A&A 528, A138 (2011) Astronomy DOI: 10.1051/0004-6361/201015288 & c ESO 2011 Astrophysics A cryogenic waveplate rotator for polarimetry at mm and submm wavelengths M. Salatino1,2, P. de Bernardis1,2, and S. Masi1,2 1 Dipartimento di Fisica, Università di Roma “La Sapienza”, Roma, Italy 2 INFN Sezione di Roma 1, Roma, Italy e-mail: [email protected] Received 28 June 2010 / Accepted 30 January 2011 ABSTRACT Context. Polarimetry at mm and submm wavelengths is the new frontier of research in cosmic microwave background and interstellar dust studies. Polarimeters working in the IR to MM range need to be operated at cryogenic temperatures to limit the systematic effects related to the emission of the polarization analyzer. Aims. We study the effect of the temperature of the different components of a waveplate polarimeter and describe a system able to rotate a birefringent crystal at 4 K in a completely automated way. Methods. We simulate the main systematic effects related to the temperature and non-ideality of the optical components in a Stokes polarimeter. To limit these effects, a cryogenic implementation of the polarimeter is mandatory. In our system, the rotation produced by a step motor running at room temperature is transmitted down to cryogenic temperatures by means of a long shaft and gears running on custom cryogenic bearings. Results. Our system is able to rotate a birefringent crystal at 4 K in a completely automated way and dissipates only a few mW in the cold environment. A readout system based on optical fibers allows us to control the rotation of the crystal to better than 0.1◦. Conclusions. This device fulfills the stringent requirements for operations in cryogenic space experiments, such as the forthcoming PILOT, BOOMERanG and LSPE. Key words. instrumentation: polarimeters – techniques: polarimetric – dust, extinction 1. Introduction of these sources is necessary to perform a precise measurement of the polarized cosmological signal. This is made difficult by Diffuse dust in our Galaxy is heated by the interstellar radiation the small amplitude of the polarized signal of these sources and field at temperatures ranging from 10 K to 100 K, depending on by its strong angular and wavelength dependence (Tucci et al. the dimensions of the grains and the radiative environment. Their 2005). / emission is therefore in the submm FIR range and is partially po- Several experiments are planned to measure dust polariza- larized owing to the alignment of grains in the Galactic magnetic tion at high galactic latitudes. field (see e.g. Draine 2003). The Planck satellite (Tauber et al. 2010) and in particular the While FIR emission of interstellar dust has been mapped High Frequency Instrument (Lamarre et al. 2010) is performing quite accurately even at high galactic latitudes by the IRAS, ISO a shallow whole-sky survey of dust polarization at frequencies and Spitzer surveys and is investigated in deep detail by the cur- up to 345 GHz. Here, the signal caused by interstellar dust at rently operating Herschel observatory, its polarization properties high galactic latitudes is roughly similar to the level of the CMB are almost unknown (see e.g. Hildebrand & Kirby 2004). anisotropy (Masi et al. 2001, 2006; Ponthieu et al. 2005). Higher The interest in studying dust polarization is twofold. An ac- frequency surveys are more sensitive to the polarization signal curate measurement of dust polarization at different wavelengths of interstellar dust (see Fig. 1). is important to better understand the nature and structure of dust PILOT is a stratospheric balloon-borne experiment for the grains and to probe the magnetic field of our Galaxy (see e.g. measurement of the polarization of the continuum emission in Vaillancourt 2008; Hildebrand et al. 2000). the diffuse interstellar medium at frequencies around 545 and Moreover, polarized dust emission is an important contami- 1250 GHz, with a resolution of 3.29 and 1.44 , respectively nating foreground in precision measurements of the polarization (Bernard et al. 2007). The experiment is optimized to perform of the cosmic microwave background (CMB), the current ambi- surveys of the polarized dust emission along the Galactic plane tious target of CMB measurements. Measuring it at wavelengths but also at high Galactic latitudes, where previous experiments where dust polarization is dominant is mandatory to correct the have provided only upper limits (Benoit et al. 2004; Ponthieu cosmological signal at the level required to measure B-modes et al. 2005). PILOT will probe the large-scale distribution of the (see e.g. Lazarian et al. 2009). Diffuse Galactic sources act Galactic magnetic field and the alignment properties of the dust as a foreground, mimicking the cosmological polarized signal grains, providing strong constraints for dust models, in particular (Hanany & Rosenkranz 2003; Ponthieu & Martin 2006): at fre- via the dependence on frequency of the degree of polarization. quencies