Astronomical Polarimetry 2008: Science from Small to Large TelescopesASP Conference Series, Vol. 4**, 2009Bastien and Manset

Instrumentation for Far-IR and Submillimeter Polarimetry

Giles Novak

Department of Physics and Astronomy, Northwestern University,2145 Sheridan Road, Evanston, IL 60208, USA

Abstract. Dramatic advances in far-IR/submillimeter detector technologyhave been underway for several decades and are likely to continue. Polarimetersthat take advantage of these advances have been developed by several groups,and are being applied to study important problems like star formation. Herewe restrict the discussion to ground-based or stratospheric polarimeters operat-ing on single-dish telescopes. We review some of the technical challenges andwe explain how they are being overcome, using the polarimeters SPARO andSHARP as examples. We also discuss several advanced polarimeters now underdevelopment.

1. Introduction

Far-IR/submillimeter polarimetry is a broad field. Much work has been doneusing single-dish submillimeter telescopes located at the summit of Mauna Kea(JCMT and CSO), on other mountaintops, and at South Pole station. Inter-ferometric polarimetry is now routinely done at SMA at wavelengths below onemillimeter. Cosmologists concerned about foreground contamination are carry-ing out balloon-borne large-beam polarimetry over a range of frequencies, some-times extending to wavelengths shorter than one millimeter (Benoıt et al. 2004).Far-IR polarimetry was carried out at 60 and 100 microns from the airborneKAO telescope, and plans for SOFIA polarimeters are now being developed.PLANCK will do all-sky polarimetry at 850 microns.

Polarimeters for single-dish telescopes that operate from the ground, bal-loons, or aircraft face a similar set of challenges. These include operating de-tectors at temperatures below 1 K in remote environments and dealing withatmospheric “background” power that is larger than the polarized flux levels ofone’s astrophysical target by many orders of magnitude. For example, Chuss etal. (2004) calculate that for ground-based submillimeter observations in the 350micron window from Mauna Kea, the background dominates the signal by sixorders of magnitude. By operating in the stratosphere one can bridge this gap tosome extent, but in this case one often moves toward shorter wavelengths wherethe atmospheric opacity is relatively higher, so background removal usually re-mains very challenging even for stratospheric polarimeters. In this paper, we willdiscuss these challenges and how they have been tackled, using two ground-basedpolarimeters that were built at Northwestern University as examples. These areSPARO and SHARP. Along the way, we will briefly discuss the U. Chicago in-struments STOKES for KAO (Platt et al. 1991) and Hertz for CSO (Schleuninget al. 1997; Dowell et al. 1998), and we’ll touch on polarimetry at JCMT using

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Figure 1. Submillimeter polarimetry of the starless core L183 (Ward-Thompson et al. 2000; Crutcher et al. 2004), carried out at JCMT usingthe SCU-POL polarimeter. Vector orientation shows inferred B-field direc-tion, and vector length is proportional to polarized flux. Contours indicatetotal intensity.

SCU-POL (Greaves et al. 2003) and soon POL-2 (see contribution by P. Bastiento these proceedings).

Far-IR/submillimeter polarimetry is and will continue to be a rapidly mov-ing field, thanks to dramatic improvements in detector technology (more pixelsand better efficiency per pixel) and new telescopes at better sites (e.g., S. Pole,Atacama, SOFIA). One can distinguish between two classes of polarimeter: astand-alone polarimeter includes its own detectors and only requires a telescopeto begin collecting data, while an add-on polarimeter brings polarimetric capa-bility to an existing camera. SPARO was a stand-alone polarimeter. It incorpo-rated two 9-pixel Helium-3 cooled bolometer arrays and was used with the Vipertelescope at South Pole station during 1998-2004 (Novak et al. 2003; Renbargeret al. 2004; Li et al. 2006). SHARP is an add-on polarimeter for the 384-pixelSHARC-II camera at CSO (Li et al. 2008).

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2. Scientific Motivation for Far-IR/Submillimeter Polarimetry

One of the primary motivations for far-IR/submillimeter polarimetry is to mapthe magnetic fields that permeate star forming molecular clouds. Here oneexploits the fact that magnetic alignment of interstellar grains is ubiquitous,and seems to persist even in dense well-shielded environments (Hildebrand etal. 2000; Crutcher et al. 2004). Even though we have not yet established thephysical mechanism that causes grain alignment, we can be reasonably sure thatthe orientation of the E-vector is perpendicular to the projected field directionfor most environments (Lazarian 2007).

