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WFC3 Grism Resources

WFC3 offers slitless spectroscopy with both the ultra-violet/optical (UVIS) and infrared (IR) channels. The table below gives a brief summary of the WFC3 grism capabilities.

WFC3 Spectroscopic Capabilities

range (nm)
Direct Filters
UVISG280190-45070 @ 300nm1.3F300X or F200LP
IRG102800-1150210 @ 1000 nm2.45F105W or F098M
IRG1411075-1700130 @ 1400 nm4.65F140W or F160W

For point sources. For extended sources, the extent of the target in the dispersion direction effectively determines the spectral resolution.

A direct image is required as part of all grism observations in order to set the wavelength zero-point.

Frequently Asked Questions

If you encounter any problems related to the WFC3 grisms, please consult our FAQ page. The guide is based on questions submitted to the Help Desk and is updated with user feedback.

For further questions, do not hesitate to contact our Help Desk.

Latest Developments

  • LINEAR is a new software package for simulating WFC3/IR grism observations at multiple rotation angles.
  • New source-dependent master sky images have been made available for the WFC3/IR grisms: ISR 2015-17 (Brammer et al., 2015). Further information and downloads are available on the IR grism master sky images page.
  • IR Grism Wavelength Solutions Using the Zero Order Image as the Reference Point: ISR 2015-10 (Bohlin et al., 2015)
  • Enabling Observations of Bright Stars with WFC3 IR Grisms: ISR 2014-15 (Deustua et al., 2014)
  • Time-varying excess Earth-glow in the WFC3/IR Chanel: ISR 2014-03 (Brammer et al., 2014)

   A full list of WFC3 instrument science reports (ISRs) can be found here and grism-related ISRs are listed below.

   A full list of Space Telescope Analysis Newsletters (STAN), sent to observers with updates which may affect observation planning or analysis, can be found here.

   Up-to-date advisories regarding the status of the instrument are available here.

Table of Contents

I. Proposal Information & Documentation Links to manuals and web pages relevant to Phase 1 & Phase 2 proposals preparation.
II. Data Analysis Provides links to manuals and web pages for analyzing WFC3 grism data.
III. Additional Resources Further information regarding the WFC3 grisms, including resources outside STScI.
IV. Grism-related Instrument Science Reports List of ISRs directly relevant to the WFC3/Grism.

I. Proposal Information & Documentation

WFC3 Instrument Handbook Overview of instrument capabilities.
HTML and PDF (see Chapter 8)

Slitless spectroscopy with spatial scanning is a recently introduced mode of the instrument. To plan observations in this mode, please refer to Chapter 8.6 and resources therein.
WFC3 Data Handbook Guide to using the data obtained with WFC3. Chapter 9.4 is specific to grism modes.
Proposal Tools A list of tools useful for planning proposals is available here.
Relevant for WFC3 grism observation planning are the Astronomer's Proposal Tool (APT) and dithering information.
Exposure Time Calculators Used for estimating the exposure time for WFC3/UVIS and WFC3/IR grism observations. For planning slitless spectroscopy with spatial scanning, please refer to Chapter 8.6 of the WFC3 Instrument Handbook and resources therein.
aXeSIM We highly recommend that users utilize aXeSIM to simulate their WFC3 grism observations. In addition to the version available for download, a limited-feature version of aXeSIM is available as a web application.

II. Data Analysis

Detailed information on the data analysis of WFC3 grism data is provided in Chapter 9.4 of the WFC3 Data Handbook. This is the recommended starting point for all novice users.

CALWF3 Pipeline Processing

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 information 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.
Do I need to recalibrate? Calibration files improve incrementally as we acquire more data and understand the instrument better. To find out if improved calibration files are available for an old data set, follow this guide.
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.

aXe Spectral Extraction Software

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 aXe, which was developed by the Space Telescope European Coordinating Facility (ST-ECF). This package enables automatic and reliable extraction of large numbers of spectra from individual images.

Installation aXe is included in the most recent versions of UREKA. The installation directions can be found here.

IMPORTANT FOR SUB-ARRAY MODE: if your data (grism and/or direct image) were taken in this mode, you must embed it in a full-frame before analyzing with aXe. A python script to do this is available here along with instructions.
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.

