Ripple-free Spectropolarimetry at UKIRT

Andy Adamson

Spectropolarimetry carried out with waveplates has traditionally suffered from periodic ripple structure in the P spectrum, arising from interference in the multiple reflections in the rotating waveplate used to build up the information necessary to generate the Stokes parameter spectra (e.g. Tinbergen 1994). At UKIRT the observed ripple is at least bi-periodic in wavelength and exhibits beats, as shown in Figure 1. It can have a magnitude of a few tenths of a percent.

Figure 1. CGS4 spectropolarimetry of the Galactic Centre (Adamson et al. 1999), showing the polarization ripple; right panel shows the result of removing this ripple using a Fourier filter. The binned spectrum in the right panel was offset vertically for clarity.

Aitken and Hough (2001) analysed the causes of spectropolarimetry ripple and showed that it could be removed by observing eight position angles of the waveplate - the original four, plus four additional angles 90 degrees further around (90, 112.5, 135, 157.5 degrees). With the increasing prominence of spectropolarimetry projects (in the current semester we are servicing four such proposals) this method has now been implemented with IRPOL spectropolarimetry; after initial tests in 2005, a spectropolarimetry run was undertaken in February which committed to taking science observations with this method. The results are more than encouraging: Figure 2 shows a polarization spectrum of the BN object taken using two passes through the first four angles in the full set; the right-hand panel in Figure 2 shows the equivalent spectrum taken using one pass through the whole set of eight angles. Clearly the ripple has been all but totally suppressed, as predicted by Aitken and Hough.

Figure 2. Left: UKIRT spectropolarimetry of the BN object in Orion at 5 microns, using the traditional four waveplate angles (0, 22.5, 45, 67.5 degrees) and the "ratio" method for reduction to Stokes parameters. Right: Spectropolarimetry of the BN object, using eight waveplate angles (0, 22.5, 45, 67.5, 90, 112.5, 135, 157.5 degrees).

Two of the additional waveplate angles correspond to IRPOL micro-switches and can be accessed quickly (the initial four angles plus those at 90 and 135 degrees). The remaining two have to be accessed by step counting. The standard sequence for the final four angles is 90-135-112.5-157.5 degrees, and because the IRPOL waveplate always moves in the same direction, a slightly greater overhead is imposed on observations taken using 8 angles. However for most normal observing, the difference is small (particularly when comparing the resulting data quality) and we anticipate that all future spectropolarimetry at UKIRT will employ the additional four angles. This change will be implemented in the 06B release of the UKIRT polarimetry template library.

References

Adamson A.J., Whittet D.C.B., Chrysostomou A., Hough J.H., Aitken D.K., Wright G.S., Roche P.F., 1999, ApJ, 512, 224
Aitken D.K., Hough J.H., 2001, PASP, 113, 1300
Tinbergen J., 1994, Spectrum, 1, 26

Coronagraphic Imaging Polarimetry with UIST

Chris Davis¹, Brad Cavanagh¹ & Malcolm Currie²

In late 2005 UIST was opened for some general maintenance and to replace unused slits in the slit wheel. Since one of the slots in the wheel was potentially vacant, we thought it might be a great idea to try something new with UIST...

Coronagraphic imaging polarimetry is an exciting field which allows users to study the scattered light very close to bright point sources, for example, in circumstellar disks around young or evolved stars. With the tried and trusted capabilities of IRPOL, the versatility of UIST, and the excellent seeing routinely achieved at UKIRT, this seemed to be a niche area worthy of investigation.

The slit wheel is located at a focal plane within the instrument. This gives sharp edges to the slits and the other masks in the wheel, including the normal imaging polarimetry mask. The slit wheel is therefore an ideal spot for an occulting mask.

Figure 1. A raw exposure of blank sky through the coronagraphic mask and the Wollaston prism. E- and o-beam images are dispersed ALONG the 20x120 arcsec apertures; the e- and o-beam images of the occulting wire in the right-hand aperture are separated by ~20 arcsec.	Figure.2. Raw 10 second H-band images of the Red rectangle. E- and o-beam images of the target appear adjacent to each other on the array. The two images are separated by 20 arcseconds.

