GMOS Optical Properties

The optical elements of GMOS (e.g. filters, gratings and slits) are described in detail on the main GMOS pages.

The filter menu includes all available filters (for GMOS North or South as appropriate). For imaging, select one of the filters u', g', r', i', z' or narrow-band. For spectroscopy, select a filter appropriate for the grating or select "none" if no order sorting or limitations on the spectral range is required.

To cover different wavelength regions with a given grating, change the input central wavelength. In general, the gratings can be use over a larger wavelength range than can be covered simultaneously, see the GMOS gratings page for details.

A change in central wavelength corresponds to a change in the instrument grating angle. The GMOS Integration Time Calculator does not apply constraints on the input central wavelength, but the user should in general not attempt to specify central wavelengths very far from the blaze wavelengths given on the GMOS gratings page. For example, the GMOS Integration Time Calculator (ITC) will allow the user to specify a central wavelength of 400nm and either grating R150_G5306 or grating R400_G5305. However, in reality this is not a useful setup.

The wavelength range returned by the ITC corresponds to a centrally located slit. In the MOS mode the location of the spectrum on the detector depends on the position of the slit-let. Thus, the wavelength range for a specific slit-let may be limited by the spatial extent of the CCDs.

The hexagonal field of view of an individual IFU element is approximate as a square aperture (size 0.21 arcsec) which gives the same effective area.

GMOS Detector Properties

GMOS is equipped with three CCDs. For computational simplicity the average properties of these CCDs are used in the ITC. For details on the individual CCDs, see the GMOS North detector or GMOS South detector pages.

In the spectroscopic mode of the GMOS ITC, the signal at the wavelengths corresponding to the gaps between the CCDs is set to zero.

The GMOS CCDs can be used in either low gain or high gain mode. Further, they can be read out in a fast or a slow mode. For the purpose of the ITC, it is assumed that the detectors are operated in slow readout mode. An average read-noise of 4.1 electrons was assumed. This is the average read-noise for the two gains used in the slow readout mode. The ITC warns about possible saturation for the low and high gain modes (software saturation), and possible hardware saturation. The mode of operation is not a user input parameter for the ITC, but the saturation warnings will be sufficient for the user to assess if the CCDs should be operated in low or high gain modes for the proposed observations.

The GMOS CCDs can be binned. The GMOS ITC allows binning in the spatial and spectral direction independently. The results from the ITC are derived using integer pixels. Thus, in some cases of strong binning the use of integer pixels will give a larger (or smaller) aperture than requested. In general, the binning in the spatial direction should be chosen such that the point-spread-function is still sufficiently sampled for the chosen image quality. Binning in the spectral direction may result in degradation of the resolution.

In the imaging mode, the spectral binning is ignored and the spatial binning results in square (binned) pixels.

Spectroscopic resolution and sampling

The ITC samples the spectral distributions at 0.25nm. However, many of the input spectra have a lower resolution, see description of the spectral distributions used by the ITC. Therefore, the output spectra from the ITC does not correctly reflect the resolution that can be achieved with GMOS.

Last update: 5 March 2003; Phil Puxley
In original form: March 2, 2001; Inger Jørgensen