RSS Slitmask Management Tool (RSMT)

Last update: 12 April 2021

Download the RSS Slitmask Management Tool:

Please also see the MOS page if you’d like to try the experimental new mask preparation software, which features several improvements over the RSMT. The MOS page also contains other essential tips and tricks for preparing MOS observations.

Version History

  • Version 1.99 fixes a bug due to which slit mask data was shown incorrectly for the masks opened from file.
  • Version 1.98 fixes a bug which made saving slit tilts impossible.
  • Version 1.95 fixes a bug which made moving of masks with a positive declination impossible.
  • Version 1.94 uses improved plate scale values.

Important: Due to what seems to be a JSky issue, RSMT versions prior to 1.912 may calculate wrong RA/Dec values. Please use version 1.912 or higher. Also note that a PC matrix is not supported for world coordinate definitions in FITS files.

Important: JAI (and hence the RSMT) under Windows is running with 32-bit Java only.

Important: The RSMT might not run under Windows 7.

Note regarding Position Angles (PA) of the MOS masks: due to the way SALT operates there are restrictions in the available PAs for any given RA,DEC.  We are in the process of making this information available automatically in the Visibility and MOS tools. Until then, you may find the required information in the SALT Position Angle Visibility document.

Just contact us in case you have any questions about this.

The RSS Slit Mask Tool is an interactive application to design RSS multi slit masks from preimaging pictures. It has the ability to import catalogs of objects, to add or remove objects, to define reference stars for slit mask alignment, and to check for overlap of spectra within a slit mask. Future versions of psmt will include the possibility to optimize the pointing and rotation of the instrument to maximize the number of spectra per slit mask.
RSMT is a 100% Java application, and should therefore run on all systems where Java is available.

Note: RSMT absolutely requires that JAI (Java Advanced Imaging) is installed on your system. JAI can be downloaded from

The Java version on Mac OS X ships with JAI out of the box, but only if you are using the Oracle JDK (not the OpenJDK).

See for installation instructions.

The installation instructions for Linux mention a directory “$JRE” or “$JDK”. In order to find out what this directory is, start by executing

which java

in a terminal. This should return something like


which in all likelihood is a symbolic link. So use

ls -l /bin/java

(where /bin/java must be replaced with whatever “which java” returned) to check whether the target of the link is yet another link or an actual file. Repeat this procedure until you get to a link whose target is an actual file.

Assume that this file is located at /some/platform/dependent/path/bin/java

Then /some/platform/dependent/path is the directory to use for $JRE or $JDK.

Chances are that you get an error like

tail: cannot open `+245′ for reading: No such file or directory

when running the installation script. If so, open a bash shell, execute


and then try again.

If all else fails, download the CLASSPATH installation, unzip/untar the downloaded file and copy the jar files from the lib directory into




And if that still doesn’t work, consider paying a visit to your local IT gurus. (But also send an email to, please.