As a first step toward trying out this technique, chief instrument engineer Tim Chuter was asked to attach a fine wire across one of the two rectangular apertures in a spare imaging-polarimetry mask. A wire thickness was chosen that closely matched the median seeing at UKIRT. The mask was then installed in the slit wheel before UIST was cooled down and put back on the telescope, ready for the Spring 2006 Cassegrain observing period.

Images of blank sky through the mask - without the Wollaston prism in the beam - showed two rectangular imaging polarimetry apertures and the wire occultor, well placed across the right-hand (southern) aperture. The width of the wire was measured to be ~ 0.6 arcsec, as expected. With the Wollaston in the beam, ordinary and extra-ordinary (e- and o-) beam images of both apertures were projected on to the array (Figure 1), resulting in two images of the wire.

Nighttime tests were conducted on two well-known evolved stars, IRC 10216 (H~3.0) and HD 44179, the Red Rectangle (H~5.1). With each extremely bright source tucked behind the occulting wire, 10-second exposures were possible. Saturation was encountered only within the central ~1 arcsec core not covered by the wire (Figure 2). Notably, the images did not suffer from complex internal reflections, latency issues, or bleeding. The longer exposures also meant that more sensitive background-limited imaging was possible, and of course the observations were more efficient than would have been the case with multiple coadds and short exposures.

The existing imaging polarimetry ORAC-DR recipes were adapted to cope with the new mask. The data in Figures 3 and 4 were reduced solely with the pipeline. The centro-symmetric pattern and high degree of polarisation observed by Murakawa et al. (2006, in "Astronomical Polarimetry", ASP Conf series, 343, 255) is reproduced in IRC 10216. A similar pattern is observed in the Red Rectangle.

Figure 3. The reduced H-band linear polarimetry data for the Red Rectangle and IRC 10216. N is up and E is left in each image.

The coronagraphic mask is at best a prototype. In particular, the rectangular apertures of the imaging polarimetry mask should be rotated through 90 degrees (orientated in the same direction as the normal imaging-polarimetry mask regularly used with UIST), so that the mask does its job of masking overlapping e and o beams. However, this prototype does illustrate the possibilities, while the observations are testament to the versatility of UIST.

Potential users of a coronagraphic mask, for use with imaging or imaging polarimetry with UIST, are encouraged to contact Chris Davis (c.davis@jach.hawaii.edu) with their views and experiences.

The UKIDSS Early Data Release

Steve Warren (Imperial College London), UKIDSS Survey Scientist, on behalf of the UKIDSS consortium

The first ESO-wide release of UKIDSS data took place on 10 February 2006. The data are available at the WFCAM Science Archive (WSA) surveys.roe.ac.uk/wsa. All UKIDSS data are proprietary to ESO astronomers for 18 months, before world release. Users need to be registered to be able to log in. The registration process is described at www.ukidss.org/archive/archive.html.

In broad outline the contents of the EDR are summarised in Table 1. This lists the area covered in each survey with the full filter complement of the survey. In the EDR, the data summarised in Table 1 correspond to the `EDR database'. This is a subset of the larger `EDR+ database', which contains all data that have passed quality control, and includes fields observed where the full filter complement has not yet been completed. The total area of the EDR+ database, i.e. sky coverage in any filter, is 220 sq. degs.

There are three UKIDSS papers on astro-ph, as follows:

• Lawrence et al. (astroph/0604426) `The UKIRT Infrared Deep Sky Survey', MNRAS, submitted (astro-ph/0603nnn). This paper describes the design and implementation of the surveys, and the final (7-year) goals.
• Dye et al. (astroph/0603608), `The UKIRT Infrared Deep Sky Survey Early Data Release', MNRAS, submitted (astro-ph/0603nnn). The purpose of this paper is to provide a self-contained guide to the EDR.
• Hewett et al. (astroph/0601592), `The UKIRT Infrared Deep Sky Survey ZYJHK Photometric System: Passbands and Synthetic Colours', MNRAS, in press (astro-ph/0601592). This paper was described in the previous edition of the UKIRT newsletter. It provides response curves for the five broadband filters, and synthetic colours of a wide range of stars, galaxies, and quasars.

