To illustrate the expected performance of the instrument we present the limiting magnitudes in direct imaging mode. In the calculation the following data was taken into account (transmission and reflection losses only):
the dark-of-Moon sky brightness for the Observatorio del Roque de los Muchachos;
the central wavelengths and FWHM of the Johnson filters;
the aerosol-free atmosphere of the site (to estimate atmospheric extinction);
the flux of a magnitude zero star at the different wavelengths;
a detector readout noise of 2.2 electrons in direct imaging mode.
With the previous considerations, and assuming a symmetric 2D gaussian containing 90% of the energy, the OSIRIS limiting magnitudes at the Nasmyth focus for 0.5arcsec and 1.0arcsec FWHM-seeing conditions are evaluated. The best image quality theoretically attainable with the GTC (seeing and telescope image quality) is 0.44arcsec FWHM. With an OSIRIS polychromatic mean image quality of 0.25arcsec, this implies 0.51 arcsec FWHM. Hence the value of 0.50arcsec would correspond to best possible conditions, while a value of 1.0 arcsec represents a worse-than-average case.
| Filter | 0.5" FWHM | 1.0" FWHM |
| U | 26.0 | 25.2 |
| B | 27.4 | 26.7 |
| V | 27.1 | 26.3 |
| R | 26.5 | 25.8 |
| I | 25.8 | 25.1 |
Last update August 8, 2005, by Héctor Castañeda