Graduate student Justin Moore and the 24-inch diameter PROMPT-8 telescope at Cerro Tololo Inter-American Observatory, Chile.

Graduate student Justin Moore and the 24-inch diameter PROMPT-8 telescope at Cerro Tololo Inter-American Observatory, Chile.

PROMPT Polarimeter

We designed and are now building a polarimeter to be attached to a 24-inch diameter telescope at Cerro Tololo Inter-American Observatory in Chile.  The telescope was funded by the National Astronomical Research Institute of Thailand (NARIT), and consists of a f/6.8 corrected Dall-Kirkham telescope mounted on a PlaneWave Instruments Ascension 200HR mount and enclosed in a 16-ft Astro Haven clamshell dome.  The telescope was completed and integrated into Skynet in 2013.  The polarimeter consists of two (V-band and I-band optimized, respectively) rotating half-wave plates and beam-splitter cubes from the Karl Lambrecht Corporation, and two Apogee/Andor CCD cameras that simultaneously image orthogonal polarization states through a set sequence of wave-plate rotation angles.  

Our control software interfaces with both custom and commercial components of the instrument.  An Arduino-powered microcontroller interfaces with the custom components:  (1) a linear actuator, to move the wave-plate/beam-splitter assembly across the optical path; (2) two stepper motors, to rotate the half-wave plates (one for each band); and (3) limit switches, to establish the home position for each moving component.  We have developed custom firmware for the Arduino and a custom serial protocol to interface with the Arduino from an external computer, which we do via an ActiveX scripting interface, to allow for looped movements and observation-specific movement orders.

The Skynet/PROMPT polarimeter consists of two rotating half-wave plate/beam-splitter cube assemblies (V- and I-optimized, respectively) on a linear actuator so either can be moved into the optical path, and two cameras that simultaneously image the resulting orthogonal polarizations through a set sequence of wave-plate rotation angles. This instrument will be used to measure the polarizations of gamma-ray burst afterglows more rapidly than has been achieved to date, within tens of seconds of spacecraft detection, when some afterglows are expected to be significantly polarized (and simultaneously significantly bright).

The Skynet/PROMPT polarimeter consists of two rotating half-wave plate/beam-splitter cube assemblies (V- and I-optimized, respectively) on a linear actuator so either can be moved into the optical path, and two cameras that simultaneously image the resulting orthogonal polarizations through a set sequence of wave-plate rotation angles. This instrument will be used to measure the polarizations of gamma-ray burst afterglows more rapidly than has been achieved to date, within tens of seconds of spacecraft detection, when some afterglows are expected to be significantly polarized (and simultaneously significantly bright).

For the commercial components (the cameras and the filter wheels), we have developed a custom client-server software package to achieve synchronous firing of the two cameras, via Diffraction Limited’s Maxim DL 5 (commercial software), installed twice on separate computers, and the local TCP/IP connection.  (Maxim supports dual cameras, but not synchronous imaging.)  This package operates each copy of Maxim DL, which can be scripted for polarization measurements.  For example:  move the wave-plate/beam-splitter assembly into position (V-band or I-band), rotate to 0º and take simultaneous images, rotate to 45º and repeat, rotate to 22.5º and repeat, and rotate to 67.5º and repeat.  This results in eight images, from which a single linear polarization is measured. Integration into Skynet is being achieved by treating the entire instrument as a custom filter wheel, with each filter/rotation-angle combination treated as a custom filter in Skynet’s database.

Once integrated into Skynet, and commissioned, the instrument will be used by UNC-Chapel Hill to measure polarization histories of gamma-ray burst afterglows on faster timescales than have been achieved so far, within tens of seconds of spacecraft detection, when some afterglows are expected to be significantly polarized (and simultaneously significantly bright).  Otherwise, the instrument will be broadly available to Skynet’s user community.  Already, users have expressed interest in using it to study a variety of polars; reflected light in a variety of binary systems, including systems including hot subdwarfs; and AGB stars in asymmetric planetary nebulae.