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Band- and Micro-shuffling

There are two shuffling modes that one can choose from when observing with Nod & Shuffle: band-shuffling and micro-shuffling. With band-shuffling the detector is divided into horizontal regions (or bands) of equal height which alternate between science and storage regions. Science regions are those areas of the detector which are allowed to be illuminated, and storage regions are never illuminated. Storage bands alternatively store science data when the sky spectra are being obtained and store sky data when the science spectra are being obtained. There must always be a storage band at the top and at the bottom of the detector. Science Bands can contain any number of slitlets of various slit-lengths. The limiting case where each science band contains exactly one slitlet and all slitlets have exactly the same slit-length is known as micro-shuffling.

Below we show three schematic layouts for Nod & Shuffle (click on each image for a larger view). The large, inset red square represents the GMOS imaging field of view (this is the area within which one is allowed to place slits in classical MOS observing). The science regions are shown as white with black filled rectangles representing the slits. The red and blue shaded regions represent storage regions. Normally the red shaded regions store the science data when the sky position is being observed, and the blue shaded regions store the sky data when the science target position is being observed. Because the sky and target are not observed simultaneously, the red and blue storage regions are allowed to overlap on the detector. These examples are not to scale and are for illustrative purposes only.

[Single Band Shuffle]

[a] The simplest possible layout is band-shuffling with a single science band. Here the middle third of the detector is the only science region, and the top and bottom thirds of the detector are used for storage exclusively. This band layout allows one to place a very high density of slits of variable length in a centralized region; however, one loses 2/3 of the field of view for slit placement.

[Two Band Shuffle]

[b] This example has two science bands and three storage bands. Because storage regions between science regions can be shared, one starts to win back field of view as the number of bands is increased. Here we are using 40% of the GMOS detector for science (available for slit placement) and 60% for charge storage. Once more each of the science bands can contain a high density and number of slits of variable length. Each band must have the same height and there must be two storage regions immediately adjacent to each science region; however, they do not need to be regularly spaced or fill the entire detector as shown here. This can be seen better in the next example.

[Micro Shuffle]

[c] The limiting case of many bands where each science region contains exactly one slit and, therefore, each slit has the same length. This special case is knows as micro-shuffling. As the number of micro-shuffled bands increases and the size of the slits decreases one can use nearly 50% of the GMOS field of view for slit placement.