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LDSS-3 Nod & Shuffle Cookbook

This is a cookbook to perform N&S observations with LDSS-3. Updated on July 2017 by G. Blanc



Nod & Shuffle (N&S, Glazebrook & Bland-Hawthorn 2001) is a technique that allows the user to shuffle the charge from a science spectrum to a dedicated empty region on the detector, then nod the telescope, and observe a sky spectrum over the same pixels and through the same part of the slit with which the science spectrum was originally observed. Doing this repeatedly allows the user to average over short term sky fluctuations during the duration of the full exposure, and store both science and sky spectra in the integrated frame after readout. The technique can produce superb sky subtraction residuals, and is recommended for the observation of faint objects in the red region of the optical window where sky contamination is very strong. 

N&S comes in two flavors: "macro-shuffle" in which the charge from the central third of the detector is shuffled to the upper and lower thirds to store science and sky spectra respectively, and "micro-shuffle" where the charge from each short-slit is shuffled to the upper and lower regions adjacent to the slit. Micro-shuffle is the most commonly used N&S mode and is the one explained in this cookbook (we have not tested a macro-shuffle mode in LDSS-3). Hereafter when we talk about N&S we always refer to a micro-shuffle mode. Using this mode requires the users to design their multi-slit masks accordingly, leaving enough space between slits to ensure the charge from adjacent slits is not mixed during the shuffling process. Depending on the nature of the program, it might be recommendable to nod from an "A" position in which the object is on one side of the slit to a "B" position in which the object is on the other side, instead of noding from an "on source" to an "off source" position. This effectively increases the exposure time by a factor of two, but requires longer slits which can affect the number of targets that can be placed in a mask. 

Dewar Orientation for N&S Mode:


Observing in N&S mode with LDSS-3 requires the instrument dewar to be rotated by 90 degrees to the N&S position. This must be done by the day crew, and therefore must be requested when submitting the Instrument Setup Request Form (which must be sent 3 weeks in advance to your observing run). In this orientation the dispersion direction is parallel to the x-axis of the 2K x 4K CCD, which implies that the wavelength range falling on the detector for N&S observations is shorter than in regular operations. In this orientation the slits will be aligned vertically instead of horizontally when displaying a frame.

Designing N&S Masks:


The process of designing masks for N&S observations is identical to that for designing regular multi-slit masks, except that care must be taken to space the slits sufficiently apart in the spatial direction to leave enough space for storing the charge during the shuffling. For micro-shuffle the inter-slit spacing must allow for a full image of a slit to be stored and leave a small gap to avoid overlap between this image and its neighbors. It is recommended that this gap is 10 pixels (~2") wide on each side. Therefore the spacing between the slits on the mask should be (4+h)/0.189 pixels, where h is the slit height in arcseconds and 0.189 is the LDSS-3 pixel scale. This parameter is controlled during mask design by the "Overlap pixels" field when running intgui. The value must be given in negative pixels. For example, if designing a N&S multi-slit mask with 10" tall slits, the value in "Overlap pixels" should be -(4+10)/0.189 = -74 pixels.

It is also essential to select the "LDSS N&S Micro" option in the "Instrument" field in intgui, as this informs the mask design software about the 90 degree dewar rotation and the right orientation of the dispersion direction on the CCD.



Figure 1: INTGUI window, showing the fields that must be modified when designing multi-slit mask for N&S.


Observing in N&S Mode:


The LDSS-3 GUI in guanaco must be initialized in N&S mode. After launching the GUI, choose "Nod & Shuffle" in the "LDSS-3C" drop-down menu in the initial Configuration window. If N&S observations were requested in the Instrument Setup Request Form, this should have already been done during daytime by the Instrument Specialist. But if for some reason the GUI must be restarted during the night, it is important for the observer to be aware of this step. The N&S dewar orientation must also be updated in the parameter "orient" of acquisition routines like "lcobject", "ltoslit", "lalign", and "lfalign" (orientation of dewar: 0=normal, 1=N&S). Read the long slit and multi slit cookbooks for instructions on how to do the acquisition.

LDSS3 NS Startup

Figure 2: LDSS3 startup configuration window. Choose "Nod&Shuffle" mode in the CCD: LDSS3-C field.


To take an exposure in N&S mode the observer must select the "N&Shuffle" option in the "ExpType" drop down menu. This will open a "Nod&Shuffle" window in which the parameters for the N&S observations are set:

  • Shuffle: size of the shuffling distance on the detector. The user enters this value in arcseconds and the shuffling distance in pixels is automatically reported given the pixel scale of the instrument and the binning. For micro-shuffle this distance should be equal to (2+h), where h is the slit height in arc seconds. The latter assumes a 2" gap was left between all slit images (see above).
  • Binning: desired detector binning.
  • Telescope-Nod: these are the telescope offsets in RA and DEC that must be applied to go from the reference position to the nod position. They are given in arcseconds, and can be calculated for a desired nod distance along the slit using the "lnodshuffle" routine in the "ldss3" iraf package.
  • Dwell Time: this is the exposure time during each nod. During the observations it is recommended to check which exposure time the Telescope Operator is using for the Shack Hartmann (SH) star, and ensure the telescope has enough time to apply the SH corrections between nods. Faint SH stars can require longer dwell times. 
  • N-Cycles: this is the total number Nc of reference+nod position cycles. The total exposure time per frame will then be equal to 2*(N-Cycles)*(Dwell_Time).
  • Guiding: here the user must enter which Guider camera is being used for guiding and which one is being used for the Shack-Hartmann corrections.



Figure 3: LDSS3 GUi. Choose "N&Shuffle" mode in the ExpType field.

LDSS3 NS Window

Figure 4: LDSS3 N&S parameters window.


After setting these parameters, a N&S exposure can be started from the LDSS-3 GUI. The instrument and telescope will work automatically in a coordinated fashion, repeating the following cycle of actions: exposing at the reference position, closing the shutter, shuffling the charge up on the detector, making a coordinated telescope offset to the nod position, exposing again, closing the shutter, shuffling the charge back down on the detector, and offsetting the telescope back to the reference position. Readout happens after this sequence is repeated N-cycle times. 




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