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SPeckle Polarimeter: a short description

Schematic view of the instrument is presented in picture to the right (not to scale). The baseline optical design is standard: the collimator (9) forms a parallel beam, which is then focused by the objective (16) on the detector (17). The focal length ratio of the collimator and the objective increases effective focal length of the telescope 8 times. Several essential optical elements are installed in parallel beam, the Wollaston prism (15) is among them. Thanks to it optics forms on the detector two orthogonally polarized images side by side. Resulting effective field of view of the instrument is rectangular: 5″х10″. The overlap of orthogonally polarized images is prevented by the slit field diaphragm (8). It is installed in primary focal plane. Right before this plane the prefocal unit is located, the linear translation stage (2) being its main part. Two flat relay mirrors (3) and linear polarizer (4) are installed on this stage.

The stage has three working positions. While it is in position off relay mirrors and linear polarizer are drawn out of the beam and light from the telescope (1) passes to the instrument freely. In position linpol the light from the telescope passes through the linear polarizer (4), which is needed for calibration. In position find one mirror directs light from the telescope to auxiliary camera (4,5), which is used for finding and centering the object. In the meantime second mirror directs light from internal calibration source (7) to the instrument.

Rotating half-wave plate (10,11) is used for rotation of polarization plane of incoming radiation, which allows to measure both Stokes parameters describing linear polarization. This is also neeed for realization of spatio-temporal modulation in polarimetry mode. It is possible to change HWP automatically using the special turret (10). In one of positions of this turret the knife diaphragm is installed, it is used for alignment of the axis of rotation stage (11) and center of exit pupil of the instrument.

The atmospheric dispersion compensator (ADC) (12, 13; made by RIVoptics ) is also located in parallel beam. The ADC comprise two independently rotatable direct vision prisms (Amici prisms). Each of prisms consists, in turn, of two prisms, one made of F1 glass, another - of K8 glass (Lytkarino). Apex angles of prism components are chosen so that the beam passes through the prism without large deviation, and in the meantime it acquired dispersion. By rotating the prisms one can create the dispersion of any direction and amplitude in some range. This values are set so that the atmospheric dispersion is compensated for current altitude, position angle and in observation wavelength.

The standard Bessel filters (Asahi spectra) and middle-band filters centered on 550 (Edmund Optics), 625 (Edmund Optics), and 880 (Thorlabs) nm are installed in filter wheel (14), which is also located in parallel beam.

We use Electron Multiplying CCD as a main detector. Detectors of this type are almost free of readout noise (at the price of two-fold increase in photon noise) and allow to obtain series of images with relatively high frame rate (35 frames per second). The detector has quite small amount of cosmetic defects and high quantum efficiency.

The instrument has 6 motorized degrees of freedom: 4 rotation stages and 2 actuators (Standa). These drives and main detectorare controlled by software Sparkle2, which runs as daemon in background. Auxiliary camera control software bullseye2 runs in background either. Observer can interact with Sparkle2 and bullseye2 through GUI called specktate or directly, sending text commands over TCP/IP protocol. The first is conventional mode of operation during observations, the second is engineering mode.

The series are being saved in FITS datacubes on the control computer. Database is employed for accounting the data. Processing pipeline is written in Matlab. All software (controlling and processing) is stored in git repositories and can be provided on request.

The instrument can be mounted both in Cassegrain focus and Nasmyth focus (station 2). The process of mounting takes 2 hours. In both foci the focal ratio of the telescope is the same: F/8. Cassegrain focus is prefferable for precise polarimetry and polaroastrometry. The measurements in other modes can be conducted in Nasmyth focus. Correction of instrumental polarization in Nasmyth is possible down to absolute accuracy of 0.15 percents. Correction for differential polarimetric aberrations (which are important for differential speckle polarimetry) arising at Nasmyth focus is described here.

Methods

The following method can realized with SPeckle Polarimeter:

1. Speckle interferometry was suggested as a method of restoration of absolute value of object Fourier transform (absolute value of visibility function) from series of short-exposure images disturbed by the atmosphere (Labeyrie, 1970). Later a method of restoration of visibility function phase was also proposed. It uses averaging of bispectrum over series (Lohmann, Weigelt, Wirnitzer, 1983). Speckle interferometry is applied mainly to measurement of binary stars parameters with separations from 0.05″ to 2″.
2. Polarimetry. The method allows to measure the polarization of radiation of celestial bodies. While mounted in Cassegrain focus the instrument allows to measure polarization with precision if 1e-4 from the V=12 in 15 minutes of accumulation. As far as the field of view of the instrument is small, for most of objects the simultaneous measurement of polarization and flux is not possible. Polarization in astrophysics arises universally: during scattering, for non-thermal radiation, after propagation of radiation in plasma, etc. The measurement of polarization dependence on time, wavelength, coordinate provides essential auxiliary information on the object.
3. Differential Speckle Polarimetry. The method of acquiring of information about distribution of polarized flux with diffraction limited resolution and beyond (Safonov et al, 2019). This relatively new method, very similar to suggested by (Norris et al., 2012), is useful for investigation of circumstellar environment, active galactic nulcei, supernova and other objects, for which it is difficult to localize the source of polarized radiation. One of interesing flavor of DSP is polaroastrometry – measurement of deviation of centroid of polarized flux from centroid of total flux. The effectiveness of this method was demonstrated by us during activities connected with speckle polarimeter development (Safonov, 2015).
4. Fast photometry. Main detector of speckle polarimeter allows to obtain images of source with frequency up to 1000 frames per second. These images can used for flux estimation. Fast photometry can be applied, e.g. in observations of lunar occultations. Fast polarimetry is also possible.