For those working on the EDR, the first paper provides important background material, and the third paper will be helpful for interpreting scientific results. But for beginners, we strongly recommend a careful reading of the second paper, which is designed to be a self-contained guide to the contents of the EDR, and the structure of the data and its quirks.

The second paper includes a summary of the observing protocols followed for each survey. It explains the different data structures used in the five surveys, e.g. whether data are interlaced, and whether data are rebinned. A brief overview of the pipeline is provided (which refers to a more detailed paper, Irwin et al, in preparation). There is also an important section describing data artefacts, that can trap the unwary user, and a section on UKIDSS quality control (QC). Although all WFCAM data follow the same pipeline and archive data-train, in collaboration with the Edinburgh Wide Field Astronomy Unit UKIDSS have developed a set of QC procedures to remove corrupt (i.e. meaningless), bad (i.e. unusable because e.g. trailed), and substandard (i.e. outside the survey requirements on e.g. seeing) data. We found this removed a surprisingly large fraction (~20%) of the data, in preparing the EDR for release. But we expect this fraction to drop substantially, as much of the EDR data come from the short 05A semester, when image quality was somewhat inferior, and when some changes to observing protocals (e.g. guide star selection) were still being implemented. The final section of the paper describes how to use the WSA, with some worked examples.

Figure.1. 5-sigma depth of point sources in Y, plotted against ellipticity, seeing, sky brightness, and zero point.

An impression of the quality of the data is provided in Figure 1 above, which plots point-source 5-sigma depth in the Y band, against the quantities ellipticity, seeing, sky brightness, and zero point, for LAS fields in the EDR. Data in future releases should be marginally better than these, because of improvements in image quality achieved following instrument realignment between the 05A and 05B semesters.

The EDR is a small fraction of UKIDSS, about 1% of the seven year plan, but rather impressively is similar in scope to 2MASS in terms of numbers of photons collected. The next release, called Data Release 1, is set for 1 July 2006, and will be several times larger.

JAC in the Community

Chris Davis

The Sky at Night

January and February proved to be a very busy time for JAC staff in terms of public relations. A film crew from the BBC Sky at Night programme arrived a few weeks after Christmas. They spent a very busy week at HP and atop Mauna Kea, filming everything they (and we) could thing of. A number of JAC staff were interviewed at UKIRT and JCMT, and spectacular day-time and night-time shots were taken of the two telescopes, the summit ridge, Submillimeter Valley, dawn over Mauna Kea and the moon and stars shining down on the telescope domes.

Some great "live" shots were taken inside each telescope dome and in each control room, while Andy Adamson and Thor Wold observed with WFCAM, and while Richard Hills and John Richer commissioned HARP at the JCMT. WFCAM and the UKIRT primary were filmed; before, during and after a CO2 clean, and a wonderful evening sequence was captured showing the UKIRT dome opening and rotating while the sun set in the background.

Andy Adamson awaits his 15 minutes of fame while the BBC set up in the UKIRT control room.

After only a few weeks of "to-ing and frow-ing", with images and suggestions passing between JAC staff and the programme's producer/editor, Jane Fletcher, the programme aired on BBC 1, BBC 2 and BBC 4. The show will be permanently available on the Sky at Night web site (http://www.bbc.co.uk/science/space/spaceguide/skyatnight/), and will be included on a DVD with the Sky at Night magazine, probably in the April edition.

As a summary of life on Hawaii's tallest mountain, the exciting science being done at both telescopes, and the present and future instrumentation being developed at both sites and at the UKATC, the show was a great success, and we hope you all enjoyed it.

Although many JAC staff were involved in the production of the show, special thanks are due to Harold Butner who "buddied" the team around the mountain for the entire week. Thanks also to PPARC whose financial support made the show possible.

Journey Through the Universe

No sooner had the BBC team left than the "Journey Through the Universe" (JTTU) team arrived. JTTU is an educational initiative led by The Carl Sagan Center for Earth and Space Science Education. Using experts gleaned from all aspects of astronomy and space exploration, JTTU offers a week of fun-filled events designed to engage the local community and forge links between local scientists and schools. The core of JTTU is the classroom visits by the JTTU scientists and local observatory staff, with presentations and activities covering broad areas of astronomy and space research.

