Multi-Wavelength Observing with TRISPEC

Shuji Sato

TRISPEC (Triple Range Imager and SPECtrograph) contains three optical channels, each with its own detector, to cover the wavelength range 0.46 to 2.5 microns simultaneously. One CCD and two near-IR InSb arrays are used. The incoming beam from the telescope is focused, collimated and then split into three beams by two dichroic mirrors (Fig.1). Each of the three channels has a filter wheel-box containing three or five wheels equippped with broad-band filters, grisms and polarization analysers (Wollaston prisms). TRISPEC is thus capable of imaging, spectroscopy and spectro/imaging polarimetry at three bands simultaneously. Spectral resolutions are around 100, depending on the slit widths (more details are available for the UKIRT "visitor instruments" web page). Linear and circular polarimetry can be carried out using an upstream (warm) waveplate(s) followed by Wollaston prisms (cold) inside TRISPEC. The polarization mode is provided by Prof. J. Hough (University of Hertfordshire, U.K.).

FIGURE 1: TRISPEC mounted on the mirror cell at UKIRT
(click on the figure for an expanded view).

The first trial with TRISPEC at UKIRT was carried out on February 4-7, 2000. Imaging and spectro-polarimetry data were obtained, although some aberration and light loss were experienced in the infrared channels, due to the misalignment of the optical axes, especially the pupil stops and focal planes, relative to the secondary mirror. Nevertheless, this was a positive first visit for the instrument to UKIRT.

FIGURE 2: The Optical and Infrared trains within TRISPEC . Two dichroic mirrors mounted split the incoming beam into three channels (click on the figure for an expanded view).

To illustrate the unique capabilities of TRISPEC we show some of the spectropolarimetric data obtained last year at UKIRT in Figure 3. We will carry out the second trial at UKIRT on August 23-30, 2000, when adjustable trusses, similar to those used with CGS4, will be used to support the 450kg weight of TRISPEC and thereby alleviate the misalignment problems experienced in February.

FIGURE 3: Spectropolarimetric data obtained at UKIRT in February 2000.

Technical specifications

The detectors used by TRISPEC are an SITe 512x512 pixel CCD and two SBRC 256x256 pixel InSb arrays (engineering grade). The simultaneous wavelength coverages are; optical 0.46-0.90 microns; J-H band 0.90-1.90 microns; K-L band 1.90-2.50 microns. The Field of View measures 56 arcsec on UKIRT (F/36: f=135m). Pixel scales are 0.1 arcsec/pixel in the optical and 0.2 arcsec/pixel in the infrared. Note that 1) the beam of F/36 or the focal length of 135 m of UKIRT currently makes the pixel scales slightly oversampled for the typical slit-width of 0.9arcsec in the spectrophotometry mode, and that 2) atmospheric dispersion degrades photometric accuracies at zenith angles larger than 30 degrees due to the wide coverage of wavelengths.

See the UKIRT-TRISPEC web page for the latest information.

Acknowledgements: Since 1996 TRISPEC has been developed by three graduate students, H. Nakaya, M. Watanabe and T. Yamamuro, from Nagoya University.

What the Fast Guider Strip-Chart can do for you

Tim Hawarden

UKIRT, Joint Astronomy Centre

The UKIRT Fast Guider has an option to display, in a strip-chart format, signals from the fast guider CCD. The "stripchart" display can be very informative and its general use by observers is encouraged. The TSS can set up the display for you: whenever the Fast Guider is operating, a strip-chart display is available.

There are two traces: the first trace is updated about once a second. It shows the GUIDE SIGNAL, the number used in the fast guider signal processing. It represents the value, from one readout, of the total signal from the sixteen superpixels on the Fast Guider CCD less the BACKGROUND SIGNAL. This last is the second signal, which is measured from the four corner pixels of the sixteen and smoothed over a 10 second period.

CLOUD DETECTION

The most important use of the guide signal strip chart is as a transparency monitor. The presence of cloud is all but unmistakable as the signal level changes over timescales of a few seconds. Quite thin cirrus cloud (see the Figure inset) is readily detected, which is a boon in a dark sky when thin cloud is undetectable by eye, even with extensive dark adaption (especially since hypoxia on Mauna Kea impairs the low-light performance of the retina). The strip chart should probably not be relied upon absolutely if one is attempting to do 1% photometry, but it is a very adequate indication of how things are going as one pursues one's favourite galaxy down into the murk in search of a spectrum with improved S/N. (Generally a mug's game, as the strip-chart display will rapidly convince you, if your spectra have not done so already.)

Excess light (i.e. an upwards excursion of the strip chart) in the "background" signal accompanies the downward swing of the guider signal when thin cloud intervenes, even in a dark sky. This is because of scattering by the cloud of guide star light into the corner pixels. Bigger excursions happen in bright time, as the cloud is also illuminated by the moon, and an increase in thickness is signalled by a rise in background brightness (noted in the figure).

Another effect which can mimmic the signature of thin clouds is windshake. This is punctuated by rather rapid downward excurions of the guide signal.

SEEING MONITORING

On moderately bright objects the Guide signal can be regarded as the signal from the star through a 3.77 arcsec aperture. Photon noise reaches ~5% at a bit below V=15, i.e. anything from the GSC should give a moderately good trace.

The effective aperture is small enough that even with excellent Mauna Kea seeing and the stabilised images delivered by UKIRT, there is usually some loss of light from the 16-pixel box as the image size varies. The fluctuations of the level of the line reflect this, and the size of its wiggles gives an immediate (subjective) indication of the seeing: small fluctuations = good seeing, large fluctuations (but on a generally flat trace) = poor seeing (Modulo the photon noise).

