necessary preprocessing steps, including astrometric registration, background subtraction, and cosmic-ray flagging.

An extensive documentation suite describes the algorithms, calibrations, and relevant internal and external data structures. Slitlessutils also provides many examples for analysis and simulation of in-flight ACS and WFC3 data. These examples are provided as both a submodule that may be directly imported into Python and as Jupyter notebooks to provide interactive data exploration. The primary codes have docstrings that describe their APIs for users who may wish to extend Slitlessutils or develop additional capabilities.

There are several improvements planned for future versions of Slitlessutils. These may include support for ACS SBC observations; extension to Webb, including reorganization of the Hubble reference file structure to align with Webb’s; cleaner control of the fixed-aperture widths, the contamination model, and the preprocessing utilities; implementation of the IFU-like flux-cube spectral extraction; and non-uniform cross-dispersion weights either from calibrations (for point sources) or observed models (for extended sources).

Slitlessutils may be obtained directly from GitHub or installed from PyPI.