aXeSIM aXeSIM web application

The Slitless Simulation package aXeSIM


A dedicated simulation package, aXeSIM, has been developed as part of the support for WFC3 slitless spectroscopic modes. While the package was initiated for exploitation of WFC3 slitless grism modes, it is equally applicable to other slitless spectroscopy modes of HST, such as ACS and NICMOS.

aXeSIM as a web application for WFC3

In addition to the aXeSIM task that is available in STSDAS, aXeSIM can also be run as a web application for the WFC3 grism modes. The web application offers only a subset of all the options available in aXeSIM. The user can select a WFC3 slitless mode, choose or adjust additional parameters and start the simulation. Once ready, the simulated images and data can then be downloaded from the web. The web interface targets new, less experienced or occasional users and offers a subset of all options existing in aXeSIM.

The WFC3 web-based version of aXeSIM is known as aXeSIMweb.


aXeSIMweb was developed independently of the STScI WFC3 spectroscopic Exposure Time Calculators (ETC). While every effort has been made to ensure that aXeSIMweb provides similar estimates of detected counts as the ETC, differences are bound to result. The STScI ETC is to be taken as the official tool and should be used to determine exposure times for the purposes of critical time justification in HST Phase I and II proposals. The WFC3 ETCs can be found here.

What is New?


aXeSIM is distributed along with aXe as a sub-package of STSDAS (from version 3.10 on). The latest version of aXeSIM (version 1.4) was released in STSDAS v3.12 in June 2010. Starting with aXeSIM version 1.4 and aXe version 2.1, the two packages have been united at the code level.

aXeSIM Documentation

The aXeSIM User Manual gives a detailed description of how aXeSIM works and how to perform simulations with it. It is available for browsing and download in various formats:

aXeSIM test packages

To verify an aXeSIM installation and to help users taking their first steps with aXeSIM, we have composed test packages for some HST instruments with slitles spectroscopic observing modes.

Please download the tarball, unpack it and follow the instructions in the README.

Help and more information

For further help with aXeSIM and its use, send an email to Please include "aXeSIM" in the subject line.