Chris Davis speaks to primary/elementary students at the Ka`Umeke Kaeo Hawaiian Charter School during JTTU week.

The east and north sides of the Big Island are currently one of 10 communities around the US that are designated Journey through the Universe sites. This year,

1700 Big Island school kids enjoyed a visit from the team. The JTTU week was topped-off with a number of family science nights in Hilo and Waimea. JAC was represented by Chris Davis, who managed not to embarrass his 13-year old son (too badly) in front of his classmates!

Opening of the `Imiloa Astronomy Center

Finally, it's open! Hilo's spectacular `Imiloa Astronomy Center of Hawai'i opened its doors to the public late-February, after a week of sneek peeks for Big Island dignitaries, business and tourism representatives, and of course Hilo's astronomers. The $28 million facility boasts a state-of-the-art planetarium and permanent exhibitions covering both the astronomical and cultural significance of Mauna Kea (which at `Imiloa is spelled in a traditional form, Maunakea). Visitors will recognise familiar faces in the astronomy exhibition, including our very own Sandy Leggett.

The building itself comprises a flat-roofed exhibition hall (housing the astronomy and cultural exhibits) and three titanium cones, representing the three mountains on the Island; Mauna Kea, Mauna Loa and Hualalai. The largest cone houses the planetarium, while the smaller cones cover the entrance and shop, and the cafe.

The `Imiloa Astronomy Center in Hilo (across from the Gemini HQ).

The planetarium, even before the installation of the laser projector (temporary equipment is being used for a month or two), is nothing short of spectacular. Early-bird visitors were shown dizzying previews of up-coming shows, including the 22-minute feature, "Maunakea: Between Earth and Sky", which links native Hawaiian culture with the astronomical activities on the mountain.

The entrance fee for the planetarium and exhibition is $14.50 ($7.50 for kids), though Kama'aina rates are available. Opening hours are from 9:00 a.m. to 4:00 p.m., Tuesday through Sunday. Access to the shop and cafe will be free, though note that the opening of the latter will be at least a few months down the line (the room and kitchen have yet to be furbished!).

Visit the `Imiloa web site at: www.imiloahawaii.org

Outreach at the JAC

Outreach efforts continue at the JAC. Staff members Frossie Economou and Iain Coulson once again acted as judges in the State Science Fair at the end of February. Each year the JAC sponsors a prize for the best project in astronomy or a related discipline. The recipient of this year's prize was a 7th grader (a 12 year old) from St Joseph's School, who presented a project entitled "Cosmic Ray Penetrability". The winner, Steven Hammer, built his own bubble chamber and measured the rate of cosmic rate activity when shielding the chamber with a variety of materials. His ultimate purpose was to discover which material would make the best "solar sail" for interstellar propulsion!

Finally, after three years of service to both UKIRT and JCMT (many of you will have read our many press releases, and who can forget the UIST image of Mars on the front page of the Times!), Douglas Pierce-Price moved on to a new position as Education Officer at ESO. Douglas will be replaced in March by Inge Heyer, who joins us from STScI. We look forward to working with Inge, and hope you all won't be shy at coming forward with your exciting science results. (And don't forget this Newsletter - ed.)

The JAC outreach department has its own web page at: outreach.jach.hawaii.edu.

View from the Top

Thor Wold

There have been lots of public-oriented events occuring in the past six months, as described earlier in this edition of the Newsletter. I, for one, have been awaiting the opening of the newly re-named 'Imiloa Center in the UHH Science Park. I did manage to get in to the final free preview, being you-guess-where when the other openings happened the previous week and was quite impressed by the planetarium. This was not even the final configuration; their laser is still on the mainland being tuned - and yet it was really awesome. The planetarium should certainly make a huge impression on the folks that live here, as well as on our visitors, and I suspect this will become a major tourist attraction. At the moment, though, the hours of operation are definitely not local-friendly (except on weekends), with their 9AM to 4PM schedule. The planetarium is being touted as the most advanced on the planet, until a newer one is opened in Southern California this summer.

