Fundamental factors pushing the development of infrared instrumentation are:

• The low interstellar absorption which allows the regions of highest extinction in the Galaxy to be probed, amongst them the Galactic Centre and major star-forming regions.
• The possibility of making diffraction-limited observations, without the need for adaptive optics, in the wavelength range beyond 6 microns. Observations using OSCIR in the Keck-II Telescope have shown that the atmospheric seeing at 10-microns is practically negligible and that a PSF FWHM, equivalent to the telescope diffraction disk, can be attained.
• The possibility of optimising the use of telescope time by observing in twilight, or even in daylight, with little or no loss of sensitivity.

The mid-infrared range offers the possibility of very high-resolution imaging from a ground-based instrument, without the need for costly additional optics. The mid-infrared performance of the GTC should far exceed its visible performance and may even be superior to its near-infrared performance, even after adaptive optics are available, due to the strong wavelength dependence of the seeing. Two key issues for future observers using the GTC are:

At 8 microns the diffraction limit for the GTC will be 0".18. Even at 25 microns the diffraction limit will be 0".60. In both cases the figures are more than an order of magnitude better than anything that has been achieved to date from space. The figures rival what can be achieved in the visible, even with adaptive optics.

With a highly stable PSF, image improvement techniques can be applied which can improve the FWHM of the processed image by a factor of 2-3 giving a final image quality significantly better than the telescope diffraction limit. Such techniques have already been successfully applied to NASA IRTF images taken with OSCIR.

Thus, for the highest quality spatial resolution with the GTC, observers will need to image in the thermal infrared, at least until the future incorporation of adaptive optics. Based on experience with OSCIR in the NASA IRTF, the image quality which is achievable with CanariCam on the GTC, at 8 microns, after the application of image enhancement techniques (0".06-0".09) will be close to that achievable with the HST (0".055 at 5500Å).