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Observing Strategies

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This page contains information that might affect decisions on observing strategy in various GRACES observing modes, and provides guidelines/tips to maximize observing efficiency and avoid some common errors. For information about actually setting up observations in the Observing Tool, please see the Observation Preparation section of these pages. If you discover errors or inconsistencies, please let us know!

Factors the PI may wish to take into account when designing observing sequences:

Read modes

The read mode is specified in the OT using a (yellow) Note in the Science sequence. The choice of readmode is given in the Detector page. The faster read modes are noisier. It is recommended to use the Normal mode for when a high signal-to-noise ratio (~100) is reached per frame. The Slow mode is recommended for fainter targets. You can use the GRACES ITC to estimate the effect of the choice of read mode on the final signal-to-noise reached.

Note that the Fast read mode is not offered.

Spectroscopic modes

GRACES offers two different spectroscopic modes. The best resolution(R~62.5k) is achieved with the 1-fiber mode. However, ths mode does not allow any sky subtraction. If you plan to observe a faint target (of the order of V~18-19 mag), the 2-fiber mode is recommended, assuming a resolution power of R~40k is sufficient for your science.

Maximum exposure times

Generally, exposure times longer than 2400sec are not recommended with ESPaDOnS to limit the effects of cosmic rays and stray light. But if needed, the exposure times can be as high as 7200sec.

Note that the current version of the extraction pipeline does not allow combining 2D spectra before extraction yet. We aim at allowing that in the future.

Central wavelength

It is preferable to choose a strategic central wavelength. The central wavelength does not affect the spectral coverage or the wavelength intervals covered by the orders. It is only used as the wavelength at which guiding is optimized.  At high airmass, the central wavelength is the wavelength at which the light loss due to the atmospheric differential refraction is at a minimum.  It should therefore correspond to the wavelength around which most of the lines of interest are situated.

You can find an interesting discussion about the potential impact of Atmospheric Refraction here (note that this is a GMOS page). 

Correct acquisition sequence 

The acquisition sequence is decribed here. It is preferable to not saturate the target in the field image. This is why the choice of filter and exposure time should be appropriate. In general, a g', r' or i' filter can be used for the acquisition. In case of a faint target, it is better to use the filter corresponding to where the target is the brightest. But if a 1sec exposure time would saturate the target, it is preferable to use a narrow-band filter. The supported ones are HeII, HeIIC, Hα and HαC. You can check this table or play with the choice of filter and exposure time with the GMOS-N ITC.  Aim for a minimum S/N of 100.

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