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ISIS focus offset caused by gratings

We have found that some of the gratings in use with ISIS cause a focus offset on the RED arm detector when the focus was fixed for a specific grating. The origin of this is currently unknown, but the results are repeatable. Support Astronomers and observers should pay attention to this fact. The table bellow summarise the results obtained using the 5400 dichroic. 
ISIS Focus                         R158R/B    R316R/R300B    R600R/B    R1200R/B  
-----------------------------------------------------------
Red Arm (Dichroic in)          -1114                0             -2341         -4214    
Red Arm (Dichroic out)          -998           -1498          -2327         -4052    
Blue Arm (Dichroic in)           -785             -447            -665              0    
Blue Arm (Mirror)                    741             1071              748            606    

These values are in Tek4 (Red arm) and EEV12 (Blue Arm), and the focus was fixed 
using R316R (red arm) and R1200B (Blue arm) with the 5400 dichroic in the beam. 
The plot below shows the spectral resolution degradation due to a wrong collimator
 position (focus offset).










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Last Updated: March 2003

Javier Licandro licandro@ing.iac.es