Polarimetry
 
This page documents the original automated polarimeter I have developed for the Epsilon Aurigae campaign used in 2009 and 2010. Is based upon a savart plate analyzer, a usb controlled broadband achromatic waveplate, photometric filters and an ST6 camera attached to a C8.  The C14 provides targeting and autoguiding.  It has been replaced with DIFPOL, a double image folded polarimeter for the C14, which operates at a higher level of accuracy.

This 14 pound instrument assembly can preform 0.1% polarimetry down to about 9th magnitude.

There are two key elements to this instrument.  1) The rotating achromatic waveplate and 2) the calcite savart plate analyzer.  

The Optec filter slider holds Johnson BVR photometric filters and is controlled by one of the ST7’s.  There is also a 400-700nm filter that slightly limits the B and R bandpasses to match the achromatic waveplate and a remote focuser.

Polarimetry_files/Polarization.HTML

Assembled Polarimeter.  The camera is an old SBIB ST6.  Although it has only 90K pixels, they are 23x27 microns and have 400K wells, just the spec for this application.   For stellar polarimetry, only the center of the field will be used.

Removing the nosepiece reveals the half wave achromatic waveplate manufactured by Bolder Vision Optik in Colorado.  This amazing multilayer polymer material has an almost flat retardance from 400 to nearly 750 nm. 

Below the waveplate is a 400-700 blocking filter. Below that is the 13 mm thick, 10 mm diameter octagonal Savart plate made by Halbo  optics in the UK.   Incoming light is split into two beams of opposite polarization that are separated by 1 mm.  Hence every star becomes a double!   The ratio of difference in brightness of the two images is the polarization across the axis of the crystal.   By rotating the waveplate, the incoming light’s polarization is rotated.  This is more convenient than rotating the camera. 


The waveplate assembly began with a “skylight” filter whose glass was removed.  A slot cut in the side allows a thin shaft to connect a rotatable retaining ring with a large external gear originally intended for a model helicopter.  To this is mated a smaller gear connected to the disk face of the servo motor.   The servo motor is controlled by a “Phidget” servo control. The USB cable connects to USB extender and then via 50 ft of RJ14 cable to USB computer block.