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Observing Strategies

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This page brings together information that might affect decisions on observing strategy in various GNIRS observing modes, and provide 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 also the noisier. It is recommanded to use the Normal mode for when a high signal-to-noise ratio (~100) is reached per frame.The Slow mode is recommanded 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 offeres two different spectroscopic modes. The best resolution(R~67.5k) 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 recommanded, assuming a resolution power of R~40k is suffisient for your science.

 

Maximum exposure times

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

Note that the current version of the extraction pipeline does not allow to combine 2D spectra before extraction yet. But we aim at allowing that in a near future.

 

Central wavelength

It is preferable to choose a strategic central wavelength. The central wavelength is not affecting the spectral coverage or the wavelength intervals covered by the orders. It is only used as the waveltngth at which guiding is optimized. In cases of high airmasses, the central wavelength is the wavelength at which the light losses due to the atmospheric differential refraction is 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). 

 

Best 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 has to 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 instead a narrow-band filter. The supported ones are HeII, HeIIC, Hα and HαC. You can check with 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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