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Defining a N&S Observation
In this section we outline a few definitions and features of a GMOS Nod & Shuffle observation.
- cycle: A Nod & Shuffle cycle is defined as the smallest useful quantity in the Nod & Shuffle exposure. In theory this is one exposure in the target position and one exposure in the sky position.
- A = B: The exposure time in the target position during one cycle is defined as A. The exposure time in the sky position during one cycle is defined as B. For Nod & Shuffle observations with GMOS the exposure time A will always be equal to B.
- A is even: Because of the slightly uneven performance of the GMOS blade-type shutter, Nod & Shuffle exposures are always taken in pairs in order to preserve photometry. Therefore the exposure time A must be an even integer number of seconds.
- B/2, A/2, A/2, B/2: In order to obtain the best sky subtraction, the sky exposure brackets the target exposure. Therefore each Nod & Shuffle cycle with exposure time A is composed of 4 equal length sub-exposures taken in the following order - sky, object, object, sky.
- observation: A Nod & Shuffle observation is defined to have a set number of cycles, each with observing time A. Therefore the total observing time (total open shutter time) for a Nod & Shuffle observation with a number of cycles C is equal to 2 × A × C.
- shuffle distance: The shuffle distance defines the height of the bands (or the length of the slits for micro-shuffling). Normally the shuffle distance is a positive number, the sign determines whether the charge is shuffled up or down for the sky exposures. The object is always observed with the charge in the normal (no shuffling) configuration. For a positive shuffle, the resulting spectral images will have the sky spectrum located beneath the object spectrum with a separation equal to the shuffle distance. The shuffle distance is always specified in unbinned pixels, regardless of whether or not the spectral data is binned in the y-direction. For example, for a single-band Nod & Shuffle observation the shuffle distance is +1536 pixels and for a micro-shuffled image with 1 arcsec slits the shuffle distance is 14 pixels. However if the spectral data is binned in the y-direction, one must make sure that the shuffle distance is a multiple of the binning (e.g. one cannot have a shuffle distance of 21 pixels if binning by 2 in the y-direction, but a shuffle distance of 20 or 22 pixels would be OK).
- charge traps: The presence of local defects (charge traps)
in GMOS detectors #2 and #3 lead to low-level horizontal stripes in Nod
& Shuffle spectral images. Efforts to
correct these charge traps are underway. Meanwhile there are two
approaches to minimizing the effects of these defects.
Since semester 2003B we have providing a special kind of dark exposure
for Nod & Shuffle. Taking these dark exposures is very time
consuming, and they are therefore only guaranteed to be made available for the
following Nod & Shuffle configurations:
- A=60, 15 cycles, shuffle distance=31 pixels, CCD binned 2x1
- A=60, 15 cycles, shuffle distance=70 pixels, CCD binned 2x1
- A=60, 15 cycles, shuffle distance=1536 pixels, CCD binned 2x1
Any Nod & Shuffle darks defined by PIs in their Phase II proposals other than the above configurations will be taken on a best-effort basis. The Nod & Shuffle darks are effective for correcting for most of the effect of the defects. However, for very deep observations it is recommended to also translate the detector between Nod & Shuffle observations by ± a few pixels in the y-direction. This allows the defects to be removed in the data reduction stage via a suitable rejection algorithm. Nod & Shuffle programs that aim to go very deep should have their observations split into at least three dithered exposures. The translation of the detector must be defined by the PI in the OT using a GMOS sequence to change the DTA-X offset.
- baseline calibrations: For Nod & Shuffle programs all baseline calibrations are taken in exactly the same manner as for classical long-slit and MOS spectroscopy programs. Additional calibrations or calibrations taken in Nod & Shuffle mode must be requested explicitly in Phase I and Phase II proposals and Nod & Shuffle Darks must be defined by the PI in their Phase II OT program.