Gemini Observatory Press Releases

Background on N&S: Dr. Cuillandre's narrative

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The first paper to present the Nod & Shuffle technique was in 94 and it was my first scientific paper ever. It was the result of my first year of PhD work. The original idea was actually from my PhD director Bernard Fort who suggested the basic principle for me to play with in the lab. We simulated a big (8m) telescope spectrograph setup on our bench with a strong sky with many emission lines. The object being a simulated distant quasar. It was fun! Unfortunately, the paper received little attention at the time, probably because it was more of a concept idea with some lab checks. What you need to make a point is at least some real sky data, or even better, a scientific result! During the year I spent at CFHT in 95 I worked on my PhD to implement the first CCD mosaic at CFHT. I also had the wonderful opportunity to implement and develop the technique on the CFHT multi-object spectrograph (MOS). That's when I really started having fun investigating the various potentials of the N&S technique.

When I came back to CFHT a year later I continued my investigations on the capabilities of the technique for efficiently dealing with CCDs suffering from strong fringing in the red. This fringing is a serious issue for normal spectroscopic mode on CCDs that are currently made by E2V, but not a problem at all for the N&S mode that takes care of fringing like a charm.

The big trouble with the engineering time I obtained on CFHT was that I ALWAYS got bad weather. Usually cirrus clouds, which allowed implementation and testing so we could produce some frames which I could at least look at in terms of sky subtraction over itself.

But I had no chance to pull off some significant science, so that's why I am so happy to see the technique working so well on GMOS with these nice results (and also the Supernova discovered on the CFHTLS which were also observed in that mode)! Bernard Fort and I are especially happy to see our initial prediction come to realization.

It's also important to give our own references! Bernard Fort got the idea himself from a polarimetry trick by Ian S. McLean where charges were shifted up and down under a big mask while a rotating polarization wheel was moving. That technique itself was based on infrared observing techniques (of which McLean is very proficient). His paper was published in 1981!

As for the name, we picked first a french name: "Va-et-Vient", but that did not sell well it seems. I later transformed it into the English expression "Back-and-Forth" to maybe get it better registered. It now seems it'll be "Nod and Shuffle" which will stick thanks to the scientific results. I knew it, it's the science that would sell the case!