UFTI Latest

Andy Adamson, Sandy Leggett & Chris Davis

UKIRT/Joint Astronomy Centre, Hilo, Hawaii

Introduction

UFTI - the UKIRT Fast-Track Imager - is a state-of-the-art camera designed to take advantage of the improved image quality being delivered by UKIRT after the upgrades programme. Designed at the ATC in Edinburgh and built at the University of Oxford, UFTI incorporates a 1024-square HgCdTe array. It was delivered in late 1998 and has just completed its first full semester of operations at UKIRT. Apart from a change to the data acquisition system to use the ORAC control system (which will take place in October 1999), UFTI will be out of shared-risk status in semester 1999B.

Vital statistics

The UFTI pixel scale of 0.09 arcseconds is just sufficient to fully sample UKIRT's images on nights of good seeing. While we have yet to see the return of the 0.25 arcsecond seeing which enthused us so greatly in September 1998, the combination of UFTI and the new secondary mirror (see next article) will allow us to cope with this when it returns, and 0.5 arcsecond imaging is now very common. At this pixel scale, UFTI has a field of view of 1.5 arcminutes. In terms of area coverage in a given time, UFTI on UKIRT is competitive with the Keck in the near-infrared.

The sensitivity of UFTI's array falls off with decreasing wavelength through the near-infrared bandpasses; in the K band UFTI is 20% more sensitive than was IRCAM3, while at 1.2mm it is less sensitive by a similar margin. The table below gives representative sensitivity figures.

Other parameters and their relationship to recommended observing strategies are given on the UFTI web page UFTI web page. Specific problems and/or queries should be forwarded to Sandy Leggett (skl@jach.hawaii.edu).

Latency

UFTI's HAWAII array exhibits latent images left by bright sources. These areas of enhanced dark current decay with time, and for point sources they are flat-topped. Latency in the UFTI array is 0.3%, around the median of those in use in astronomy. The image of Saturn's rings (in the methane absorption) shown at the bottom of page 10 may well be the only demonstration in existence of methane absorption in Saturn's atmosphere taken with the camera shutter closed ! Latency can be minimised by applying a number of resets to the array before exposing, and ensuring that the array is blanked when not exposing (the standard system execs do this for you). In normal jitter-mode observations, median filtering is an effective defence against latent images.

FIGURE 1: A latent image left by Saturn on the UFTI array. This image also shows the four quadrants of the array, and the smattering of hot pixels (these are not saturated and subtract out well). The enhanced region to the top left is a reset feature which also flatfields out satisfactorily.

The DR Pipeline

UFTI uses the ORAC-DR pipeline and reduction recipes set by an "exec" (the exec system looks similar to that used with IRCAM3). The pipeline combines together a considerable number of Starlink monoliths to carry out reduction matched to the incoming data. In fact the recipe applied is set in the observing exec and carried along with the image headers; the only ORAC-DR command line parameter is an overriding recipe name. The brief of ORAC-DR is restricted to giving the astronomer a good idea of the quality they are likely to achieve offline, but in many cases it can produce near publication quality results.

Sample Images

The previous newsletter showed examples of small mosaics in star forming regions. We show below two examples of the power of UFTI applied to large extended objects and wide fields. The image in Figure 1 is of the spectacular Sombrero Galaxy; the data in Figure 2 are of the Galactic Centre. Both data sets were reduced with available, on-line, ORAC-DR reduction scripts.

FIGURE 2: M104, an image resulting from two applications of the EXTENDED_5x5 exec and DR recipe. There are many joins in this image, all but one of them virtually imperceptible. Data courtesy Stuart Ryder.

FIGURE 3: The Galactic centre, demonstrating the power of UFTI for wide-field imaging. This mosaic is the result of five applications of EXTENDED_5x5, in the J, H and K wavebands. Exposure time per point totals 30 seconds. This tiff RGB representation of the results is some four times undersampled compared to the original image which is 7 arcminutes (and some 5000 pixels) across. Data courtesy Antonio Chrysostomou and Chris Davis. The original data, taken in engineering time, will be made publicly available. Contact Chris Davis for further details (c.davis@jach.hawaii.edu).

