Basic design and current development status of IRC: an infrared camera onboard the IRIS (Infrared Imaging Surveyor) is presented. IRC is one of the focal-plane instruments of the 70cm cooled telescope of the IRIS. IRC utilizes recently developed large-format IR arrays for imaging and low-resolution (resolving power ~ 50) spectroscopy at wavelength 1.8~26 micron. IRC consists of 3 camera channels: NIR (1.8~5 micron), MIR-S (5~12 micron), and MIR-L (10~26 micron). These 3 channels simultaneously observe different fields (10'x10' FOV each) of the sky, with diffraction-limited spatial resolution. One critical component limiting the performance of the IRC is the performance of large-format arrays: 512x412 format InSb (NIR) and 256 x 256 format Si:As IBC (MIR-S, MIR-L) arrays operating at very low temperature (<15K). Performance test of the Si:As array manufactured by the Hughes/SBRC is under way, and the preliminary results is presented. Design of the camera optics and the optical components is also presented. IRC is operated under the pointing observation of about 500 sec exposure time, and the development goal is to achieve high point-source detectivity limited nearly by the confusion due to faint astronomical sources.

IRIS[1] is the first Japanese infrared astronomical satellite dedicated for general sky survey, and is now scheduled for launch in early 2003. The observing instruments consist of a 70cm cooled telescope[2] and two focal-plane instruments, an Infrared Camera(IRC) and a Far-Infrared Surveyor(FIS)[3]. A major scientific objective of the IRC is to investigate the process of birth and evolution of galaxies by means of deep photometric and spectroscopic survey in near- and mid-infrared wavelengths over a wide area of the sky(~100deg²). The unique characteristics of IRC onboard IRIS are:

In table 1 and figure 1, the characteristics of IRIS/IRC is compared with that of the infrared camera onboard ISO and with that of the SUBARU/IRCS. ISO is an infrared space observatory recently launched by ESA[4], and IRCS is an infrared camera(and spectrograph) at the focus of SUBARU, the Japanese ground-based large telescope[5]. These characteristics demonstrate that IRIS/IRC is a very powerful tool for general sky survey in the infrared.

Table 1. Comparison of Characteristics of IRC with Infrared Cameras of ISO(space) and of SUBARU(ground-based)

* 0.023”/pixel is also available.

The ultimate goal of general sky survey is reached when the detection limit of the survey is nearly determined by source confusion due to faint astronomical sources(ex. distant galaxies). Within the limited observing time, this requires high throughput(optical efficiency and detector response) , as well as low detector noise. In figure 2, various noise sources limiting the detectivity of IRC are compared. Assumed optical and detector parameters for figure 2 are discussed in the following (tables 2 and 3). In the near-infrared, the detection limit is close to the confusion limit for a pointing observation of 500sec total exposure time. In the mid-infrared, however, the detection limit is equally contributed by the detector read-out noise limit and by the background photon limit. Hence the development of higher throughput and lower detector noise than those assumed here will enable further improvement of sensitivity of the mid-infrared cameras.

Figure 1. Expected 5 sigma detection limit for point sources of IRC. The detection limit of ISOCAM, an infrared camera onboard the ISO(infrared space observatory) which is now in operation by ESA, and of SUBARU/IRCS, an infrared camera and spectrograph at the focus of a ground-based telescope, SUBARU, are shown for comparison. An example of the spectrum of very young, ultraluminous galaxy at z=5(i.e. located at the early universe of only 10% of the cosmic age) is also shown.

　Figure 2. Comparison of various noise sources limiting the point-source detectivity of IRC(figure 1).

IRC consists of 3 camera channels: NIR (1.8~5 micron), MIR-S (5~12 micron), and MIR-L (12~26 micron). Detailed specifications of these channels are shown in table 2. The IRC is operated under the pointing mode[1], in which the satellite attitude is fixed for about 10 minutes in order to observe one target direction on the sky with typically 500sec exposure time. The 3 channels of the IRC simultaneously observe different fields (10'x10' FOV each) of the sky, which are separated by about 20' each other, with diffraction-limited spatial resolution. This pointing observation can be made 1~3 times per revolution of the satellite.

