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Nodding

When observing with Nod & Shuffle, one needs to define a nod vector and distance. This is the direction and the distance that the telescope will move when changing pointing from the object position to the sky position. In theory there is no restriction on the size and direction of the nod, though in practice there are several factors one should consider:

  • How much additional overhead can the observations tolerate? Both the size and direction of the telescope nod will affect the amount of extra overhead incurred by the Nod & Shuffle observations compared to classical spectroscopic observations. While minimizing these extra overheads shouldn't be the primary consideration when selecting a telescope nod configuration, they can have an important impact on the observing efficiency.

  • Does one wish to nod along the slit and observe the target in both the sky and object positions? This is a useful mechanism by which one can double the exposure time on the target and greatly reduce the overheads associated with Nod & Shuffle observing. However, this may not be possible or desirable for certain applications, e.g. observing crowded fields or extended objects. When nodding along the slit there are certain additional considerations to take into account when designing the Nod & Shuffle focal plane mask.

  • If one is not nodding along the slit, one must define a sky position which does not place any objects in the slitlets. This can be tricky especially if one is using a large nod to a position for which deep imaging does not exist. It may be beneficial to request pre-imaging for both the target and sky positions if one anticipates using a very large (several arcmin) nod with Nod & Shuffle. Or perhaps more useful would be to request a short exposure (5min) at the sky position through the mask once it has been designed - this is the best way to make sure there are no bright objects contaminating the spectral image of the sky.

  • Can one still guide in the sky position? For large nods (several arcmin or more) it is unlikely that the OIWFS will still be able to reach the guide star (the OIWFS patrol field is 3.54 arcmin x 4.15 arcmin). At this time, guiding with two separate guide stars in the target and sky positions is not a supported GMOS operational mode. It is still possible to take the sky spectra unguided (specify OIWFS in FREEZE mode for the sky position) but one must make doubly sure there are no bright objects that can wander into the slits during the unguided sky exposure. Note that observing with the sky position unguided will also help to reduce the Nod & Shuffle overheads.

  • One is restricted to a single nod definition for each Nod & Shuffle observation. This means that the telescope will continuously nod between one target position and one sky position; this is known as the standard Nod & Shuffle mode. There are plans to support a pattern mode in a future implementation where the telescope nods between the target position and any number of user-specified sky positions in the same Nod & Shuffle observation.