Polarimetry with UFTI

Polarimetry, with support from the staff and resources at the University of Hertfordshire, has always been an important part of observational capabilities at UKIRT. Happily, UFTI is no exception!

UFTI is installed with an a-BBo (alpha-Barium-Borate) prism. When used together with a half-wave retarder (the same waveplate used for IRCAM3 and CGS4) we are able to conduct linear polarisation measurements of extended objects. Because both "e-" and "o-beams" (the orthogonally polarised beams) are projected onto the array, to avoid overlap, we must also use a focal plane mask. A consequence of this is that the e- and o-beams occupy horizontal (east-west) strips on the top half of the array. Sky observations are simultaneously obtained in the bottom half of the array. The field of view of UFTI with IRPOL2 is thus roughly 90" x 15".

Commissioning observations conducted in July 1999 show that the prism and mask are well aligned with the array. Observations of the polarised reflection nebula GSS 30 (Fig. 4) reveal the well-known centro-symmetric polarisation pattern. As a bonus, we also detected linearly polarised line emission from the nearby Herbig-Haro object, HH 313!

FIGURE 4: Imaging polarimetry of the star forming region GSS 30 and the nearby Herbig-Haro bow shock HH 313. These data were obtained through a narrow-band 2.122 mm filter. The familiar linear polarisation pattern around GSS 30 is observed (see Chrysostomou et al. 1996, MNRAS, 278, 449). A new result is the detection of polarised line emission from the HH object. Remarkably, the vectors are aligned with the axis of the outflow (indicated by the arrow) that powers HH 313; the associated magnetic field may therefore also be aligned with the flow axis! (Data: Antonio Chrysosotmou & Chris Davis).

We also sought to check whether array latency (mentioned above) would affect the apparent polarisation of a source if, for example, the source revisited the same location on the array too frequently. To do this, 3- and 5-point jitter patterns of the same two bright sources were obtained. Although this was not an exhaustive test, it did suggest that any effects on the measured polarisation would be small (of the order of a few tenths of a percent). Nevertheless, it is probably prudent to observe with a 5-point jitter pattern when observing bright point sources; faint, extended sources are unlikely to be affected by this phenomenon.

Finally, during a more recent engineering night, measurements at I,J,H and K of the instrumental polarisation and polarisation efficiency were obtained These data have yet to be properly reduced, although because we use the same waveplate for UFTI, CGS4 and IRCAM3, tests with these latter two instruments suggest that the instrumental polarisation and efficiency with UFTI are likely to be low (0.2-0.4%) and high (~99%) respectively. The UFTI results will be posted on the IRPOL - UFTI web pages in the near future.

Table 1: UFTI sensitivity parameters

UKIRT gets an even better secondary mirror

Tim Hawarden

UKIRT, Joint Astronomy Centre, Hilo, Hawaii

In August 1996 the UKIRT Upgrades Programme equipped the telescope with a new top-end, secondary mirror positioning system, momentum-compensated fast tip-tilt system and a new, lightweighted secondary. The immediate effect was a substantial improvement in the telescope's imaging performance as more light went through smaller spectrometer slits, and telescope wobbles and wind-shake was effectively eliminated, along with image degradation by misalignment coma. The better images reduced toaround 0."3 arcsec FWHM. These improvements were greeted with enthusiasm by the users.

However, the secondary was found to suffer from a significant defect. The mirror had a turned down edge (TDE). This was enough to reduce the maximum attainable Strehl ratio (the ratio of the observed central intensity of a point-source image to that in a perfectly diffraction-limited image) from 100% to ~70%.

The mirror was made of very uniform and transparent zerodur and the optical surface was figured by testing from the back, through the glass, using a null lens system. Only when this was complete were lightweighting holes machined out of its rear surface by an ultrasonic abrasion process. Preliminary experiments on a test flat, lightweighted by this process after figuring, had shown no distortion of the front surface, so it was with surprise that strong print-through of the lightweighting pattern was seen when the mirror was tested. The effect of this defect was to scatter some of the light out of the central peak into a halo with a hexagonal symmetry. The relatively steep and short-spaced surface errors would also make the use of adaptive optics very difficult.