Due to the limitation of volume of the focal plane space, we decided to use the refractive system(i.e. lenses) for the camera optics. Hence to achieve high optical efficiency, it is very important to apply good anti-reflection coatings on the lenses with large refractive index (Si, Ge, KRS-5). Figure 3 shows the configuration of optical parts of NIR and MIR-L. The optical configuration of MIR-S has not been fixed yet.

Figure 3. Optical configuration of IRC: a) NIR (2-5 micron) b)MIR-L(12-26 micron)

At the focus of the cameras, large format infrared array detectors manufactured by Hughes/SBRC are placed(NIR: InSb 512x412, MIR: Si:As IBC 256x256). For NIR, only 412 x 412 portion of the InSb array is used to observe the 10' x 10' field. The advantages of observations from space for infrared astronomy are the negligible atmospheric absorption and the emission (1~5 photons/s for NIR, and 100~1000 photons/s for MIR). The latter enables us to use the detector arrays with relatively small full well capacity(~1e5 electrons), and also to use relatively long charge-integration time, reducing the onboard data generation rate.

At the aperture stop(=image position of the telescope primary mirror, 11~12mm in diameter) of each camera, a 6-position filter wheel is placed to select the observing wavelength band. In addition to a few broad-band photometric filter bands, there is also a blank position to measure the dark level, and positions for the grisms for the slit-less spectroscopy. The grism surface area is 20 x 20mm² and the thickness is 6~7mm. The material of the grisms is KRS-5, and 4~14 grooves are ruled on one surface(closer to the detector) per 1mm in order to disperse the first order light onto 100 of the detector pixels. A stepping motor specially designed for low-temperature use drives the filter wheel. Heat dissipation to the cold stage from the motor during its motion is less than 10mW.

For the in-flight calibration of response of the detectors and for the in-flight flat-fielding, a light source with uniform intensity is equipped. A rough, light-scattering surface of the blank position of the filter wheel faced to the detector array is illuminated by a light source before and after each pointing observation. This method will be useful to establish the flat-fielding with about 1% accuracy. However, for the MIR cameras, the flat-field accuracy better than 0.1% is required. For this purpose, each pointing observation (~10 minutes) is divided into 8~10 independent observations in unit of the exposure time of NIR (=64s), and among them the telescope position in the sky is moved by 10"~30" from the origin of the observing field.

Table 2. Specifications of IRC

* Si is not transparent for > 9micron and thus configuration using 4 Ge lenses is under consideration.

Figure 4. Structure of the Hughes/SBRC sensor chip assembly. The detector is either an InSb array with 512x412 pixels or a Si:As Impurity Band Conduction(IBC) array with 256x256 pixels. Each of the detector pixels is individually connected to a corresponding input of the Si readout IC[6].

The overall design of optical system has almost been fixed. However, in order to perform the detailed design, the accurate value of refractive index for some lens material (especially KRS-5) at cryogenic temperature is needed. We plan to fabricate a test (pre-flight) model of optical system until the end of 1998, and evaluate their performance to find a solution for this problem.

The most critical part in the development of IRC is to establish the performance of the large-format detector array system. In table 3, the required performance of the detector arrays for IRC is summarized. Since the telescope as well as the focal plane plate of IRIS is cooled to 5~6K, the detector arrays have to work at a similar temperature with very low heat dissipation to the liquid He stage of the cryostat. This is relatively severe requirement for the InSb array. The array should further satisfy two other requirements: low dark current and low read noise as discussed in §1. Hughes/SBRC has developed a low-noise 256x256 pixel Si readout, the CRC-744 multiplexer designed for low temperature(~5-15K) operation[7]. For the MIR cameras, detector arrays in which Si:As IBC detectors are bump-bonded to the CRC-744 multiplexer (figure 4) are used. For the NIR camera, the InSb detector array is used. The InSb photodiodes will be similarly bump-bonded to the inputs of a low-noise multiplexer which will be developed based on the same techniques as that of CRC-744.

