CIRSI: The Cambridge Infra Red Survey Instrument

M.G. Beckett, C.D. Mackay, R.G. McMahon, I.R. Parry, F. Piché and R.S. Ellis
Institute of Astronomy, Cambridge University, Madingley Road, Cambridge CB3 OHA, UK

Abstract:

We are currently building a panoramic wide field near infrared imaging camera based on 4 Rockwell Hawaii HgCdTe 1024² detectors. The survey instrument will operate in the J and H bands and will be as scientifically versatile and as easy to use as a large format CCD camera. It is expected to be ready for astronomical use by late 1997. It will be particularly well-suited for surveys of star-forming regions, low mass stars, distant galaxies, clusters and QSOs. The camera will be commissioned at the prime focus of the 2.5m Isaac Newton telescope, where the image scale is 0.45''/pixel, giving an effective field of view of 14.6 $\times$ 14.6 arc minutes. The field of view of this camera with 0.15'' pixels is 5.1 $\times$ 5.1 arc minutes and is thus $\sim$60 times larger than the current near-infrared imager on Keck (NIRC). When combined with a 4.0m class telescope, the combination is $\sim$10 times as powerful as the Keck 10.0m, when the apertures are taken into account. The options for upgrading the camera into a wide field spectroscopic survey instrument are currently being investigated.

Keywords: Infrared, Array, Astronomy, Surveys

Introduction

The Institute of Astronomy (IoA) Instrumentation Group (URL http://www.ast.cam.ac.uk/~optics/) is currently building a panoramic infrared imaging camera. The instrument, which will consist of a mosaiced array of four Rockwell HgCdTe 1024² (18.5$\rm \mu m$ pixels) (Hodapp et al. 1995, Kozlowski et al. 1995 ) detectors, will be used to carry out deep imaging surveys in the near-infrared spectral region. The survey instrument will be as scientifically versatile as a large format CCD camera. It will be particularly well-suited for surveys of star-forming regions, distant galaxies, clusters and QSOs. It is intended that the camera be ready for astronomical use by late 1997.

The program includes the option of upgrading the camera at a later date to a wide field spectroscopic survey instrument to carry out large redshift surveys that probe to high redshifts (z>2). This spectroscopic upgrade is currently at the design stage with a variety of options being explored, including the use of optical fibres.

Scientific Objectives

Astronomy is primarily an observational science and wide field surveys are a basic foundation of our subject. Wide field sky surveys can be used to select objects in a quantitative manner for statistical studies in themselves, or for subsequent investigations with a large telescope. They are also useful in the follow-up of sources detected at other frequencies, e.g radio, infrared and X-rays. The impact of large format CCDs is clear from the substantial efforts many observatories have invested to secure such devices. Yet it is only in the last year that thinned high QE 2048² CCD devices have become widely available. A similar technological capability is now available at infrared wavelengths via the first 1024² HgCdTe arrays. The mosaic camera discussed here consists of 4 $\times$ 1024² detectors which will fill a significant fraction of the unvignetted focal plane on existing 2-4m class telescopes. Such a device will represent a major leap ($\simeq$50-100) in our survey capability at infrared wavelengths compared with the widely available 256² NICMOS IR arrays. A similar jump in survey capability in the NIR is unlikely to occur again.

The near-infrared wavebands are increasingly important for both galactic and extragalactic surveys. Distant galaxies are optimally studied at these wavelengths because the k-correction is smaller and less uncertain. For QSOs, a significant increase in redshift coverage (i.e. z>6) requires NIR observations since cosmologically distributed HI absorbs away most of the optical waveband. Further physical drivers include minimising dust obscuration at low Galactic latitudes as well as searching for cool sources with temperatures $\simeq$2000 K.

The range of scientific programs that can be attempted with a wide field infrared imaging camera is substantial ranging from searches for the lowest mass stars to the searches for high redshift galaxies and quasars.

Description of the Instrument

The Wide Field Camera

The camera will be remarkably simple and compact (see Figure 1). It is an extension of current IOA capabilities, e.g. the development of a NICMOS HgCdTe 256² device for the IoA/MRAO collaborative programme on the Cambridge Optical Aperture Synthesis Telescope (Baldwin et al., 1996), and does not involve significant technological risk. The prime difficulty in moving in this direction has been the non-availability of large NIR arrays.