The mirror also suffered temperature-dependent distortions. This was because its three INVAR mounting pads, which are glued into hexagonal pockets in the back, had a slightly different coefficient of expansion than the zerodur. In cooling from the gluing temperature to operating temperature the shrinking INVAR produced about 300 nm of 3rd-and 5th-order trefoil and some (4th-order) spherical aberration.

FIGURE 1: Wavefront sensing images of the primary, before (left) and after (right) the installation of the new secondary mirror.

The 5th-order trefoil could not be corrected by the active figure control of the primary mirror, and so had to be tolerated. The 3rd-order trefoil and spherical were correctible, but were high enough up the order of the mirror's bending modes that considerable force was needed to do this: consequently the ensemble of UKIRT's optical errors exceeded the authority of the primary figure control actuator systems to correct them and we had to compromise as to which errors got backed out and which had to be lived with.

It was therefore apparent that a replacement mirror was necessary. The new secondary should have no TDE, no light-weighting print-through and would be equipped with new, a-thermal, mounting pads. The same manufacturing process was adopted, though with two important modifications: First, the mirror would be manufactured and figured slightly oversized, so that the excess could be machined away to remove the turned-down edge. Second, the lightweighting would be done by conventional grinding and thereafter the rear surface would be stress-relieved by acid etching to eliminate microcracks, etc. And, of course, a-thermal mounts would be installed.

Ralf-Rainer Rohloff of the MPIA undertook once again to manage the manufacturing process in exchange for a further MPIA share of observing time on the telescope. Horst Kaufmann's initial optical figure, checked for us by Eli Atad of the UKATC, was excellent, this time with no peripheral TDE and a very smooth finish. The lightweighting was done at ZNBM, University of Jena, and the acid-etching at Zeiss. On its return the mirror figure was checked for print-through distortion with a subdiameter spherical testpiece. It was with great jubilation that Eli Atad and I received the test images in his office at the ATC: the Newton's rings were perfectly smooth over the lightweighting pocket walls.

The mirror was installed on UKIRT on 14 June, 1999 for testing with the UKIRT curvature sensor - there's nothing like an out-of-focus image for showing up small-scale defects! Figure 1 shows such an image taken with the earlier mirror (left): the print-through is dramatically visible. A similar image with the new mirror is shown on the right. There is no trace at all of print-through from the lightweighting or of distortion by the mounts: indeed the surface is so good it now shows up the slight defects of the Primary in too-vivid relief: the small craters from an aluminising accident in its early days and the low-amplitude ring-pattern from the final figuring of this, the world's first, large, thin mirror, by the late David Brown at Grubb Parsons in the early '70s.

With the first mirror, UKIRT's typical RMS wavefront error was about 350 nm. The new mirror has reduced this to around 180 nm, almost all of this being left-over spherical aberration. This surprised us at first, but we surmise that the sign of the spherical component introduced by the thermal distortion was actually such as to correct for a conic-constant mismatch between the secondary figure and the primary. The ultimate goal is to reduce this below 140 nm, which would make UKIRT formally diffraction limited at 2 mm wavelength (>80% Strehl ratio at 2 mm).

We plan to achieve this, and other benefits, by partial rehabilitation of the first secondary. The protective plate for this mirror is still in the shop at Prazisions Optik. So we will remove the old mounts and return it to Horst Kaufmann. He will install the cover plate and send it to Zeiss to get the back etched for stress-relief, which should remove the print-through. When we get it back we will retrofit it with a-thermal mounts and recoat it, thereby acquiring a backup secondary nearly as good as the one on the telescope. THAT one can then be removed and sent off for ion figuring to correct the residual spherical aberration, and maybe be given a very low-emissivity coating, before being re-installed. That should bring the RMS error down to between 50 and 100 nm (about like HST...), for diffraction-limited intrinsic performance practically down to the J band.

Having two interchangeable secondaries may have many potential advantages!

Auto-focusing has arrived!

