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CGS4 Exposure TimesTom Kerr1 & Tom Geballe21 : CGS4 Instrument Scientist2 : Head of UKIRT OperationsJoint Astronomy Centre, HiloSince the installation of the long focal length camera in August 1997, many observers have requested that optimum exposure times be placed in the CGS4 web pages and in the manual. This is not a trivial task, as the optimum exposure time is dependent on many factors such as resolution, wavelength, object brightness and weather conditions. In this article, we provide a guide, consisting of tables and discussion, on how to select appropriate exposure times for each grating given the above factors.150 l/mm GratingThe figures in Tables 1 & 2 assume a 1-pixel wide slit, the long camera and that the light falls on one row of the array. Typically light falls over three rows (but is mostly in one row) and these exposure times can be increased by about 30%. Halve these times when using the 2-pixel wide slit.
Background limited exposures at JHKThis medium-to-high resolution 150 l/mm grating enables one to work in between OH lines in many regions in the 1.1 to 2.3 micron spectrum (OH line emission is not a factor beyond 2.3 microns).In simple terms, in order to be background limited in the non-thermal regime (<2.3 microns), the sky noise must be greater than the array read noise. For multiple non-destructive reads (NDR), the read noise is approximately 23 electrons. A typical value for the continuum background (from the telescope, sky and long wavelength leaks) using the 1-pixel wide slit in J, H and K is 30 counts in 100 seconds, and with a gain of six, corresponds to 180 electrons and a sky noise of approximately 13.5 electrons. Therefore, in most cases, an exposure time of approximately 300 seconds is required for the sky noise between OH lines to equal the read noise. For longer exposures than this the array is background limited and best s/n is achieved in principle. With the two pixel wide slit (which still gives high enough resolution to work between many OH line pairs) the exposure time must be greater than about 200 seconds. Beyond about 2.2 microns, the background increases rapidly as the thermal background from the sky and telescope begin to increase, and the background-limited exposure time drops rapidly. The drawbacks to using such long exposures are variations in the sky background and OH line intensities, OH line saturation (at H and K), and increasing likelihood of spikes on individual or small groups of detectors. If the critical wavelengths are well clear of the OH lines, then you probably don't have to worry about their approximate 5-10 minute variation timescales or strength for OH variations, but if you are close to one than these can become problems (but see the two paragraphs below for ways to minimise these). Add to this that you will probably need to oversample your spectra, your time on source will become at least 600 seconds before you nod to sky. If you are using the 1-pixel wide slit and 2x2 sampling, it will be 20 minutes before you can nod the telescope. In addition to the dangers of sky variations these long times mean that, although in principle maximum sensitivity is achieved, a lot of time is wasted if something goes wrong. For extended sources, nodding to sky is required, and the brightness
of the source and stability of the sky background on that night will effectively
determine the time between nods and hence the exposure time; these may
be considerably less than the above ideals.
The frequency of spikes is difficult to judge and their effect difficult
to assess, because spikes sometimes are severe, sometimes are only somewhat
above noise levels, sometimes effect only one pixel, sometimes effect a
few adjacent pixels, and their frequency may vary. Clearly they are more
likely to affect observations of an extended source than a pointlike source.
Empirically they do not appear to be a serious problem when observing point
sources with exposures of a few hundred seconds.
40 l/mm GratingThe figures in Tables 3 & 4 assume a 1-pixel wide slit, the long camera and that the light falls on one row of the array. Typically light falls over three rows (but is mostly in one row) and these exposure times can be increased by about 30%. Halve these times when using the 2-pixel wide slit.
Background limited exposures at JHKIn most cases the 40 l/mm grating is background limited at much shorter exposure times at all wavelengths than is the 150 l/mm grating due mainly to its lower resolution, which ensures that an OH line is present in almost every resolution element.Typical background-limited exposure times are about 30 seconds at H
and K (less than 2.3um), giving 2 minutes between nods with 2x2 sampling.
