Maximize the Polarization Efficiency

Determine your POSANG (p2pp) approprietly to have the derotator angle in such a way that you maximize the instrumental polarization efficiency (Figure 22. of the User Manual). 

SPHERE_IRDIS_DPI_POSANG_v1.xls

(Prepared by H. Avenhaus, modified by J. Girard. Analysis of cross-talks performed by R van der Holstein, J. de Boer and J. Girard).

USAGE:

1. Type in the declination of your object (left blue cell in column C)
2. Start with a zero position angle or "field PA" (right blue cell in column D)
3. Look at columns E F G for to find out at which time (difference from the meridian at 00:00) the efficiency is the best (green or yellow).
4. Iterate by changing the position angle  until you center the best observing slot around you the time at which you plan to observe your target.

EXAMPLE:

1. Declination = -40
2. With 0 degree position angle, the optimum efficiency seems to occur about 2 hours after the meridian 
3. Column E says that the optimum position angle to observe the start at around the meridian (00:00) is 105 degrees
4. Indeed, by modifying the position angle (field PA) to 105º, the green zone is centered at 00:00. This star transitiing only ~15 degrees from zenith, there is only 1h10 around the meridian to observe this star with the best possible efficiency. Changing the field PA to 80 degrees, one can find that such object can be observed longer with good efficiency starting ~2 hours before the meridian.