Technical description

The spectroscopic mode is implemented by inserting a spectroscopic slit into the image plane i1 and one of the four gratings into the pupil plane p2. The spectroscopic dispersion is along the long axis of the detector array. Preliminary grating properties are listed in Table 1, below. The Lo-Res-10 grating permits full coverage of the 10 microns atmospheric window (spanning approximately 8-14 microns) in one setting of the grating with a spectral resolving power of at least 100. The Lo-Res-20 grating spans the 20 microns atmospheric window (approximately 16-25 microns) in one setting of the grating, with a resolution of at least 60. Two additional gratings provide spectral resolving powers about a factor of ten higher (see Table 1) in these two atmospheric windows and can be rotated to center any wavelength on the array.

Performance

The current baseline performance at PDR is the following:

The optical design of CanariCam has been optimised with respect to T-ReCS to give the best spectral resolution compatible with the requirement for diffraction-limited imaging. A goal of R=1500 has been set for the high resolution mode in the 10-micron window (equivalent to a velocity resolution of 200 km/s) and R=80 (equivalent to 3750 km/s) for the low-resolution mode in the 20-micron window. Fulfillment of these goals is one of the tasks of the detailed design phase.

The slit widths are set to be 1x and 2x the diffraction limit of the telescope at the longest wavelength in each window. This maximises the resolution, both spatial and in wavelength. Under consideration is the possibility of implementing adjustable slit jaws that permit any slit width to be set. The excellent image quality allows a minimum of 73% and 86% respectively of the light through the slit.

Detailed additional performance requirements are:

• The positioning of the tunable grating shall be reproducable to better than 5 pixels in the dispersion direction.
• The plate scale will be the same as in imaging mode in the spatial direction.
• The slit will cover the full width of the field of view of the detector in its short axis (19.2 [± 5%] arcseconds).
• The throughput of the instrument in First Order shall be at least 15% with both wide and narrow slit.
• Calibration in wavelength will be possible to 20% of the resolution at the central wavelength being observed. This will be carried out with a combination of a polystyrene sheet (probably located in the exterior sector wheel), which has many lines particularly in the 10-micron window, telluric lines of known wavelength within the source spectrum, and emission or absorption lines in astrophysical sources.