CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
37 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
was DIT x NDIT = 20s x 1. Note that a total of four exposures were taken at each nodding
position, with the polarimeter position angles at 0, 180, 180 and 0 degrees (see §6.3.7).
Figure 30:
The Stokes Q polarization is shown for the star Gamma Equ (H=4.18) at both
nodding positions A and B, taken with the 0.2" wide slit;; for comparison, we show the
performance of the Spirou spectrograph (green line). The exposure time of one exposure
was DIT x NDIT = 20s x 4. Note that a total of four exposures were taken at each nodding
position, with the polarimeter position angles at 0, 90, 180 and 270 degrees (see §6.3.7).
Performance: A comparison with another IR spectrometer SPIROU working in the same
spectral region and equipped with an SPU unit was done using the spectro-polarimetric
standard star Gamma Equ. The polarisation signal was compared in Y to K bands for
selected lines with strong Zeeman effect. We find good match for Stokes V and Q profiles.
Using the intensity profiles it was possible to confirm that the residual differences are due
to higher spectral resolution of CRIRES.
Figure 29
and
Figure
30
show the standard star
Gamma Equ, respectively for circular and linear polarization.
Limitations and systematics: Given that the observing conditions during commissioning
were bad, it was only possible to collect S/N of 200-400 on non-polarised test stars and that
is what is seen in the intensity, Stokes and Null spectrum. It can be said that the detection
limit and the systematics of the SPU/CRIRES are well below 0.01. Technical requirements
specify 10
-4
, but there was no way to check this without collecting spectra of non-polarised
