Slitmask Design Tips

• 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 available slots.
• 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 EPOCH of the mask coordinates.
  Note the EPOCH of the coordinate for the center of the mask. If you did not specify an EPOCH in the autoslit mask parameter file, the EPOCH of the mask coordinates output from autoslit is "todays" EPOCH.
• Pickoff mirrors are recommended.
• Aligning a slitmask is technically quicker using pickoff mirrors than aligning without them. This is because the course alignment is achieved by putting a guide star at a specific location on the slit-viewing guider, instead of having to acquire an image of the field to identify alignment stars.
• The key to pickoff mirrors is having a good guide star. A guide star in the range of 15-19 mag is preferred. The same magnitude range should be applied to alignment boxes used to fine tune the course alignment.
• If you specify using pickoff mirrors in the autoslit3 software, the software prohibits placing a slit in the area of the pickoff mirror. You can and should specify targets above the pickoff mirror. You will only take a hit if you don't have targets above a guide star. If you design the mask with care, the location of the pickoff mirror will not affect your science.
• If using autoslit3 was used to design the mask and you offset the field, note the offsets. The offsets are required to predict the location of the guide star in the slit-viewing FOV.
• If the first alignment using a pickoff mirror fails, we can revert to aligning slitmask without pickoff mirrors procedure easily. Subsequent mask alignment can then use the pickoff mirror.
• Including pickoff mirrors permits potentially more stable guiding. There is some flexure in the LRIS offset guider, which is used for slitmask observations without pickoff mirrors. With the pickoff mirrors, you guide on the more stable slit-viewing guider, and thus have less risk of having the slitmask drift relative to the targets during the exposure. .
• To review the procedures for aligning slitmasks see the alignment procedures at:
• Slitmask Alignment with Pickoff Mirrors
• Slitmask Alignment without Pickoff Mirrors