Table 3. Required Performance of Infrared Detectors for IRC

* The NIR channel should work even after the boil-off of the liquid He(~30K).

We have developed an array control and data acquisition system for the IRC detector arrays. This system consists of low-noise preamplifiers, an 8ch low power A/D board, a clock-driver board, and a data handling and array control system with VMEbus interface, called as 'COGITO-3'[8]. The data acquisition and the clock pattern generation are executed from a SUN workstation. We first evaluated a CRC-744 multiplexer without the detector array at 6-20K. At 6K we found that the multiplexer output is quite sensitive to the temperature of the multiplexer, and that it is necessary to stabilize the temperature within 1mK. To do this the heat dissipation from the multiplexer is kept to be almost the same between the running(read-out) and the park(charge-integration) mode with enabling the clamp-bias. This technique has already been reported in testing the InSb 256 x 256 using the CRC-744 multiplexer[9]. Another problem encountered at 10K and lower temperatures is a level-jump (30~60mV) of the output after passing a bad-pixel, a unit cell which is not working because the output is always ~ 0 V. This level-jump will disappear at 20K. We still do not fully understand the reason, however, since the number of the bad-pixels is only a few, we may simply skip all rows containing the bad pixels. Then no appreciable level jump was observed in the outputs of multiplexer.

The measured performance of the multiplexer is summarized in table 4. The noise electrons have been evaluated from 10 frame measurements of dark current by the correlated double sampling with 8sec integration. Further improvement of noise electrons can be realized by using the Fowler sample[10]. Figure 5 shows the read noise measured by using Fowler-4 sampling scheme, which is good enough to achieve the development goal(figure 1).

Table 4. Measured Performance of Hughes/SBRC 256x256 Readout IC.

Performance evaluation of the science grade Si:As detector has just started at 7.4K. Figure 6 shows the preliminary result of the dark current measurement. The temperature drift may still affect this measurement. We also plan to measure the response for mid-infrared radiation as a function of the bias voltage across the detector. Moreover, especially for the IBC detectors, we will investigate the following problems inherent to the space astronomy:

In relation to (i), it is necessary to find an effective curing method for damages suffered when the satellite goes through the regions of large cosmic ray flux (ex. the South Atlantic Anomaly). As for the InSb array, it is important to find out the minimum operating temperature(10~15K), in order to minimize the heater power for the InSb array and hence to reduce the heat load to the liquid He of the IRIS cryostat.

The author thank all members of the IRC development team: Takashi Onaka, Munetaka Ueno, Katsuyuki Narita, Takehiko Wada, Hidenori Watarai, Yoshihide Takeyama, Hiroshi Murakami, & Toshio Matsumoto, for their effective works in the design and the development of IRC. Thanks also go to staff at the Hughes Aircraft Santa Barbara Research Center, especially Dr. Steve Solomon, for their kind assistance on the operation of the Si:As IBC array.

Figure 5. Plot of readout noise vs. number of the Fowler sample pairs. Filled circles are the mean value of the readout-IC at 6.2K(error bar corresponds 1 sigma of pixel-to-pixel deviation. Thin solid line represents the noise value predicted from the measured noise power spectrum of the readout-IC and the theoretical transfer function ¹⁰ of the Fowler sampling scheme.

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Figure 6. Dark current image of the Hughes/SBRC Si:As sensor chip assembly at 7.4K.(Vrstuc - Vdetl=0.11V).

Figure 7(bottom) Expected (5sigma) minimum detectable flux of velocity-integrated lines emitted from the point sources(500sec exposure time). In the spectroscopic observations, IRC uses grisms which disperse the first order light onto 100 detector pixels.