Some theorists argue that magnetic fields play very important roles in thestar formation process, e.g., by supporting clouds and/or by regulating angularmomentum flow from small to large scales. Other theorists argue that the fieldsare too weak to play crucial roles. A good summary of the current state ofknowledge is provided by McKee & Ostriker (2007).

Far-IR/submillimeter polarimetry cannot directly determine the character-istic strength of molecular cloud magnetic fields, a poorly known quantity that isthe subject of intense debate. But far-IR/submillimeter polarimetry can provideever-increasing quantities of data on the projected magnetic field directions inmolecular clouds, and thereby constrain the theories and models. For example,most strong-field models predict that cloud cores should be oblate with minoraxis parallel to the field. A few starless cores have been studied polarimetrically(Ward-Thompson et al. 2000; Crutcher et al. 2004). As illustrated in Figure 1,the predicted alignment is not observed. More observations of these cold sourcesare needed, and they should come soon. The polarimetric map shown in Fig-ure 1 was obtained using SCU-POL, a add-on polarimeter for SCUBA that had37 pixels at 850 microns. SCUBA2 will have ∼5000 pixels at this wavelength,and will have an add-on polarimeter, POL-2 (see contribution by P. Bastien tothese proceedings).

How do the magnetic fields of molecular clouds relate to the large scale fieldof the Galaxy? We computed the median difference between inferred field di-rection and Galactic plane orientation for three submillimeter/millimeter wave-length polarimetry data sets, and in Figure 2 we plot these as a function ofthe approximate size scale sampled. The leftmost data point comes from asample of source-average field directions given by Hildebrand (2002) for sourcesobserved with STOKES/KAO and Hertz/CSO. As indicated on the abscissa,the STOKES and Hertz maps generally cover spatial scales of order a few pc,corresponding to cores and clumps in GMCs. (When computing the median forthe Hildebrand 2002 sources, we omitted Galactic center sources.) The right-most data point comes from the WMAP 3 mm polarization map (Page et al.2007) where we have imposed a Galactic latitude restriction as noted on theplot. These measurements sample long sightlines through the Galaxy, as againindicated on the abscissa. The intermediate-scale data point comes from ourSPARO 2003 GMC survey (Li et al. 2006), in which we mapped sky areas thatwere larger than those sampled in previous submillimeter polarimetric studies ofGMCs. The Li et al. (2006) measurements correspond to the global, ten-parsecscale fields of GMCs, as indicated on the abscissa.

Figure 2 suggests that the orientation of the large-scale Galactic field issomehow preserved during GMC formation, even if there is significant disorder

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Figure 2. Median deviation ∆φB of B-field direction from angle of Galac-tic plane, calculated from submillimeter polarimetry of cores and clumpswith Hertz/CSO and STOKES/KAO (Hildebrand 2002), submillimeter po-larimetry of GMCs with SPARO at South Pole (Li et al. 2006), and WMAPmillimeter-wavelength polarimetry of extended Galactic dust emission (Pageet al. 2007).

Figure 3. Photo of the detectors, Helium-3 refrigerators, and 4 K cold-platein SPARO, with the optics removed. The location of the rotating half-waveplate (HWP) and the polarization-splitting wire grids is shown schematically.The two bolometer arrays detect orthogonal polarization components fromthe same sky location. The cold-plate diameter is ∼12.5 inches.

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Figure 4. Chop-demodulated output for two corresponding pixels in thetwo detector arrays of SPARO, for observations of G 333.6-0.2 made duringAustral Winter 2003. The signals from G 333.6-0.2 have been removed fromthese data, leaving only sky noise. The dashed curve has been shifted upwardfor clarity. The horizontal axis spans several hundred seconds of elapsed time.

on parsec scales. This provides a constraint on models and theories that dealwith formation of molecular clouds. The SPARO data point is derived from justfour cloud-average field directions, so it must be confirmed with more measure-ments. We will learn much more about the detailed shape of the curve crudelytraced by the points in Figure 2 from the PLANCK all-sky submillimeter polar-ization map, which will have five arcminute resolution.

