Funded by NSF awards to Northwestern University and to The University of Chicago; and by NSERC awards to The University of Western Ontario.
 

 

 
 
The SHARP polarimetry module


Detector technology for submillimeter wavelengths has undergone dramatic changes in recent decades, with steady increases in the numbers of pixels and optical efficiency (percentage of photons that are detected).  The detector array in the SHARC-II camera is currently the most advanced array available in its wavelength range, with 388 pixels (in a 32 by 12 configuration) and optical efficiency near 90%.  SHARC-II is operated at the Caltech Submillimeter Observatory (CSO) at the 14,000 foot summit of Mauna Kea on the island of Hawaii.  We have built a fore-optics module, called "SHARP", that we insert in front of this advanced camera in order to convert it into a sensitive imaging polarimeter.

         
 
 
Table of Specifications

 

Central Wavelength

350 microns

450 micronsa

bandwidth [ (delta-lambda) / (lambda) ]

0.13

0.10

field of view of 12 pixel x 12 pixel array

55 arcsec x 55 arcsec

55 arcsec x 55 arcsec

pixel size

4.6 arcsec x 4.6 arcsec

4.6 arcsec x 4.6 arcsec

pixel size, measured in terms of (lambda / D)

0.66 lambda / D

0.52 lambda / D

angular resolution

9 arcsec

11 arcsec

point source flux for (sigmaP = 1%) in 5 hoursb

3.6 Jy

2.0 Jy

surface brightness for (sigmaP = 1%) in 5 hoursb

0.63 Jy per SHARP pixelc

0.35 Jy per SHARP pixelc

Max. separation of main & reference beams

8 arcmin

8 arcmin

systematic errors, sigmaP(sys)

below 0.2%

below 0.2%

a - all estimates of required flux for 450 micron band are +/- 20%

b - assumes binning over 4 SHARP pixels, which is approximately one resolution element

c - one SHARP pixel = 4.6 arcsec x 4.6 arcsec = 21 arcsec2

 
         
 
Optical Design




At left we show two views of SHARP.  In the upper illustration, the submillimeter light beams from the Nasmyth focus of the CSO telescope enter SHARP from the left, and are relayed through an optical path including flat mirrors and curved mirrors as well as polarimetric components such as free-standing wire polarizing grids and a birefringent crystal.  The light then passes into the M4 box (blue parallelogram at right of illustration) that contains the first mirror of the SHARC-II camera.  The lower illustration shows the view looking at SHARP from the Nasmyth focus.

The detailed photon path is as follows: First, an off-axis paraboloidal mirror collimates the expanding beam from the Nasmyth focus; then a gold-coated flat mirror directs the light toward the "crossed-grid" (see photo below) that serves to separate the incident radiation into two orthogonal polarization components.  The two orthogonal components then travel in opposite directions as shown in lower illustration.  (Just before it reaches the crossed-grid, the light is transmitted through a rotating birefringent crystal quartz half-wave plate that is not shown in these illustrations).  After exiting the crossed-grid, there are more reflections by curved and flat mirrors, and then the beams come to a focus (see bottom of lower illustration) at the "beam combiner".  The effect is to reconstitute the image but with an offset between the two polarization components.  It is this divided image that is re-focused back into SHARC-II.  The advantages of simultaneous detection of two orthogonal components of polarization are explained below.


The optical design was carried out by former Northwestern graduate student Hua-bai Li.  He used the Zeemax optical design software package for ray-tracing as well as diffraction studies.
         
Dual Polarization Capability


One of the key features of SHARP is simultaneous detection of two orthogonal polarization components. As described above, the effect of the SHARP module is to split the incident beam coming from the Nasmyth focus into two orthogonally polarized beams that are then re-imaged onto opposite ends of the "long and skinny" SHARC-II bolometer array. When the (removable) SHARP module is installed, SHARC-II becomes a dual-polarization 12 x 12 pixel polarimeter. (The central 12 x 8 pixels are not used by SHARP.)

The photo at right shows the crossed-grid, a compact device that allows us to split polarization components without introducing long optical paths. This device, that was fabricated in the United Kingdom by TK Instruments, consists of two intersecting orthogonal free-standing wire grids.

The "dual-polarization" capability not only avoids wasting photons, but more importantly serves to reduce noise.  Specifically, we can reduce the effects of signal variations caused by variable atmospheric emissivity, called "sky noise".  Sky noise is a very significant source of error, but it is correlated between polarization components, so dual-polarization capability allows us to remove it during data analysis.

         
Modular Design


The photo at left shows the SHARP module installed at CSO. SHARP is the stack of four boxes, shaped like an inverted "L". The large cyllinder at right with parallelogram-shaped extension is SHARC-II.  The light from the Nasmyth focus of the CSO telescope enters SHARP from the left.  The top box can be removed to convert from polarimetry mode back to the ordinary SHARC-II camera mode.
         
Documentation for SHARP Observers