An UVIS Cookbook is not currently available. In the interim, ISR 2011-18 provides helpful suggestions on using aXe to reduce G280 data.
Visualization aXe2web is a software tool which produces an html summary of each reduced object such as in this example. Documentation and software can be found here.

The configuration and calibration files used with the aXe extraction software and the aXeSIM simulation 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 aXe and aXeSIM, 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.

G280 UVIS Grism All files:
Information on the derivation of the G280 calibration files as well as individual file downloads are available here.
G102 IR Grism All files:
Information on the derivation of the G102 calibration files as well as individual file downloads are available here.
G141 IR Grism All files:
Information on the derivation of the G141 calibration files as well as individual file downloads are available here.
Zeropoints for WFC3 Filters The zeropoint of the reference image filter is required by aXeSIM and aXe in the *.cat files:
Note: aXe and aXeSIM only accept ST and AB MAG zero-points.
Pivot Wavelengths for WFC3 filters The reference filter pivot wavelengths are required for the *.cat files used with aXe and aXeSIM:


The WFC3 IR detector is susceptible to persistence which appears as afterglow in parts of the detector that experienced high fluence (total number of photo-electrons released) in earlier exposures. Persistence is not corrected for in calwf3 or aXe. Examples of persistence and suggestions on how to mitigate it are given in the WFC3 Data Handbook.

STScI provides a persistence model for each WFC3 IR dataset through a separate persistence search form which allows users to view and download the persistence model. Search for the dataset of interest and then follow the links under "Dataset" to view the model and the links under "Visit" to download the model. A full description of the data products is provided here. A detailed description of the current modeling algorithm can be found in ISR 2015-15.

Observers and archive users are encouraged to contact the help desk if the persistence subtraction substantially limits their ability to extract science from an image or set of images.

IR Grism Master Sky Images

Master sky images have been provided for both G102 and G141, however the single images used by aXe do not take into account the full complexity of the sky background of grism observations. A more accurate background subtraction can be achieved by using separate images for each of the background components: zodiacal light, He I emission and scattered light (for G141). Further information and downloads are available on the WFC3 grism master sky page and in WFC3 ISR 2015-17. The use of the new sky images has not been implemented in aXe.

To take advantage of the new sky images, users are encouraged to follow the algorithm described in the appendix of WFC3 ISR 2015-17.

III. Additional Resources

LINEAR LINEAR is a suite of IDL and C routines designed to simulate WFC3/IR grism spectroscopy observations at multiple positions/rotation angles. 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). 1-dimensional spectral extraction tools are in development. The code will be described in forthcoming publications by R. Ryan & S. Casertano.
WAYNE WAYNE is a WFC3 IR Grism Spectroscopy simulator described in Varley & Tsiaras (2015). It can simulate both staring and spatial scanning observations. Currently the code is not public but requests for access can be sent to the author.
Webpage for AGHAST - A Grism H-alpha SpecTroscopic survey led by Ben Weiner which has obtained G141 grism observations in GOODS-N.
WISPS Team Page
Webpage for 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 site 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 Team Page Webpage for the 3D-HST Survey - a spectroscopic galaxy evolution survey with HST. 3D-HST has 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)
2010 STScI Slitless Spectroscopy Workshop The 2010 workshop highlighting scientific results of slitless spectroscopy and tutorials on using and analyzing data from the WFC3 Grism. A direct link to the archived videos of the talks can be found here.
PandExo Homepage of PandExo: A community tool for transiting exoplanet science with JWST & HST

IV. Grism-Related Instrument Science Reports

Instrument Science Reports (ISRs) are STScI in-house refereed technical papers which address specific topics related to instrument and telescope performance or data analysis techniques. A full list of WFC3 instrument science reports (ISRs) can be found here. This grouping of ISRs has been compiled to provide additional information to observers beyond what is found in the Instrument Handbook.