3. SPARO, a Stand-alone Polarimeter

By collaborating with cosmology groups to develop the Viper 2-meter SouthPole telescope (Peterson et al. 2000) that was used for ACBAR and other CMBexperiments, our group at Northwestern was able to secure a significant fractionof the Viper observing time for SPARO observations of large-scale Galactic fields.During Austral winter 2000 we studied the Galactic center (Novak et al. 2003),and during Austral winter 2003 we observed four GMCs in the Galactic disk, asdiscussed in the previous section (see also Li et al. 2006; Novak, Dotson, & Li2009).

The submillimeter atmospheric opacity at South pole is lowest during thewinter season, but operating Helium-3 refrigerators in winter at S. Pole is chal-lenging. Outdoor temperatures reach -100◦F, and since no flights to or fromPole are allowed during winter, experiments that winter-over are generally op-erated by a single team member. To cope with these technical challenges, weintroduced three novel features into the cryostat design (Dotson et al. 1998;Renbarger et al. 2004, see also Fig. 3): (1) a capillary-fed continuously-pumped

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1.5 K Helium-4 stage for re-condensing Helium-3 without pumping on the mainHelium-4 bath, (2) dual vapor-cooled radiation shields for long Helium-4 holdtime and low cryogen usage rate, and (3) all-metal seals for cryostat reliabilityin case of power failure. The SPARO cryostat worked well, and our 2000 obser-vations yielded the first astronomical result ever obtained using Helium-3 cooleddetectors at South Pole during winter (Novak et al. 2003).

SPARO’s detectors, optical design, and data acquisition scheme were allpatterned after the U. Chicago polarimeters STOKES and Hertz (see section 1).In particular: We used two detector arrays for simultaneous detection of orthog-onal polarization components (Fig. 3), we employed a rotating half-wave plateused in stepped mode, and we modulated a telescope mirror (“chopper”) at ∼3Hz as a first step in removing the huge atmospheric background (see section 1).Just as for the Chicago instruments, the second step is to difference chopper-demodulated signals measured in corresponding pixels of the two arrays, sincethey are viewing orthogonally polarized signals from the same sky position. Thisimportant step cancels correlated “sky noise” (see Fig. 4). Data were collectedin chop-nod mode, with one photometric chop-nod integration being carried outat each of six half-wave plate positions. This “Chicago data acquisition method”(Hildebrand et al. 2000; Dowell et al. 1998) has proven to be very successful,provided that one carries out the division of polarized flux by total flux onlyafter enough data have been accumulated to ensure that the latter is well mea-sured (Kirby et al. 2005; Li et al. 2005). For SPARO, this occurs after one“set” of nine HWP cycles. The SPARO experiment was optimized for mappinglarge-scale fields. For our 2003 observations, the beam size was four arcminutesand the chopper throw was 40 arcminutes.

Given the six orders of magnitude that separate the polarization signal fromthe background (section 1), it is interesting to ask whether SPARO observationsreached the sensitivity limits set by fundamental noise sources. For our 2003observations, this fundamental noise was dominated by a roughly equal combi-nation of photon fluctuations and detector noise. The level of this fundamentalnoise is easily determined from the scatter in the corresponding-pixel differencesignal on 1-10 second time scales. We will refer to this scatter as the several-second time scale fluctuations, or “STS” fluctuations. Each sky position wasobserved via ten or more nine-cycle sets. The above question can now be re-stated as follows: Do the values of the normalized Stokes parameters q and uthat are derived from each set agree with one another within the expected levelsof error, where the expected error levels are calculated by error propagation fromthe STS fluctuations?

Quantitatively, the ratio of the measured variance in q (u) to the expectedvariance in q (u) is given by the reduced χ2 of q (u). For a clean data set (nonoise beyond the expected level) the reduced χ2 should be χ2

r ∼ 1 (Bevington& Robinson 1992).

For the SPARO 2003 observations reported by Li et al. (2006), we computedχ2

r in two steps: (1) For each set, calculate χ2

r for the polarized flux values andinflate the errors in the set-average Q (U) by the square root of this χ2

r, and (2)after propagating these new set-average Q (U) errors to form new set-averageq (u) errors, calculate χ2

r for the group of ten or more set-average q (u) values.The results were satisfactory. For example, for the Carina-Southeast field the

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Figure 5. The photo at left shows the 384-pixel Helium-3 cooled bolome-ter array used in the SHARC-II camera at CSO, and at right we show thesame photo with a superposed image illustrating the principle of operationof SHARP (Li et al. 2008). SHARP is a fore-optics polarimetry module thatsplits the incoming radiation into orthogonal polarization components andthen directs these to opposite ends of the array. The superposed image is aSHARP observation of Uranus with markings indicating E-vector orientation.