In the J band the OH lines are weaker and exposures of ~75 seconds are
required to reach the background limit (5 minutes between nods). The same
concerns and optimal procedures regarding OH fluctuations as discussed
for the 150 l/mm grating apply here, except that spikes are less of a problem
because super-long exposures are not needed
The EchelleBecause the echelle has not been used while both the long focal length camera and 256 x 256 array have been installed in CGS4, optimum and maximum exposure time are as yet unknown. An added complication is that it is planned to install new wedged CVFs in CGS4 by the time the echelle is once again available, and these may slightly affect the optical performance. However, because of the echelle's higher resolution, background limited performance between the OH lines may require longer exposures than with the 150 l/mm grating. Once the echelle exposure times are known, they will be put on the CGS4 web pages.Back to the ContentsStatus Report for UKIRT CamerasSandy Leggett1 & Tim Hawarden21 : IRCAM Instrument Scientist2 : Head of UKIRT DevelopmentJoint Astronomy Centre, HiloThe new UKIRT camera, the Fast Track Imager UFTI, will be a basic 1-2.5 micron imager with a fixed pixel scale (PFOV) of 0."09 /pixel and overall field of view (AFOV) of 90" (the array is a 1024x1024 HgCdTe). This pixel scale will exploit UKIRT's improved image quality and enable the observer to fully sample images with FWHM <=0."3 (such as we now see quite often at the telescope). It had been planned to commission UFTI in May 1998 but the schedule has now slipped such that telescope commissioning will have to take place in July 1998. UFTI should be available for semester 98B, initially in shared-risk mode.Once it has been determined that UFTI is fully functional, UKIRT staff will modify IRCAM to improve its performance in the thermal regime. IRCAM's warm fore-optics will be replaced with a new cold snout which will both reduce the thermal background and change the PFOV to match that of UFTI, 0."09 /pixel. The lower background levels will enable thermal data to be taken much more efficiently, and images can be fully sampled. Although the array field of view will only be 23" this is still larger than the field given by the 64x64 pixel subarray which is currently required to achieve short enough integration times. This instrument should be available in late 98B or early 99A. Back to the Contents10 Micron Camera Spectrometer Available for UKIRTAntonio Chrysostomou1 & Tom Geballe21 : UKIRT Support Scientist2 : Head of UKIRT OperationsJoint Astronomy Centre, HiloMICS (Mid-Infrared Camera Spectrometer) is a prototype of an instrument being built by the National Astronomical Observatory, Japan, for the SUBARU telescope. It operates as a camera and a spectrometer in the 7.6 - 13.6 um window (in addition, a K band filter is available for imaging) and is outfitted with a 128x128 Si::As BIB array. The camera has a 49 x 49 arcsec field of view (0.385 arcsec pixel scale), while the spectrometer has a 0.8 x 49 arcsec east-west slit with a resolving power of R ~ 77 - 128 (i.e., a resolution of about 0.10 um). In March and September of 1997, MICS underwent extensive commissioning tests at the UKIRT with a view to offering the instrument to the UKIRT community. The performance of MICS was such that UKIRT has made the instrument available for general observing (including service mode) to the UKIRT community for this semester (98A). The first PATT observations will be carried out in May 1998.Observers are invited to submit their own research programmes for use with MICS. Prior approval from the MICS team in Japan is not required, however, members of the MICS team are eager to collaborate with prospective users. In any event, the MICS team will provide assistance in the use of the instrument at the telescope and with data reduction. If you wish to get in touch with members of the MICS team to discuss details of the instrument, or possible collaborations, then either visit the MICS web site or contact Miyata Takashi (miyata@mtk.ioa.s.u-tokyo.ac.jp). For further details of the instrument and its sensitivities on UKIRT
visit, the MICS
section of the instrument availability web page for semester 98A.
