Slitmask Design Tips
Below are tips and suggestions regarding the design of slitmasks for LRIS.
- Typically, eight slots are available for user slitmasks.
Although 10 slots are available for user slitmasks, one is reserved for "direct" imaging mode,
and a second holds the focus holes mask. Direct imaging mode is necessary to identify the
target field, and is used during the aligment process. The focus holes mask is used to
focus the instrument in the afternoon. When designing masks for your run, keep in mind that
there are only eight slots and that if longslits are desired, the longslits use one of the
- A minimum of three alignment stars are required to align a slitmask.
Four to six alignment stars is recommended.
A few extra alignment stars, allows you to ignore a particular alignment star if, for example,
the astrometry for that star is poor.
- Place a few alignment stars at the edges of the mask.
The alignment stars are used to correct rotation and translation errors.
Stars placed at the edges of a mask better determine the rotation correction.
- Magnitudes for alignment stars should be between 15-19 magnitude.
- Prepare star charts for the alignment stars and the guide star if using pickoff mirrors.
- Check the EQUINOX of the mask coordinates.
Note the EQUINOX of the coordinate for the center of the mask.
If you did not specify an EQUINOX in the autoslit mask parameter file, the EQUINOX of the mask coordinates
output from autoslit is "todays" EQUINOX.
- Pickoff mirrors are NOT recommended.
- The new offset guide camera is very stable and there is
no observed flexture between the guide camera and the
science field at the slitmask. Thus, guiding with the offset
guider is very stable and the need for pickoff mirrors is
- Pickoff mirrors are also prone to jambing in the
instrument so not having pickoff mirrors makes LRIS more
- The Slitmask Alignment Tool also makes alignments
easier than using the pickoff mirrors saving time.
- To review the procedures for aligning slitmasks see the alignment procedures at: