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You are here: Home Telescopes Magellan Instruments IMACS Observing Cookbooks Observing with a multi-slit mask in Nod & Shuffle mode

Observing with a multi-slit mask in Nod & Shuffle mode

Note: Updated by D. Osip from original notes submitted by K. Luhman.

Designing Masks for Nod & Shuffle Mode:

  • Select the slit length. If nodding off to a sky position, the slits can be very short (a few arcseconds). If nodding point source targets between two positions along the slits, a minimum nod length of about 2" is necessary so that the two images of the target do not overlap when one shuffled image (one nod position) is shifted and subtracted from the other during data reduction for sky subtraction (see example in figure below). The distance between a nod position and the end of the slit should be at least 2" (the last 1" of which is not fully illuminated), producing a minimum slit length of 6". To prevent the targets from appearing too close to the end of their slits (or off the end) due to slight misalignments along the slit, and to better separate the two spectra of the targets with a larger nod distance (say 2.5-3" rather than 2"), a slit length of at least 8" is advisable when nodding targets along their slits.
  • Select the uncut length. A minimum of twice the slit length should be left uncut between neighboring slits to provide the unilluminated region for storage. The uncut length in the mask-making program "intgui" refers to the portion of the "slit length" value that is left uncut. For instance, in "intgui", if the right-half and left-half slit lengths are 9" and the right and left-half uncut lengths are 5", then the total cut slit length will be 8" and the uncut regions will be 5" beyond the end of each slit.

Observing with Multi-Slit Nod & Shuffle Masks:


  • In the IMACS CamGUI, set the Binning X and Binning Y values to 1, and the readout to full. Select N&Shuffle under ExpType, and in the resulting window enter the N&S parameters. The dwell time should be at least 30 sec, and 60 or 120 sec are good values for long exposures. Select the number of cycles, where the total exposure time = 2 x (number of cycles) x (dwell time). The shuffle length should be the average of the slit length and the uncut length on one side of the slit. Select a nod length appropriate for the length of the slits (e.g., half the slit length). You can use the inod task in IRAF to convert it to an offset in RA and DEC, and enter this offset in the N&S parameters window. 


  • Insert the desired disperser and mask and start the exposure. An example of data produced by N&S with masks is shown in the figure below.




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