Provide sequence to improve sampling of the psf

We can shift and dither the slit to improve the sampling.
I were told that the shift would need to be 0.5 pixel
That would get us nine positions :

• (-0.5;-0.5)
• (-0.5; 0 )
• (-0.5; 0.5)
• (0 ;-0.5)
• (0 ; 0 )
• (0 ; 0.5)
• (0.5;-0.5)
• (0.5; 0 )
• (0.5; 0.5)