Back to the ContentsView from the TopThor WoldTelescope System Specialist, Joint Astronomy Centre, HiloThe better part of a year has already gone by since the last of these columns. Time flies when you are having fun! There have been lots of changes at JAC, mostly in personnel, as more people move away or bolt to Gemini. Up on the mountain, we try to make the changes as transparent as possible, but the loss of key day personnel has been difficult to cope with. It takes time for the new hires to get accustomed to this machine and all its quirks.We now have two new PDRAs (Stuart Ryder and Chris Davis), who are responsible for one five-night shift each per month, and this is working very well. They got up to speed very rapidly. Tim Carroll and I and all the JCMT operators had our jobs changed last summer. We are no longer TOs, but TSSs (Telescope System Specialists). This is mostly a change in title and job conditions, allegedly in order to save admin on paperwork and streamline things. If this has succeeded for them still remains to be seen! To me, the level of paper flowing through the place looks much the same. The challenges for us continue with the arrival of our new set of instrumentation. UKIRT will be seeing 4 new instruments in the next 18 months (if they arrive on schedule), so we will have to be on our toes. First to arrive should be UFTI, followed by COHSI, UIST and (finally!) Michelle. Along with this will be ORAC, our new data acquisition and reduction system. You may need a program to know what all these acronyms stand for... One project that has met with utter failure is CICH, the Campaign to Illuminate the Cows in Humuula. The cowboys appear to resent any idea of painting the cows with reflective paint striping, so as to cut down on the number of animals that seem to want to attack vehicles in fog at night. Nobody still can explain to me why the buggers are ALL black. It is not like this is some special breed of cattle. At the fabulous Vacation Resort Hale Pohaku, cutbacks are the bill of fare. It is looking like they did not have to build Building D at all. Occupancy rates are down, more observers doing remote observing and more daycrew teams commuting instead of staying at HP. The result has been a revenue drop, reflected in some decreases in service. As if it were not already a Fun Place. Makes me a tad bitter, since I do spend 1/3 of my life there. I won't delve on this. Locally, there is finally major movement on what to do with Saddle Road. There are proposals on how to re-design both ends of it, and these will be the first segments to be worked on, along with the segment that runs through the military camp at Pohakuloa, where they are proposing to move the roadway north and up the side of the mountain to skirt the reservation. Unfortunately, work on the segment between Hilo and the Mauna Kea turnoff will be the last considered. This may not happen until maybe 8 years from now. One can only hope that they at least continue to upgrade the last section of rough and narrow road just before the turnoff. Nevertheless, this does promise to vastly improve our commuting, both in terms of time and safety, and it is certainly about time someone did something about this situation. I will keep you posted. Aloha!
Back to the ContentsPeopleSince the first issue of the UKIRT Newsletter, there have been a significant number of changes to the UKIRT staff.DeparturesAlvin Balius left the island in the summer for personal reasons. He returns to his previous occupation in Tuscon, Arizona. Al's work on UKIRT's tip-tilt guider was instrumental in its recent success, and he will remain in close contact with the UKIRT Upgrades Team in the foreseeable future.ArrivalsMichael Wagner joined the UKIRT ETS group as an Electronics Technician in the early summer.Chris Davis joined UKIRT in July as a Research/Operations Support Astronomer. Chris was previously employed at the Dublin Institute of Advanced Studies in Dublin, Ireland. Erik Starman arrived in Hilo on Friday, 5 September from New Mexico to begin his employment at the JAC as UKIRT Electronics Engineer. Junichi Meguro is another new face at the JAC who joined us as
a Mechanical Design Engineer to work for UKIRT and JCMT as well as for
GEMINI.
Back to the Contents...And FinallyThose readers who read the last issue of the UKIRT Newsletter, will be aware that the JAC Soccer tournament, contested bi-annually between UKIRT and the JCMT, finished in a 1-1 draw. The second game was played this summer on a typically wet, June afternoon. We apologise to our readers for the absence of a report of this game.Nevertheless, the editor is pleased to announce that UKIRT triumphed over the JCMT by a score of 5-0. A hat-trick from Tom Kerr (nice doughnuts!) and a couple from Antonio Chrysostomou saw off the spirited challenge from the sub-mm people. The first of the 1998 games are due sometime in March or April. We shall
keep you informed!
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