SAT (Slit Alignment Tool)

The Slit Alignment Tool (SAT) allows the three multiobject spectrographs available at Keck (LRIS, DEIMOS, and MOSFIRE), to align the mask (in case of LRIS and DEIMOS), or the CSU (in the case of MOSFIRE) in order to put precisely the fainter objects intended to be observed into their respective slits.

The alignment process works slightly different for LRIS and DEIMOS, and for MOSFIRE, given that LRIS and DEIMOS are optical instruments, and MOSFIRE works in the near infrared. A successful alignment process needs to be carried out in two steps:

1/ Guider Coarse alignment or Imaging Coarse alignment (only available for DEIMOS and LRIS). The guider coarse alignment is slightly faster, since it compares the image of the guider at the position of the science field, with the image of the guider expected for that science field as downloaded from the DSS catalog.

The imaging coarse aligment is only available for DEIMOS and LRIS, since they are optical instruments and the image of the mask and the science field can be directly compared without substracting the sky. The imaging coarse aligment is slower since an image of the field with and without mask need to be taken, and compared. It is a good option when guider coarse aligment does not work.

2/ Fine Alignment. The aim of fine alignment is to put the alignment stars as close as possible to the center of the alignment box. Fine alignment takes an image of the mask and fits a gaussian to the alignment stars on the alignment boxes. It finds their center and suggest X, Y shifts and a rotation to achieve the most optimal centering of the alignment stars into their boxes. In the case of MOSFIRE, as it is a near infrared instrument, a sky image is also taken and substracted, so that fainter alignment stars can be used for alignment.

• Further details on how to use the SAT to align LRIS are in this link: https://www2.keck.hawaii.edu/realpublic/inst/lris/slitmask_alignment_tool.html

• Details on how to use the SAT to align LRIS masks can be found here: https://www2.keck.hawaii.edu/realpublic/inst/deimos/checklist-align-sat.html

• Details on how to use the SAT to align MOSFIRE masks can be found here: https://www2.keck.hawaii.edu/realpublic/inst/mosfire/slitmask_alignment_tool.html