The upgraded UKIRT optics are now almost diffraction-limited at K (when properly focussed!) and in the best seeing conditions the telescope is capable of delivering images not far from the diffraction limit. In fact, if defocus is to contribute the same share of image degradation as the other factors in the optical error budget employed in the Upgrades Programme, the secondary mirror position has to be accurate to <0.004 mm.

Focusing by trying to determine the best image of a seeing-degraded set means, almost by definition, that the residual focus error is the dominant optical maladjustment of the telescope. (We are, after all, trying to measure a parameter at a point where the first derivative of our quality function is zero, i.e. our function has zero dependence on that parameter!) Even a local fit "around the peak" is non-optimum, and while our automatic focus measuring routines were a great improvement over "eyeballing", focus errors have continued to be a major concern. Under good conditions, results from our old focusing technique were rarely better than ±20 mm, so we had a long way to go. Furthermore, experience with measuring FWHM from UFTI frames in the conventional manner showed that the time required to achieve a (reasonably?) good focus was an unacceptable overhead. Nevertheless UFTI's fine pixel scale can properly sample the best images the telescope can deliver, so it became clear that a fully-objective, precise focusing mechanism was required: ideally one that does not use image quality as the measuring criterion.

A new auto-focus system, employing a 2 x 2 Shack-Hartmann detector in the fast guider, was implemented by Nick Rees in June 1999. This measures the positions of the images formed by the 4 sub-apertures and so does not have zero sensitivity at the correct setting! It can be used to fast-guide AND correct seeing on any star down to about V=12 and currently derives instantaneous defocus measures at 60 Hz. It was originally intended to use this information for Adaptive , i.e. fast, seeing-correcting, focusing, but the dynamic range of the secondary mirror piezos is currently much too small for this purpose. Instead the focus measurements are averaged and the resulting correction is sent to the hexapod - which positions the whole secondary, tip-tilt system and all.

As currently set up, in an automatic focus run the system integrates for 2, then 4, then 8, then 16 and finally 32 seconds, sending a correction to the hexapod after each interval. This system thus allows an accurate focus (of order a few microns) to be determined in 62 seconds, while the observer, having watched the process (all details of which are displayed in stripchart format on an EPICS display) has a good feel for the quality of the result.

Each Cassegrain instrument now has its own well-determined focus offset (applied as an internal focus setting in the Fast Guider). Once an automatic focus determination has been made, the focus can be maintained by a recently-upgraded thermal-elastic model which has been in use for some time. This may be necessary if the guide star is fainter than V ~12, but brighter than this, full guiding capability is available and the focus can be continuously upgraded (corrected every 30 s) while the observation is going on.

Seeing measurements are likely to become possible using the 60 Hz defocus measurements. The RMS of the z (focus) correction measured by the Fast Guider in autofocus mode should be a good measure of seeing in all but the very worst conditions. We are exploring the calibration of this quantity, which should be closely analogous to the differential image motion measured by a DIMM-type seeing monitor. We anticipate that this measurement will soon provide quasi-continuous seeing monitoring without the need for measuring images, since the z(rms) is continuously logged by the Telescope Control System.

View From the Top

Thor Wold

UKIRT/Joint Astronomy Centre, Hilo, Hawaii

Welcome to semester 99B on UKIRT. There have been yet more innovations since the last newsletter, not the least of which is the institution of our autofocus routine. This has cut the time for focusing UFTI from about 20 minutes to around 3 minutes!

We began the new semester by closing down for four nights to completely gut the control room. The entire contents were removed on Monday, August 2nd (see right), and Andy Adamson, Frossie Economou, Nick Rees and myself then painted the walls.

The following day, the guys in space suits came to remove the ceiling tiles (asbestos). They did not do very well, though. They foolishly came straight up from sea level without pausing at HP and so turned a bit green. Being confined in space suits is nauseating enough... I don't think they were feeling well at all at the end of the day. It seemed like they took turns writhing on the floor.

On Wednesday, the ceiling contractor came to begin putting in the new ceiling, while I entertained myself by trying to sort out all the stuff that had now been piled in the computer room. The amount of sheer junk and outdated stuff was daunting! My goal is to keep the control room simple and neat, so removing reams of papers was top priority.

