Grism Data Analysis

Data downloaded from the Mikulski Archive for Space Telescopes (MAST) is processed on the fly by the calwf3 pipeline with the most recent calibration files. Users generally should not need to reprocess their data. The links below provide more information on the pipeline.

WFC3 Pipeline

A brief description of the calwf3 reduction, and its recent updates, is provided here. A more detailed description is available in the WFC3 Data Handbook and in the wfc3tools package documentation.

WFC3 Reference Files

The most recent individual calibration files for WFC3 are available from the Calibration Reference Data System. These files are used by calwf3 and Astrodrizzle. Reference files can also be retrieved from MAST or downloaded directly from the iref directory.

The calwf3 processing of the grism observations does not include extraction or calibration of the spectra. This must be done independently using additional tools. The current software supported by STScI is slitlessutils, which can be used for both spectral extractions and simulations. Additionally, slitlessutils has several built-in utilities to prepare observed spectroscopic data for analysis, such as astrometric updates, background subtraction, and cosmic-ray rejection.

Installation

Please find the most up-to-date installation instructions in the slitlessutils documentation.

Examples and Tutorials

There are WFC3 Jupyter tutorial notebooks, hosted on the hst_notebooks GitHub, that demonstrate a step-by-step workflow for downloading, pre-processing, and extracting spectra from grism data. There are currently three notebooks:

1.WFC3/IR  -- full-frame G102 extraction

2.WFC3/UVIS – full-frame G280 extraction

3.WFC3/UVIS – subarray G280 extraction

The tutorial notebooks have linked references throughout and are meant to be easily adaptable for alternate datasets. We encourage users to explore the notebooks, and we welcome feedback from the community on the performance and code enhancements through the HST Help Desk.

The slitlessutils package also has built-in examples located in the example folder.  Some of these scripts are like the Jupyter notebooks, such as wfc3ir.py and wfc3uvis.py. Successfully running the notebooks or example scripts is one way to validate slitlessutils installation.

Configuration

The configuration and calibration files required by the slitlessutils software are available for download directly through the package itself. A complete description of slitlessutils configuration is available in the documentation. The most important step is to initialize the Config()object with:
 

from slitlessutils import config
cfg = config.Config()

When run for the first time, Config() will automatically download the latest version of the files into a new dot-directory in the user’s  home:{HOME}/.slitlessutils. Once downloaded, slitlessutils will automatically apply the correct files based off information in the data’s header, such as TELESCOP, INSTRUME, DETECTOR, and FILTER.

LINEAR

LINEAR is a suite of IDL & C routines designed to simulate & extract WFC3/IR grism spectroscopy, ideally taken at multiple positions/orients. See Ryan, Casertano, & Pirzkal 2018 for more details on the implementation, verification, and improvements. Most packages extract spectra for individual ORIENTs and stitch the contamination-corrected 1-d spectra (for each roll angle) post facto. This is the fundamental distinction with LINEAR, where we attempt to solve for the non-parameteric spectrum that best fits the entire canon of available dispersed images (that may be rotated or dithered with respect to each other).  Example usage of the code is described by Ryan, Casertano, Pirzkal 2018 and  ISR 2018-13.

Note: The technique performed by LINEAR is now included directly in slitlessutils via the multi() module. Details are provided in the slitlessutils documentation. 

Wayne

Wayne is a WFC3 IR Grism Spectroscopy simulator described in Varley & Tsiaras (2015). It can simulate both staring and spatial scanning observations, and is publicly hosted on Github.

AGHAST

A Grism H-alpha SpecTroscopic (AGHAST) survey, led by Ben Weiner, obtained G141 grism observations in GOODS-N.

WISPS

The WFC3 Infrared Spectroscopic Parallel Survey (WISPS) is a pure-parallel WFC3 survey which has obtained IR grism observations in over 1000 random fields. The current data release provides access to reduced and drizzled direct near-IR imaging (F110W, F140W, & F160W), direct UVIS imaging (F475X & F600LP), and the near-IR grism spectra (G102 and G141).

3D-HST

The 3D-HST Survey was a spectroscopic galaxy evolution survey with HST. 3D-HST obtained G141 grism spectroscopy over four of the five CANDELS fields (AEGIS, COSMOS, GOODS-S and UDS). HST and ground-based images, photometric catalogs and grism spectra for all five CANDELS fields are available for download from the team website. The data reduction pipeline employed by 3D-HST is described in Momcheva, van Dokkum, Brammer et al. (2015).

The calwf3 processing of the grism observations does not include extraction or calibration of the spectra. This must be done off-line using additional tools. The current software supported by STScI is hstaxe which is a complete Python implementation of the Spectral Extraction and Visualization Software (aXe). HSTaXe has the same functionalities as aXe but, unlike aXe, does not require IRAF/PyRAF (no longer supported by STScI). HSTaXe can be used for the calibration and automatic extraction of large numbers of spectra from HST slitless spectroscopic observations.

Installation

Please find the most up-to-date installation information here.

Data Reduction

The WFC3 IR Grism Cookbook is a step-by-step guide to reducing G141 IR grism data using aXe. The procedures described in the Cookbook are directly applicable to the G102 grism with small modifications. The grism data from the WFC3 ERS Program used as an example in the Cookbook is also available for download.

Two more Cookbooks exist for reducing UVIS grism data, as described in ISR 2023-07. They address spectral extraction for UVIS full-frame and subarray data separately:

1. Spectral extraction with full-frame data from WFC3’s UVIS channel, and
2. Spectral extraction with subarray data from WFC3/UVIS

Configuration

The configuration and calibration files used with the HSTaXe software are available for download below. The download for each grism includes sensitivity, flat-field cube, and configuration files for different direct filter - grism combinations. To use these calibration products with HSTaXe, copy them into a local directory and set your AXE_CONFIG_PATH environment variable to point to that directory. No further re-formatting is needed.