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ISIS image rotation caused by dichroics

We have found that some of the dichroics in use with ISIS cause an image rotation on the BLUE arm detector with respect to the mirror which sits in the same slide, and which is used for blue only observations. The origin of this is a misaligment, and the results are repeatable. Support Astronomers and observers should pay attention to this if if will compromise the data. A summary follows. 
Dichroic     Image rotation in pixels top to bottom  
-----------------------------------------------------
5400         1.00
5700         2.86
6100         1.33
7500         1.20
Arc lines were aligned along the detector columns with the mirrors, so that the "Image rotation" was effectively zero. The dichroics were then put in and the values for "Image rotation" in the table above refers to the offset between the arc lines at the top of the CCD and at the bottom. These values are in EEV12 pixels (i.e. 13.5 microns), the slit extent was 400 pixels, and the 1200B was used. In all cases no image rotation was found between the mirrors in different slides, only the dichroics with respect to the mirrors.
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Last Updated: March 2003
Javier Licandro licandro@ing.iac.es