average of χ2

r(q) and χ2

r(u) is 1.23. If we omit step 1, we obtain 1.53 for this field,which suggests the presence of extra errors on 1-10 minute time scales. However,the reasonably low final values of χ2

r justify our two-step process which implicitlytreats these extra errors as random errors. From these arguments we concludethat the SPARO 2003 data are not significantly influenced by systematic errors.Furthermore, the final errors derived by Li et al. (2006) via the above process areonly slightly larger what would be expected from propagation of the fundamentalSTS fluctuations.

4. SHARP, an Add-on Polarimeter

In 2003, the success of the Chicago data acquisition method as applied toSPARO, Hertz, and STOKES, coupled with the unusual “long-and-skinny”shape of the detector array in the SHARC-II camera which had just begunoperating at CSO, led us to ask whether it might be possible to convert thiscamera into a dual-polarization polarimeter in which each end of the detectorarray would observe the same part of the sky but in orthogonal polarizations.The result was SHARP, the SHARC-II Polarimeter (Fig. 5, Fig. 6, Li et al.2008). This fore-optics module intercepts the submillimeter radiation as it trav-els from the Nasmyth focus to the SHARC-II relay optics, splits this radiationinto vertically and horizontally polarized components, and then directs thesetoward two 12 × 12 pixel “sub-arrays” located at opposite ends of the 12 × 32pixel detector array in SHARC-II. A rotating (warm) quartz HWP is includedjust upstream of the polarization-splitting grids. Data are acquired in chop-nod,stepped-HWP mode (i.e., Chicago data acquisition method).

Hertz and STOKES were mounted on instrument rotators to cancel skyrotation, thereby allowing for repeated measurements of identical sky positions.SPARO enjoyed the same benefit due to its geographical location. The SHARPdesign precluded an instrument rotator so new data analysis algorithms wereneeded. SHARP collaborators at U. Chicago and U. Western Ontario (Hilde-brand and Houde groups, respectively) took responsibility for developing and

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Figure 6. The modular design of SHARP (stack of boxes at left) simplifiesits installation on the right Nasmyth platform at CSO, just in front of theSHARC-II camera (right). The SHARP optical path includes ten reflectionsby grids and mirrors and achieves a throughput of 75% (Li et al. 2008).

implementing these algorithms, which we will now briefly describe. Due tosky rotation, the sky positions corresponding to several nights’ worth of single-cycle, corresponding-pixel I, Q, and U measurements do not fall onto a regulargrid. A computer program called “sharpcombine” (written by J. Vaillancourt)interpolates these onto a regular grid using algorithms described by Houde &Vaillancourt (2007). The interpolation involves error propagation from STS fluc-tuations as well as some smoothing and consequent loss of angular resolution.Subtraction of instrumental polarization and removal of sky noise from totalintensity maps via median filtering are also incorporated into sharpcombine.

Li et al. (2008) describe the optical design of SHARP, and show that thepolarimeter achieves good image quality (9 arcsecond resolution at 350 microns,like SHARC-II), good optical transmission through the polarimeter optical path(75%), correct primary illumination for both polarization components, and lowinstrumental polarization. The signal-to-noise is optimized by directing thetime-reversed light beams corresponding to the unused polarization componentsback into the cryostat. Considering only STS errors, SHARP has a point sourcesensitivity about three times better than what was achieved with Hertz. Theimprovement is mostly due to the high quality of the detector array in SHARC-II.

We have recently implemented a procedure for reliably estimating uncer-tainties in SHARP measurements, based on χ2

r tests. We will illustrate it withan example: observations of NGC 1333 IRAS 4 (both 4A and 4B) made during

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Figure 7. Submillimeter photometric mapping of a 50 square degree regiontoward molecular clouds in Vela, carried out with the BLAST balloon-bornetelescope (Pascale et al. 2008) during an Antarctic flight in 2006-2007. Imagestaken in the three wavebands are shown in orange (500 microns), green (350microns) and blue (250 microns). (See electronic version for color image.)BLAST is being rebuilt as a polarimeter, BLAST-pol (Marsden et al. 2008).