This is a useful facility as the autofocus Zrms seeing masurement is available only when checking focus, while the "guider trace" method is available whenever observations are taking place. It is not linear (or calibratable!), but it is certainly a useful indicator.

SETTING SCALES

Currently the vertical scale of the strip-chart display must be set manually on the screen. If one is observing numerous bright sources with short exposures this can be quite onerous for the TSS and should probably be avoided. It may be possible to automate the scale-setting, but we have not as yet plucked up the courage to ask the software section to address this!

PHOTOMETRY CORRECTIONS

There have been suggestions that the guide star signal could be logged and used to correct for transparency variations when doing photometry. This could work, though only when conditions were good enough so that all the guide signal fell in the guide box on the CCD. One would also have to assume that the clouds were perfectly neutral attenuators. As an old photometrist, I would, however, view this stratagem with grave suspicion.

View From the Top

Thor Wold

UKIRT/Joint Astronomy Centre

Another semester is underway, and we are now committed to ORAC and the PTCS. As of this writing (late August), things are progressing nicely and it seems the visitors are catching on quickly. It seems the TSSs are also catching on... The hands-on assistance from our software group has been most welcome!

It appears La Nina has gone (if it were ever here). Since around June, we have had moisture occasionally at the elevation of Hale Pohaku. This has alleviated the severe years-long drought that has plagued the upcountry, and has greened up the pastures on Humu'ula Ranch. This has meant the re-introduction of my favorite black cows. However, it appears the ranchers are finally weary of having their animals driven into. They have erected a three-strand electric fence along the road. I am somewhat suspicious though, as they have set this somewhat far from the road, leaving a nice stretch of long, green grass, which I bet they turn the cows loose on should it turn dry again. So, no excuse not to drive warily.

I had been asked to make a listing of some `rules of observing etiquitte' as a possibly interesting subject, so in collaboration with some of my compatriots (not only at JAC), I came up with a rather lenghty list. When I first wrote this column, I went ahead and wrote up the entire listing. I then had third and fourth thoughts on this. I certainly did not want to seem rude and crude and did not want to imply that visiting observers did not have any sense of basic etiquitte. It is true, though, that some visitors seem to never have been trained as children in social graces. These are definitely in the small minority, though they had an overwhelming influence in the items on this list, which I thought was unfair.

And so, rather than get into the nitty-gritty of some small number of people's behavior, I thought it best to re-write this in more general terms. The laundry list of "don'ts" shall remain unpublished.

A. Safety: Please remember that your TSS is absolutely in control. This applies to everyone, regardless of their political stature. For example, in the event that the TSS has decided the conditions are dangerous enough to warrent evacuation, no dissention will be tolerated. Discussion over the merits of this decision can happen later. If you do not want to leave, you can certainly stay - by yourself! We are not in this business to put our lives in danger. If you choose to not wear your seat belt in the car, you will find yourself suddenly walking.

B. Personal space: If you are ill and infectious, you should remove yourself without being told to do so. We have all been made sick by visitors bringing exotic bugs here and being most ill-mannered in expectorating their bugs all over the place. Our standing demand is a bottle of single-malt scotch for being infected...and we have collected. There are ways to get around this situation without your infecting us, which causes great hardship on UKIRT's operations. Basically, please remember that we are spending 11 to13 hours in a small room, so have consideration for everyone's personal space in all ways and at all times.

C. General: With the consideration of the above; safety and our being confined in a small room for long periods of time (and, of course at altitude and through the night), there are some subtle ways to ensure that the whole operation proceeds smoothly. For example, remember that while we allegedly do speak the same language, a lot is often lost in the translation. It certainly does not help that sometimes people do not enunciate clearly and think that they can face the wall and speak to the other side of the room and be understood. Personally, I find it embarassing to keep having to ask people to repeat things to me; after this happens a few times - get the hint.

Finally, you are reponsible for your clutter. We are not janitors here, so please clean up your coffee cups, uneaten sandwiches, etc. And a universal pet peeve: please do not start doing something at the summit at the end of the night, after we have closed down, that you could do from Hale Pohaku.

So if you follow the safety rules and pay attention to this situation of being in a confined space, etc., I see no reason why the whole operation could not run smoothly. Then again, the reason I (and others) have brought this subject up in the first place is that there are individuals who apparently do not know how to conduct themselves sensibly. This said, I hope your next observing run is a productive and enjoyable! Aloha!

People

Arrivals

The most recent addition to the UKIRT Support Scientist team is Paul Hirst, who joined us in March of this year. Paul will largely be responsible for CGS4 as it faces its twilight years (though in the UIST era to come it will be maintained for use with the echelle). Paul obtained his PhD from Leicester University and also has an MSc from Jodrell Bank. His research interests include active galaxies, starbursts in AGN and multi-wavelength astronomy. Paul also enjoys mountaineering - rock and ice climbing - so he'll be a handy guy to have around should your car break down between HP and the Summit.

Kynan Delorey started at the JAC in June. His official title is `Computer Helper' and he's responsible for making sure that observers get their UKIRT data and that non goes astray (he'll soon be responsible for JCMT data as well). He's also been writing some nifty utilities in Perl; new on the scene at UKIRT are `whoson' and `nightlog'. Kynan is a Sega Dreamcast and DVD fanatic, and he thinks the petting zoo at Panaewa Zoo is top notch (hey, so do my kids, though when do we get to pat the white tiger, that's what I want to know! - Ed )