Performance
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The throughput of the GMOS-N IFU has been measured by comparing the total flux in a GCAL imaging flat at the positions of the IFU fields with the flux through the IFU with no disperser in the beam. The throughputs for
different wavelength are shown in the table below. The fiber-to-fiber variation in the throughput is about 6% for spectra in the center of the CCD and there is a ~30% reduction in throughput near the ends of the slits due to vignetting. Currently the flat-fielding is good to about 5%, varying between 3% and 10% depending on the slit block. The method is still being improved.
| Filter | Throughput |
| g | 62% |
| r | 65% |
| i | 62% |
| z | 58% |
Sensitivity estimates can be found on the GMOS Spectroscopic Sensitivity page which were derived using the GMOS Integration Time Calculator (ITC). The ITC reports results for single IFU elements.
Arc lines have Gaussian FWHM of about 4.2 pixels (unbinned). Thus, the effective slit width of the IFU in the dispersion direction is 0.31 arcsec.
Sky subtraction is done by averaging all spectra in a specified sky area (all the fibers in the "Sky" field, for example) and then subtracting the mean sky spectrum from all spectra in the cube. It is working well, though there can be gradients in the residuals with wavelength on the order of 5% (see figure below). Sky subtraction is sensitive to the flat-fielding accuracy, so it should improve as the flat-fielding improves.