The camera has a core detector system which consists of 4 Rockwell 1024$\times$1024 HgCdTe arrays. Unlike some optical CCDs, it is not possible to closely pack the infrared arrays (see Figure 1) and thus a minimum of four exposures is needed to give a 100% fill-factor. An advantage of the large gaps between the detectors is that it allows us to use small, relatively cheap filters for each device. This is particularly important for narrow band programs.

The camera system includes a fully integrated auto-guider system (see Figures 1 and 2). A pick-off mirror inside the dewar and a 1:1 relay system will be used to send the light from an unused part of the field to a CCD on the side of the dewar. The control computer for the CCD will lock on to the brightest star in the field and generate XY correction signals and send them down an RS232 line to the telescope control system. The CCD autoguider system will also facilitate the dithering exposures to reduce flat field noise and the effects of dead pixels. The CCD system we are planning to use is an Ultrapix 1600 system from Astrocam Ltd. which uses a Kodak KAF1600 CCD. This CCD has 1534$\times$1024 9$\mu$m square pixels and it samples images at twice the resolution of the Rockwell array.

Figure 3 shows a schematic of the detector control system with its associated computer system. Each detector has 4 quadrants which are be read out independently. A full exposure of the 4 chips requires the capture of 8Mb of data. In the H band the sky is very bright and this dictates how fast the mosaic has to be read out. Assuming a well capacity of 60,000 electrons, each pixel saturates in $\sim$15 seconds for the prime focii we intend to use (2.5m f/3.3 INT, 4.2m f/2.8 WHT), which is within the specification of the AstroCAM 4100 controller. We propose to commission the new camera initially on the 2.5m Isaac Newton Telescope or the 4.2m William Herschel Telescope but, thereafter it may be transported to telescopes elsewhere.

The filters have to have a very low transmission out-of-band especially near 2.5$\mu$m. Furthermore, because we intend to use the system like a CCD we have no cold stop and the detectors can see a large solid angle through the dewar window. The back of the field corrector therefore appears as a strong black-body source in the K band just in front of the dewar window. We expect that the cold filters in the dewar will reduce this radiation to $\sim$20% of the intensity of the OH emission from the sky when working in the H band mainly due to radiation at $\sim$1.8$\mu$m. The thermal blocking of the filters is better for the J band.

As part of the work done for this proposal a fully-transmitting field corrector was designed in detail for the Cassegrain focus of the WHT and so we are confident that successful designs for other telescopes can be achieved. In contrast to prime focus, a cassegrain option will offer higher spatial resolution with a smaller field of view.

Instrument Control System

The instrument will be controlled using a Tk/TCL GUI interface running on a PC with user defined scripts. It is envisaged that a series of dithered and interlaced exposures should take place with little user intervention. A typical cycle of 5 dithers and 4 interlaced raster positions should take one hour. If guide stars are necessary it is envisaged that no user intervention will be required whilst moving between raster positions. Single person operation is intended. Data will be written to disk as either IRAF or FITS format files and accessible to a unix workstation via nfs.

Data rates and pipeline processing

The data are transferred directly from the ADC to the PC memory through a PCI interface. The high speed of the PCI interface means that no buffer memory is required. Each exposure will produce 4 images at 2MBytes each. A typical 100sec exposure or stare will consist of a real-time coadd consisting of 10 times 10sec exposures and will result in a single 2MByte image. Therefore the maximum data rate is 8MBytes in 100seconds i.e. 288MBytes/hour. Therefore in a typical '10hr night' the data volume is $\sim$3GBytes. This will fit onto a DDS-2 DAT(4GB capacity). The 'raw' uncoadded data which will not usually be archived would amount to 30GBytes/per night. Our aim is that the data reduction pipeline will produce astrometically calibrated images and linearised catalogues of detected objects within an IRAF/FITs based environment.

Commissioning Phase - La Palma

The camera will be used directly at the prime focus of the 2.5 Isaac Newton Telescope (INT) using the existing triplet corrector which gives excellent performance in the J (1.3$\mu$m) and H (1.6$\mu$m) bands. The timely upgrade to the INT prime focus assembly and telescope control systems to facilitate a 2$\times$2 thinned Loral 2048² CCD mosaic means that integration with the IR camera will be relatively easy. The advantage of the INT prime is its very large field of view and well matched pixel scale. Each pixel corresponds to 0.46 arcsec and the total field of view is 0.07 deg². A sequence of 4 interleaved images will give a final image of 31.4$\times$31.4 arcmin. Thus in a total of 16 exposures an area of 1 degree square can be covered. For example, with individual image exposures of 300 seconds, a survey to J$\sim$21 and H$\sim$20 over a area of $\sim$20deg² could be carried out in $\sim$7 nights of clear weather. Such a survey fills the gap between the shallow all sky surveys being carried out i.e. DENIS and 2MASS and the deepest surveys feasible with Keck and HST which cover a few arc min².