September 2008 (Attard et al., in prep.). We divided the complete data set,consisting of approximately 100 cycles, into ten bins. Using sharpcombine, weproduced regular-grid Q and U maps for each bin. By comparing the ten bins,we computed χ2

r maps for Q and U. We then repeated the procedure for the caseof four bins. From each of the resultant χ2

r maps, we computed three average χ2r

values: χ2

r(4A), χ2

r(4B), and χ2

r(map). The first two averages incorporate onlyχ2

r values near the respective flux peaks, while the third includes the whole map.After averaging together results for Q and U, we find that the ten-bin case givesχ2

r(4A) = 4.6, χ2

r(4B) = 2.7, and χ2

r(map) = 2.1, while the four-bin case resultsin χ2

r(4A) = 4.2, χ2r(4B) = 3.0, and χ2

r(map) = 2.2. The χ2r values exceed unity,

so its clear that our uncertainties are not entirely due to STS errors, similar towhat we found for SPARO (section 3). However, since the corresponding χ2

r

values are quite similar for the four bin and ten bin cases, we can conclude thatregardless of the source of the extra errors, we can treat them as intermediatetime scale random errors, just as we did for SPARO. Consequently, we inflatethe formal errors given by sharpcombine by the square root of the calculated χ2

r

to obtain our final error bars.The values of χ2

r(map) given above indicate that SHARP observations comewithin a factor of ∼1.5 of the STS-limited (fundamental) sensitivity for thefaintest regions. Furthermore, our work on NGC 1333 shows that SHARP canobtain secure polarization detections for a low-mass core. Specifically, aftererror bar inflation we have 19 3σ detections of polarization (Attard et al., inprep.). Besides NGC 1333, SHARP has been used to detect polarization in thehigh-mass star forming regions DR 21 (Kirby 2009) and Orion (Vaillancourtet al. 2008, see also Fig. 8) and to set a low upper limit on the 350 micronpolarization of DG Tau (Krejny et al. 2009). The contribution by H. Shinnagain these proceedings shows preliminary SHARP results for IRAS 20126, andobservations of several other targets are under analysis.

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Figure 8. Measured (symbols and dashed lines) and theoretical (solid curve)polarization spectra from Vaillancourt et al. (2008), shown together with ver-tical bands indicating frequencies and approximate passbands of advancedpolarimeters now under development. The polarization spectra are normal-ized at 350 microns. HAWC-pol and BLAST-pol will operate in the strato-sphere (see section 5), and POL-2 is an add-on polarimeter for the ∼5000-pixelSCUBA-2 camera at JCMT (see contribution by P. Bastien to this volume).The data point shown as a filled triangle was derived from SHARP data.

5. BLAST-pol and HAWC-pol, New Stratospheric Polarimeters

The future of far-IR/submillimeter polarimetry is bright. In section 2 we notedthe big advances that will be possible with POL-2 and PLANCK. On smallerscales, ALMA will open up vast new possibilities. In this section we describetwo new stratospheric polarimeters now under development.

BLAST, the Balloon-borne Large Aperture Submillimeter Telescope, hasbeen flown three times as a photometric experiment. With its 2-meter aper-ture and its SPIRE-prototype detector arrays having 266 pixels in three bands,BLAST has been successful at discovering and characterizing hundreds of pro-togalaxies as well as at surveying the Galaxy for cold clouds at the earlieststages of star formation (see Fig. 7). The conversion of BLAST into BLAST-pol(Marsden et al. 2008) is now underway, and BLAST-pol is funded through 2013.The telescope has been replaced and the camera repaired following a very roughlanding in 2007, and the team is now in the process of installing the polariz-ing grids and half-wave plate. The photolithographic polarizing grids mountdirectly to the detector arrays and have orthogonal polarization directions foradjacent rows of pixels, and the achromatic half-wave plate (Savini et al. 2006)will operate in stepped mode at 4 K. The first flight of BLAST-pol will be inlate 2010 from McMurdo, Antarctica.

BLAST-pol will map the global magnetic fields of scores of GMCs down tocolumn densities corresponding to AV = 4 mag. Its angular resolution, whichranges from 30 arcseconds for its shortest waveband (250 microns) to 60 arcsec-

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onds for its longest (500 microns) will allow up to 1000 vectors per cloud. Bycomparing the degree of order we discover in real clouds with that seen in simu-lated GMCs developed for different assumed values of the magnetic field strength(Ostriker, Stone, & Gammie 2001; Falceta-Goncalves, Lazarian, & Kowal 2008)BLAST-pol will constrain the field strength which is a crucial parameter for starformation theory.

