Instrumentation for the La Silla Observatory                                  
                                                                              
At the ESO La Silla site is a direct imaging facility (DIFA) at the A Nasmyth 
focus of the NTT, in front of the image derotator which feeds the IRSPEC      
infrared spectrograph. Two remotely-controlled 45-degree mirrors can be       
inserted in the light beam to deviate the telescope beam either to a CCD      
camera (SUSI) or to an infrared array. In the configuration which has been    
implemented, SUSI is equipped with a Tektronix 1024**2 CCD with high quantum  
efficiency from the UV to the near infrared. The CCD pixel size corresponds to
0.13 arcsec in the focal plane of the NTT. A filter wheel and a shutter are   
located in front of the CCD.                                                  
                                                                              
SUSI imaging capabilities complement well those of EMMI on the other focus.   
While EMMI provides, besides the various spectroscopic options, large-field   
capabilities (up to 10 x 10 arcmin with a 2048**2 CCD), imaging at SUSI is    
limited to a small field (2.2 x 2.2 arcmin) during periods of excellent       
seeing. The scheduling of the observing programmes at the NTT can be adapted  
to the sky conditions. When the image size is better than 0.7 arcsec, the     
telescope can be configured for observing with SUSI. In order to take full    
advantage of this unique capability, ESO has now introduced a special category
of observing programmes which require exceptional seeing and are executed in a
service mode by ESO staff when the sky conditions are favourable.             
                                                                              
The 2.2 m Ritchey_Chretien telescope is a f/10 Cassegrain with 46 ' field.