There are no existing or any other planned IR instruments for the 2.5m INT and we expect to be able to obtain 2 to 3 weeks per semester. Within the UK context, we effectively gain an IR telescope to complement our access to UKIRT which will be primarily more useful in the K band and at longer wavelengths and in cases where small field and high image quality is the main driver. In the short term one would expect to follow up objects discovered during the INT phase using UKIRT, potentially pushing COHSI (Piché et al, these proceedings) to its limits.

Moving the camera to the 4.2m William Herschel Telescope (WHT) is another option open to us. The advantage of the WHT is the larger aperture and better image scale which will help in deeper surveys. However the WHT prime focus corrector has a lower throughput in H than the INT by $\sim50$% and bright time is more oversubscribed on the WHT compared with the INT since its bright time instrument set includes an echelle spectrograph and a J, H and K adaptive optics system built around a 256² InSb array. We are also considering putting CIRSI at the Cassegrain focus of the WHT or possibly a larger telescope using a special purpose NIR corrector.

Upgrade to Wide Field Spectroscopy

Ultimately we want to use our four 1024² arrays to do spectroscopy in the 0.85-1.8$\mu$m wavelength range to obtain redshifts for galaxies who's redshifts are so high that the familiar diagnostic spectral lines have been moved out of the optical bandpass. Although we are already planning to do this with COHSI (see Piché et al, these proceedings) we want to build a spectrograph with higher throughput by recording the spectra at R$\sim$3000 and doing the OH suppression in software. To do this with an overall observing efficiency greater than that of COHSI requires a very large number of pixels to have a large wavelength range in one shot and a large multiplex gain. With 4 1024² arrays we can get the full spectrum from 0.85-1.8$\mu$m for $\sim$50 objects on an 8m class telescope. We are currently undertaking a design study to find the best spectrograph design to satisfy our scientific needs and we are looking at both slit and fibre based systems.

Current Status of the project

In February 1996, the wide field camera phase of the program was started after a internal conceptual design and budget review. The funded program includes a design study for a wide field spectrographic survey instrument. In May 96, the engineering grade 1024² array arrived almost 3 months ahead of schedule and we are currently working towards a laboratory first light for this array by Oct 31 1996. Astronomical first light of the mosaic of 4 Rockwell 1024² arrays is expected in late 1997 on the 2.5m INT on La Palma. Further details of the project can be found at http://www.ast.cam.ac.uk/~optics/.

Acknowledgements

The CIRSI project forms part of the Deep Sky Initiative at the Institute of Astronomy and is funded by a generous donation from the Raymond and Beverly Sackler Foundation.

References

4

Baldwin, J.E., Beckett, M.G., Boysen, R.C., Burns, D., Buscher, D.F., Cox, G.C., Haniff, C.A., Mackay, C.D., Nightingale, N.S., Rogers, J., Scheuer, P.A.G., Scott, T.R., Tuthill, P.G., Warner, P.J., Wilson, D.M.A., Wilson, R.W., 1996, The first images from an optical aperture synthesis array: mapping of Capella with COAST at two epochs, Astron. Astrophys., 306, L13-L16 (1996).

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Hodapp, K., Hora, J.L., Hall, D.N., Cowie, L.L., Metzger, M., Irwin, E.M.; Keller, T.J., Vural, K., Kozlowski, L.J.; Kleinhans, W.E. ``Astronomical Characterization of 1024x1024 HgCdTe HAWAII Detector Arrays" Proc. SPIE 2475, 8 (1995).

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Kozlowski, L.J., Vural, K., Cabelli, S.A., Chen, C.Y., Cooper, D.E., Bostrup, G.L., Stephenson, D.M., Mclevige, William V., Bailey, R.B., Hodapp, K., Hall, D.N., Kleinhans, W.E. Kozlowski et al. ``2.5$\rm \mu m$ PACe-I HgCdTe 1024$\times$1024 FPA for Infrared Astronomy'' SPIE 2268, 353 (1994)

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Piché, F., Parry, I.R.. Ennico, K., Ellis, R.S. Pritchard, J., Mackay, C.D., McMahon, R.G., ``COHSI: The Cambridge OH Suppression Instrument'', these proceedings

 

 

 

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