Focal plane masks for FOCAS polarimetry

In order to avoid blending of the ordinary and extraordinary rays, the standard masks for polarimetry have eight slits (or slots) of 10.0 mm = 20.6 arcsec length aligned at equal intervals of a 11.5 mm = 23.7 arcsec length gap (see figures). The standard masks are made of poreless ceramics (fluorine mica). The currently available masks are shown in the table.

Mask name Slit width Offset Notes
for spectropolarimetry
0".4 center specpol. 0".4 center (0".0)  
0".6 center specpol. 0".6  
0".8 center specpol. 0".8  
1".0 center specpol. 1".0  
2".0 center specpol. 2".0  
0".4 offset specpol. 0".4 offset (113".4) (1)
0".6 offset specpol. 0".6 (1)
0".8 offset specpol. 0".8 (1)
1".0 offset specpol. 1".0 (1)
2".0 offset specpol. 2".0 (1)
for imaging polarimetry
Imaging pol.      

(1) for longer wavelengths only (e.g., 300B+Y47)

Figure 1. Schematic representation of standard masks for spectropolarimetry
 

Figure 2. Schematic representation of standard masks for imaging polarimetry



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