Optical design & Instrumentation

Ivan Syniavskiy

Visible Imaging Stokes Polarimeter (VISP-instrument)

The main purpose

Instrument is designed for simultaneous production of four images of the space object, followed by calculation of the three Stokes parameters describing linearly polarized radiation. As the polarization analyzer in the instrument used by the polarizing film, set in the exit pupil of the telescope, which is formed by a collimator. The analyzer consists of four segments with different orientations of 0°, 45°, 90°, 135°.

The principle of imaging Stokes polarimeter

Specification

  • Telescope
    D=360…1000 mm, D/f = 1/12...1/10
  • FOV, Angular of telescope / linear on detector
    18’ x 18’/ 17x21 mm (1.6”/px)
  • Effective focal length of the system
    1200 mm
  • Instrument aperture ratio F#
    11
  • Diameter of pupil
    24 mm
  • Detector: Pixel size / Total size
    16x16 µm / 16,5х20,4 mm
  • Spectral range
    Optimized for 0.41–0,85 µm
  • Filters
    Position for 4 filters in wheel
  • System of filters replacement
    Manual, possibility of using a stepper motor
  • Diffraction grating
    Position for the transmission diffraction grating (100 lines/mm)
  • Focusing system
    Main mirror
  • Length of instrument
    800 mm
  • Weight of instrument
    6 kg

Short description

The polarimeter consistst of:

The collimator performs several functions: partial correction of field aberrations of the telescope, the construction of main mirror image (the pupil) with a diameter of 24 mm, and collimation of the radiation itself.

The analyzer of the polarization - the polarizing film, set in the pupil, formed by a collimator. The analyzer consists of four segments with different orientations of 0°, 45°, 90°, 135°, forming 4-channels.

The Space diversity system - four pairs of achromatize wedges.

Spectral selection element - replacement filters or transmission diffraction grating with a frequency to 100 lines per mm. Light diameter of replaceable elements (filters, gratings) is equal to 24 mm.

The camera lens - the simultaneous formation of four images on a detector. It consist of seven components and builds 4 image of sky in the different polarization.

Four images of the sky on the detector

Optical conceptual design

General view of the polarimeter optical layout

Image quality

The function of the encircled energy concentration which characterizes the quality of the optical layout is presented in the Fig.4. Spot size in the spectral range of 0,41-0,85 m for each point within the field of view 18 'x 18', which concentrates 95% of the energy of 32 µm which corresponds to 2 pixels for the matrix. The maximum value of the distortion of the optical system of the Stokes polarimeter is 0.65%.

The function of an encircled energy
Distortion of the system

Mechanical conceptual design

The mechanical design of the instrument is shown in Figure 6. Structurally, the Stokes polarimeter consists of the following units. Composite tube having sufficient rigidity, in which a collimator and block of removable diaphragm and slits are placed. The diaphragm or slit entered into the system manually. For panoramic observations with the maximum field of view 18 'x 18' the diaphragm with size equal 32x32 mm is using.

Parts of polarimeter
General view of polarimeters