Bids will be going out in mid-March for a set of stoplights at the intersection of Nowelo St. and Komohana; better late than never, I guess, but perhaps this ought to have been done now, to coincide with the opening of 'Imiloa Center. I am sure there are lots of folks at JAC that will be glad to finally see this, as trying to get out onto Komohana at busy times of the day can be hair-raising. No completion target announced yet, but this might take a year.

The UKIRT crew-room, before and after its recent paint-job.

Tim Carroll and I along with the very dedicated and tasteful assistance of Marge Dougherty finished a full renovation of our crew room back last winter/spring. I can't speak for Tim, but for myself, we are indebted to Marge's taste in color and decor; I am near color-blind, or so I have been told. Reviews of the finished product have been quite positive. It is certainly a vast improvement from what was there! This was there when I began work here over 20 years ago and seemed to have been put together without any concept of color schemes or taste. Nobody would ever admit to putting those colors together and it seems like it was a matter of having a spare can of paint from something and a bare wall. So again, kudos to Marge! And of course huge thanks to Andy Adamson for budgeting this project.

And then there was Aunty Beeb. The BBC invaded us (they are very nice people, really) and I happened to be "The Victim" up at UKIRT. `Move the dome just a bit more around'... `Can you raise up the telescope just a bit more and turn the dome lights on...?' And then "Gandalf the White" (that was a quote from a review of the finished program) had to sit before The Eye and speak. Oh, well...They never asked my last name, so I guess I join those in the entertainment field with one name like Cher, Bono, Madonna, etc. I just wish I had their financial assets! Anyway, this just added to the feeling of being On Stage...

And so we have exited the first "full-length" episode of WFCAM. For myself, I am still awaiting evidence that we are actually doing as well as we are supposed to be doing. We have seen results from SV and the early stages of survey operations, but back then we were not perfectly aligned, and had guiding and focusing problems.

I do think the WFCAM semester went well, though. Early problems with galaxies being presented as "guide stars", etc., were eventually sorted out, and we ought to hit the ground running next time. We certainly provided the pipeline with mountains of data to work with!

And so we have just once again gone from WFCAM back to the cassegrain instruments...alas, just in time for winter to appear, with its usual weather! At this point we are not doing too well, losing a lot of time to weather - which is very disappointing since we will not have this window of cass instruments for very long at all. I do hope this will not make one huge dent in the number of published papers we get out, since this is how we are ultimately judged for funding, etc. The switch-over itself was seamless; it was just remembering all those quirks of three instruments vs. one (we now have some nice documentation to help).

And now the Weather Mavens say we are entering a La Nina phase, which is to last into next summer. This is where the sea surface temperatures are below normal, as opposed to El Ninos, where the temperatures are higher than normal. This is supposed to mean dry weather. Well, that is what they say for El Ninos; makes me wonder if they are just making things up as they go. We shall see what this means for us in the upcoming months. So far, its been a bit wet, to say the least.

Events at your Beloved Vacation Resort Hale Pohaku include the ongoing Chinese water torture with the installation of the outside siding on building B, first mentioned in this space in the Fall Edition of 2004; the project had started that summer. They now have *almost* completed the south wall and appear to be poised to go around the east end and then start the north wall. At this rate, maybe they might (just might) finish sometime in the year 2008. This is a crew of two, or basically one guy with a part-time helper, but they keep dragging the poor guy off this project to do other More Important things...like painting the Commons Building's outside doors a rather unsettling shade of mustard (did they need painting to start with?). Being up here is unsettling enough to some folks. I hope they are not further inspired by the colors of the doors.

If this weren't upsetting enough to those of us who must sleep during daylight, now they will be installing internet connections to all of the rooms which they say will be done in the "afternoon and early evening hours"...but which will mean blocks of rooms will still have to be set aside for this, crowding other places (please, not where they are trying to do the outside of Building B!). Since "morning" to me lasts up until 2.30 or 3.00 pm, I hope that after this is what they mean by "afternoon hours". At least this project is being outsourced and so it ought to be completed in good time.

As I have said before in this space: so much changes, yet somehow remains the same.

And so, everyone coming this way please remember to have that $14.50 in your pocket for a trip to the new planetarium. Have no fear, we shall try to keep you entertained and amused up at the top as well! Aloha!