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Doin’ The “Nod and Shuffle”

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Links and Resources are here

No, it's not a new dance...

"Nod and Shuffle" is a technique used to obtain very faint spectra and it was a critical element to the realization and success of the Gemini Deep Deep Survey (GDDS). Without this capability, the natural glow of the nighttime sky would overwhelm the extremely faint spectra of the galaxies targeted by the GDDS. For this reason, this epoch in the history of our Universe has been called the "Redshift Desert" and has long eluded astronomers wishing to obtain a non-biased survey of spectra that include the more mature, redder galaxies whose key spectral features overlap the continuum and bright spectral lines of this atmospheric glow.

Simply put, N&S allows astronomers to effectively subtract away the bright spectral emission lines and fainter continuum of our atmosphere's nighttime glow while retaining the faint spectra of dim, red galaxies. The technique also significantly reduces the amount of electronic noise introduced by the charged-coupled-device (CCD) detectors that capture the tenuous rainbow of colors from the galaxy's spectrum. In order to do this, a complex choreography of precise telescope motions (nodding) and electronic shuffling of electrical charges built up by light striking the CCD's must be precisely executed. The process involves:

  1. obtaining a spectrum of an object;
  2. moving the electrical charge built up by the image of the spectrum on the CCD to another location "buffer storage" on the CCD (Shuffling);
  3. shifting the position of the telescope slightly (Nodding) so that the spectrum of the target object shifts to a different part of the CCD;
  4. obtaining another spectrum in the nodded position;
  5. shuffling the charges moved in step 1 back to the original position (while moving the charges from step 4 to the buffer storage);
  6. nodding the telescope back to the position in step 1;
  7. repeating until enough light is collected.
The final step is to take the two spectra produced and subtract them from each other which removes the atmospheric component of the spectrum and then the resulting twin spectra can be combined to make a final spectrum for study. See this page for a PowerPoint animation of this process and other illustrations.

The history of this technique goes back to 1994 (or earlier) when two independent teams, one French and the other Australian, first began thinking about how to remove the myriad of night sky lines from their spectroscopic observations.

The first scientific paper on this technique was published in 1994 by Jean-Charles Cuillandre now of the Canada-France-Hawaii Telescope on Mauna Kea and was based on the work of his PhD thesis (see references below). The concept was based upon an original idea by Dr. Cuillandre's PhD director, Dr. Bernard Fort which they called 'Va-et-Vient' spectroscopy, which is French for "back and forth spectroscopy" because of the fact that the telescope "nods" or shifts between locations in the sky. Read Dr. Cuillandre's narrative here.

While Cuillandre and Fort's work helped to pioneer this technique, their work primarily proved the concept in the lab and they were never able to use it to produce scientific results on a telescope. Dr. Cuillandre responded to the Gemini results by saying, "Since I had no chance to pull off some significant science with this technique, I am very happy to see the technique work so well on GMOS with these nice results."

Independently, in March 1994, Anglo-Australian Observatory Astronomer Joss Bland-Hawthorn was also thinking about how to remove the effects of the Earth's atmosphere from observations with a new kind of imaging spectrograph (called an integral field spectrograph) announced that same year. Like Cuillandre, he envisioned a CCD that was larger than the observed patch of sky so that once an exposure was taken, the charge resulting from the image could be shifted into the unilluminated region. He imagined the telescope being "nodded" between an object of interest and a clear patch of sky, in synchrony with the CCD charge being "shuffled" backwards and forwards. He referred to the operation as "nod & shuffle spectroscopy" and his early development work took place at the AAT during 1994 and 1995. Read Dr. Bland-Hawthorn's narrative here.

Since 1997, GDDS team-member Karl Glazebrook and Anglo-Australian Observatory Astronomer Joss Bland-Hawthorn have successfully applied the technique to the study of faint galaxies using the Anglo-Australian Telescope. The work they did in Australia laid the foundation for the GDDS. For more details on this work see their 2001 paper (which includes many references to this technique going back to 1994.)


'Va-et-Vient' spectroscopy: A new mode for faint object CCD spectroscopy with very large telescopes Astronomy & Astrophysics, Vol. 281, p. 603, 1994 see abstract here

"Microslit Nod-Shuffle Spectroscopy: A Technique for Achieving Very High Densities of Spectra"
The Publications of the Astronomical Society of the Pacific, Volume 113, Issue 780, pp. 197-214, 2001 See abstract here.

"Back and forth spectroscopy: optimization of an optical nod-and-shuffle technique to reach fainter objects and increase the multiplex gain on multi-object spectrographs" Proceedings of the SPIE, Vol. 4841, p. 1531, 2003

"Accurate sky Subtraction of Long-Slit Spectra: Velocity Dispersions at Sigma(v) = 24.0 Mag/arcsec2"
Sembach & Tonry's work on the N&S technique can be found here