Another stratospheric polarimeter currently under development is HAWC-POL, a polarimetric upgrade for HAWC, SOFIA’s four-band 384-pixel far-IRcamera. HAWC is being built at U. of Chicago (Harper et al. 2000) and aHAWC-pol insert consisting of a replacement for the cold pupil-wheel and con-taining one half-wave plate and one polarizing grid for each HAWC band isunder construction at JPL by C. D. Dowell and collaborators (Vaillancourt etal. 2007). To combat sky noise, the half-wave plate will be rotated continuouslyat ∼0.25 Hz while the secondary mirror chops at ∼10 Hz. HAWC-pol will allowthe first far-IR polarimetry since the KAO decommissioning more than 15 yearsago, and will benefit from a three-fold increase in telescope size compared toKAO.

Together, the advanced polarimeters HAWC-pol, BLAST-pol, and POL-2 will cover the entire far-IR/submillimeter spectrum with angular resolutionranging from 5 to 60 arcseconds. This is illustrated in Figure 8. Note thatSCU-POL did not have good sensitivity at 450 microns but POL-2 observers doplan to attempt observations at this wavelength, which would provide overlapwith BLAST-pol’s longest waveband. In the final section of the paper we reviewwhat can be learned by observing the polarization spectrum of thermal dustemission from interstellar clouds.

6. The Far-IR/Submillimeter Polarization Spectrum

The first far-IR/submillimeter polarization spectra of molecular cloud envelopeswere published by Hildebrand et al. (1999) and showed surprisingly strong struc-ture, now revealed more clearly by Vaillancourt et al. (2008) as shown in Fig. 8.These observational studies avoid cloud cores, where optical depth effects caninfluence the results. We still do not have an accepted theoretical explanationfor the structure evident in Fig. 8, but initial progress has been made (e.g., seeLazarian 2007).

Observational and theoretical studies of polarization spectra may one daytell us which physical conditions are conductive to grain alignment. For ex-ample, we may learn how the degree of alignment is affected by the ambienttemperature, the ambient density, and/or the ambient radiation field. Thisinformation is vital for quantitative comparisons between cloud models and po-larimetry observations such as the comparisons described in sections 2 and 7of this paper. If far-IR/submillimeter polarimetry is to fulfill its promise toadvance star formation theory by providing strong tests that can discriminatebetween magnetically-controlled and turbulence-controlled models, it is likelythat we will first need a better understanding of grain alignment. The strongspectral structure seen in Fig. 8 provides us with a good observational handleon this thorny issue.

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When developing a stand-alone polarimeter one can devise a plan for con-trolling systematic errors at the outset, and follow it to the end. When buildingan add-on polarimeters (like SHARP and POL-2) or modifying existing cameras(as in BLAST-pol and HAWC-pol) one inherits both undesirable and desirablefeatures, and one must improvise. With SHARP we were able to implement the“Chicago data acquisition method” but the other three instruments listed aboveuse different approaches. As explained in section 7, BLAST-pol’s pixels alter-nate polarization directions across the array to gain immunity from backgrounddrifts, and HAWC-pol’s half-wave plate will spin reasonably fast for the samereason. The plan for POL-2 is also involves rapidly spinning a half-wave plate.

Far-IR/submillimeter polarimetry can involve work in remote locations,long wait times for cutting-edge detectors, and bothersome systematic errors.But better detectors and better telescopes coupled with clever ideas and hardwork should allow investigators to provide answers for long-standing theoreticalquestions in astronomy.

Acknowledgments. The author acknowledges support from the NationalScience Foundation for his work with SPARO (award OPP-0243156) and SHARP(awards AST-0243156 and AST-0505230) and from the National Aeronauticsand Space Administration, via a subcontract from the University of Pennsylva-nia, for his participation in BLAST-pol. The SPARO and SHARP observationsdescribed here would not have been possible without important contributions byM. Attard, P. Calisse, D. Chuss, J. Davidson, J. Dotson, C. D. Dowell, G. Griffin,R. Hildebrand, M. Houde, L. Leeuw, L. Kirby, M. Krejny, H. Li, R. Loewenstein,T. Matthews, R. Pernic, T. Renbarger, H. Shinnaga, and J. Vaillancourt.

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