Thor wonders how he's going to run up the telescope for that night's observers...

I had enough to entertain myself well into the following day, while the ceiling contractor finished installing the new ceiling and the air conditioning guys put in a whole new set of ducting. The pile of papers to be recycled was enormous. The daycrew began to put up the new countertops after the contractors were finished around 4pm. They did not leave the summit until after 10pm, making for a VERY long day, and then they were back up at 7:30 the next morning, to make sure that everything got put back, including of course the computers (so that's what's missing in the picture! - Ed.).

We were back on the air Friday night without a hitch, Martin Ward being our first guinea pig! The only complaint he had was it got a bit cold the following night: we are still adjusting to the new environment.

In fact, the project at this writing is only about 90% finished. We have gotten the most disruptive stuff out of the way, but it will probably take us another month or so to get everything done. We still have some cabinetry to get made and installed, etc. Now that the basic stuff is done, we can see how other things will be integrated, including more Pretty Pictures! Please remember that this is still a work in progress, though.

As you saw in previous articles in this newsletter, UFTI is going Great Guns. This will certainly give us some nice images to get framed and hung on the walls. Perhaps we might even have room for some `historical' images—the `before and after', so to speak. All in all, we certainly hope that this will make your observing visits more pleasant. The place was certainly starting to look very shabby. It is difficult enough to just BE up here, so making the environment a bit more pleasant has to be a plus. A warm mahalo to the painting crew of Andy, Nick and Frossie, to the army of daycrew who did the long hours of installation and, above all, to Erik Starman for organizing this whole endeavour. Hope you enjoy! Aloha...

AN ODE TO IRCAM3

On the occasion of the last scheduled observations with IRCAM3, in recognition of its years of sterling service, an "anonymous observer" composed the following poem:

IRCAM 3 : RIP

You've been a great detector,
Nobody could perfect a
Better infrared array than IRCAM3

Over the semesters
With data sent to test us,
You've given sterling service to Astronomy.

From J band to the M band,
You've got the wavelength range spanned,
With filters broad and narrow and even an FP.

With read-outs that are stable,
And no remnance we are able
To observe with dual-beam polarimetry.

For objects that are too bright,
And would saturate then we might,
Use your fast readout to get them easily.

You've even got your own AO,
It's ownly shift and add I know,
But it lets us freeze the seeing when it gets windy.

But your pixels are a bit too big,
After the top-end re-jig
It's time to step aside in favour of UFTI

And so we bid a fond "adieu"
As TUFTI you'll be born anew,
But we will still remember you as IRCAM3.

People

Arrivals

Watch this space in the next issue of the newsletter for news of new staff arriving at UKIRT soon....!

Departures

Antonio Chrysostomou, the UKIRT Support Scientist responsible for polarimetry, wave front sensing and this newsletter, has returned to the UK after two and a half years at JAC. We wish Ant, Mila and baby Mira-Sofia good fortune for the future.

Yaguang Yang left JAC in mid August. He has moved to Maryland where he will join his family.

And Finally... a note from the Editor.

As the new editor of the UKIRT Newsletter, I'd just like to say thanks to those who contributed science articles to this edition. I'm a firm believer in the need for an observatory newsletter and will continue to pester you all for contributions (so beware if I'm supporting your run!). Should you have a successful run at UKIRT, please seriously consider writing a few paragraphs; the more concise the better.

Publications like our's very much represent the "glossy face" of British Astronomy. The Science Minister, the folks at PPARC, your colleagues in other fields and your Granny for that matter are all more likely to browse through our Newsletter than your latest publication in MNRAS. We don't referee articles, we have a quick turn-around and publish in glorious technicolor. Need I say more..?

The back page

HRH Prince Andrew takes time out from the recent Gemini Dedication Ceremony to visit UKIRT and JAC. July 1999

Chatting with Ian Robson and Ant Chrysostomou in the UKIRT control room.

Viewing UFTI data taken just 36 hours prior to his visit, with UKIRT staff astronomers.

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UNITED KINGDOM INFRARED TELESCOPE

Newsletter

Issue 5, September 1999

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