ISR 2017-01: A More Generalized Coordinate Transformation Approach for Grisms Describes an updated version of the grism dispersion parametrization.
ISR 2016-15: Trace and Wavelength Calibrations of the WFC3 G102 and G141 IR Grisms Description of new calibrations of the field dispersion of the trace and wavelength solutions of +1, -1 and 2 orders of the G102 and G141 WFC3 grisms.
ISR 2015-17: Source-dependent master sky images for the WFC3/IR grisms Improvements to aXe master sky images.
ISR 2015-10: IR Grism Wavelength Solutions Using the Zero Order Image as the Reference Point Discusses calibration of grism wavelengths using zeroth order images.
ISR 2014-15: Enabling Observations of Bright Stars with WFC3 IR Grisms Demonstrates the use of the IR grisms and spatial scan mode to observe stars of up to zeroth V-magnitude.
ISR 2014-14: Attempts to Mitigate Trapping Effects in Scanned Grism Observations of Exoplanet Transits with WFC3/IR Describes tests to characterize and ameliorate the apparent rise in flux at the beginning of IR grism transiting exoplanet observations.
ISR 2014-03: Time-varying Excess Earth-glow Backgrounds in the WFC3/IR Channel Investigates the cause of strong time-dependent variations of the background levels observed in a variety of filters of the infrared (IR) channel of the WFC3 as due to metastable helium emission at 10,830 Angstroms. Characterizes the effect on archival observations and recommends mitigation strategies.
ISR 2012-06: Flux Calibration Monitoring - WFC3/IR G102 and G141 Grisms Confirms stability in +1 order G102 and G141 flux calibrations to within 1% over WFC3 lifetime.
2011-09: IR Detector Timing & Persistence Information on persistence in the IR detector for full & sub-arrays.
2011-05: Revised Flux Calibration of the WFC3 G102 & G141 Grisms Update of flux calibrations, aperture corrections for the +1 order, and throughput variations across the FOV.
2011-04: WFC3/IR Banding How to deal with background pixels a few DN above of the image in full & sub-array frames, including observing techniques to avoid.
2011-01: Master Sky Images for the WFC3 G102 & G141 Grisms Details regarding construction of new master sky frames for data analysis and discussion of possible time-variations of background sky brightness.
2010-17: WFC3/IR Persistence as Measured in Cycle 17 using Tungsten Lamp Exposures Measurement of persistence from internal Tungsten Lamps to assess impact of bright illumination on observations.
2010-16: IR Channel Subarray Dark Current Behavior Discussion of the effects of, and how to deal with, dark current for sub-arrays.
2010-13: WFC3/IR Bad Pixel Table: Update Using Cycle 17 Data Information regarding updated bad pixel tables, including values assigned to known bad areas (i.e. blob, death star, etc...)
2010-12: Proposal 11913-IR Filter Wedge Check Information on offsets in pixel position for all IR filters. This ISR is important for programs using direct images obtained in filters other than F140W.
2010-09: Dithering Strategies for WFC3 Guidelines for dithering strategies. This ISR is useful for Phase 2 proposal preparation.
2010-07: First On-orbit Measurements of the WFC3-IR Count-rate Non-Linearity Discussion of non-linearity in the WFC3 IR detector and comparison with similar phenomenon in ACS and NICMOS.
2010-06: The WFC3 IR "Blobs" Discussion of blemishes which have appeared in the WFC3 IR detector. This ISR is useful for developing dithering strategies for Phase 2 proposal preparation.
2009-18: WFC3 SMOV proposal 11552: Calibration of the G102 Grism Provides calibration information on the trace, resolution and wavelength solution of the G102 IR Grism.
2009-17: WFC3 SMOV proposal 11552: Calibration of the G141 Grism Provides calibration information on the trace, resolution and wavelength solution of the G141 IR Grism.
2009-01: The ground calibration of the WFC3/UVIS G280 Grism Provides calibration information on the trace, resolution and wavelength solution of the G280 UVIS Grism.
2007-03: Performance of the WFC3 Replacement IR Grisms Summarizes the preformance of the G102 and G141 grisms. This report includes technical specicifications of the grisms, their installation, throughput, and dispersion.
2001-07: WFC3 Data Formats in OPUS & STSDAS Describes content and structure of WFC3 science data files generated by OPUS and used in processing with STSDAS tasks.