Dipol-UF: Three-band high-speed polarimeter
Dipol-UF polarimeter is capable of making simultaneous measurements in three passbands, B, V,
and R, with high sensitivity. The detection limit of polarization is at the level of 10
-5
(0.001 %), set
in practice by photon noise. A schematic layout of the instrument is shown in Fig. 1.
With the dichroic beam-splitter system, polarized images can be recorded simultaneously in three
colors. The instrument is equipped with three high-speed ANDOR iXon Ultra 897 EM-CCD cameras
(Fig. 3). The cameras can be used in two amplification modes, 1) conventional amplifier: widest
dynamical range, readout rate up to 3 MHz, best suitable for bright targets, and 2) EM amplifier:
low readout noise, readout rate up to 17 MHz, best suitable for faint targets. For observations of
bright targets and the highest S/N measurements, defocusing is used to spread light over a large
number of pixels, to avoid saturation and collect up to 10
8
photons in one stellar image. The plane-
parallel calcite plate works as the polarization analyzer, splitting each stellar image into two
perpendicularly polarized images in the focal plane (Fig. 2). The distance between the two images
is 2 mm (~14.6 arcsec at the NOT). This limits the size of extended sources to about < 10 arcsec, to
avoid overlapping of the polarized components.
Figure 1: Dipol-UF layout
1) Retarder plate (λ/2 or λ/4) rotated by
a stepper motor
2) Retractable calcite unit
3) Dichroic beam-splitter #1, blue
reflector
4) Dichroic beam-splitter #2, red
reflector
5) Negative lens
Three CCD cameras take exposures
synchronously in the three passbands at each
position angle of the retarder plate.
Figure 2:
Left: Image of the area of stellar
sky with calcite retracted.
Right: The same area of sky with
calcite inserted.
The field size is 1 x 1 arcmin in the
B-band and 0.7 x 0.7 arcmin in the
V and R bands.
For measuring linear polarization a superachromatic half-wave plate (HWP) is used. For circular
polarization, a quarter-wave plate can be installed. Changing the plate takes about 15 min and
should be done daytime. One measuring cycle for linear polarization consists of sixteen exposures
taken at different orientations of the HWP separated by 22.5 degrees steps, i.e. at 0, 22.5, 45,
67.5, etc. This gives four measurements of the Stokes parameters q and u over one full rotation of
the HWP. Exposure time for each passband can be set individually.
The Dipol-UF is equipped with a retractable calcite unit and can be used also as a high-speed
simultaneous three-band photometer. The calcite can be moved away and put back to the optical
path remotely. Switching between polarimetry and photometry modes, done via control software,
takes approximately 20 seconds.
Dipol-UF is controlled by three industrial-grade mini-PCs. The process of observations is highly
automated and all observations can be done remotely. Software based on the ANDOR CDK has
been developed for the instrument control and data acquisition. It works under Windows OS
running on the main control mini-PC. The observer operates the instrument during observing
session via network connection.
Figure 4:
Dipol-UF mounted at the Nordic Optical
Telescope during commissioning two-
nights run in July 2019.
Figure 3:
Andor iXon Ultra 897 EM CCD camera.