above 100 GHz interstellar dust is the dominant fore- BLAST-pol, the polarization sensitive version of the very ground. Therefore, an accurate knowledge of the polarization successful BLAST balloon telescope, will also search for Article published by EDP Sciences A138, page 1 of 8 A&A 528, A138 (2011) Fig. 2. Contour levels of the upper limits for the errors in the polariza- tion degree σΠ (left) and in the orientation angle σψ (in degrees, right), caused by a positioning error σθ, versus the values of the Stokes param- eters S 1 and S 2 normalized to S 0. The figure refers to N = 8. The thick contours identify the normalized values of S 1 and S 2 for which Π is, from bottom left to top right, 3%, 5%, and 10%. Fig. 1. Measurements of rms fluctuations in interstellar dust emission ◦ at Galactic latitudes above 20 (data points with error bars) from In PolKa a reflective system equivalent to a reflective wave- BOOMERanG (filled circles, Masi et al. 2001, 2006) and Archeops plate is composed of a rotating polarizer close to a reflector at (empty circle, Ponthieu et al. 2005). The continuous line is a thermal 2 room temperature (Siringo et al. 2004). For a review of polariza- spectrum with Td = 18 K and ν emissivity. The dashed line represents rms CMB anisotropy for ΔT/T = 10−5. The dash-dotted line represents tion modulation techniques see Ade et al. (2009). the spectrum of the polarized signal from a dust cloud with brightness In parallel to the development of techniques for polarization ff of 1 mKRJ at 353 GHz and 10% polarization (see Benoit et al. 2004). modulation, a vigorous theoretical e ort attempts to describe the The symbols mark the frequency bands explored by forthcoming polar- behavior of real dielectric HWPs (see e.g. Savini et al. 2009; ization measurements from several experiments: Planck (up triangles), Bryan et al. 2010a; Bryan et al. 2010b); moreover very promi- PILOT (squares), BLAST-Pol (stars), SPIDER (circles), EBEX (dia- sing metal-mesh waveplates have also been proposed (Pisano monds), SCUBA2 (down triangles), SOFIA (upgraded HAWC: pen- et al. 2008). tagons). In this paper we describe the system we developed for PILOT, BOOMERanG, and LSPE (de Bernardis et al. 2009a), where the HWP will rotate in steps and will be kept at cryogenic polarized dust emission in the galactic plane in the southern temperatures. The paper is organized as follows: in Sect. 2 we hemisphere (Marsden et al. 2008;Fisseletal.2010). briefly review the principle of operation of a HWP polarimeter SPIDER (Crill et al. 2008) is an ambitious balloon-borne in- and prove the necessity of cooling it down to cryogenic temper- strument aimed at the measurement of the polarization of the atures (Sect. 3). After listing (Sect. 4) the experimental require- CMB by means of large arrays of polarization-sensitive detec- ments for the necessary cryogenic waveplate rotator (CWR), we tors. Working in the stratosphere, SPIDER can cover high fre- present our implementation of the system; we describe the hard- quencies (in particular two bands at 225 GHz and 275 GHz) to ware structure (Sect. 5.1) and the tests we performed (Sect. 5.2). monitor polarized emission from interstellar dust where it is not We conclude in Sect. 6 summarizing the main features of our negligible with respect to the polarized component of the CMB. system. EBEX is also a balloon-borne CMB polarimeter, aiming at smaller angular scales and with a dust monitor at 410 GHz (Oxley et al. 2004). 2. The HWP polarimeter There are different instrumental techniques to detect faint polarized signals. Polarization-sensitive bolometers (PSB, Jones A polarimeter sensitive to linear polarization is composed of a et al. 2003) have been used in B03 (Masi et al. 2006)&Planck HWP, rotating at frequency f0, followed by a stationary polari- (Delabrouille & Kaplan 2001). zer. Monochromatic linearly polarized light, passing through the Orthomode transducers have been used in WMAP (Jarosik waveplate, emerges still linearly polarized, but with its polariza- et al. 2003) and other coherent systems. tion vector revolving at 4 f0. Unpolarized radiation is unaffected A stack of a rotating birefringent crystals followed by a static by the waveplate. If we place a polarizer between the waveplate polarizer implements a Stokes polarimeter, and has been used in and the detector, only the polarized component of the incom- the BRAIN-pathfinder experiment, where the waveplate opera- ing radiation is modulated by the rotation of the waveplate. The tes at ambient temperature (Masi et al. 2005). amplitude of modulation depends on the polarization of the ra- In MAXIPOL, instead, a DC motor outside the cryostat ro- diation: it is maximum (null) for radiation that is totally (not) tated a cryogenically cooled Half Wave Plate (HWP) by means polarized. The signal of interest is encoded at 4 f0, far from the of a fiberglass drive shaft crossing the cryostat shell and the ter- spectral region where 1/ f noise is important.