CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
72 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
·
Polariziation Stokes Parameter
(TSF:
INS.POL.TYPE
) sets the desired Stokes parameter
(Q, U or V). Note that a sequence of multiples of four exposures is always taken (i.e., for
SEQ.NEXPO
= 1, four exposures are taken), with the following polarimeter rotation angles:
For Stokes V at 0, 180, 180 and 0 degrees,
for Stokes Q at 0, 90, 180 and 270 degrees, and
for Stokes U at 45, 135, 225 and 315 degrees.
Note that these values are for
INS.ROT.POSANG
=0.
·
Polarimeter: rotation angle
(TSF:
INS.ROT.POSANG
) sets the initial reference angle on the
sky for linear polarization (Stokes Q and U) - this can be useful when observing extended
objects. This value is added to the aforementioned rotation sequence;; the allowed range is
from -180 to 180 degrees. Note that this parameter must be kept at 0, as currently extended
objects cannot be observed in polarimetry mode.
·
Number of exposures per nodding position
(TSF:
SEQ.NEXPO
): gives the total number
of Stokes cycles per nodding position, with one cycle always being 4 exposures. For
example, for
SEQ.NEXPO
= 2 and
INS.POL.TYPE = Q
, eight exposures are taken, with the
polarimeter rotation sequence 0, 90, 180 and 270 executed twice.
Figure 43: Illustration how the polarized image is split (corresponding to Decker24).
