We report the evaluation results of a commercially available InGaAs image sensor manufactured by Hamamatsu
Photonics K. K., which has sensitivity between 0.95μm and 1.7μm at a room temperature. The sensor format was
128×128 pixels with 20 μm pitch. It was tested with our original readout electronics and cooled down to 80 K by a
mechanical cooler to minimize the dark current. Although the readout noise and dark current were 200 e- and 20 e-
/sec/pixel, respectively, we found no serious problems for the linearity, wavelength response, and intra-pixel response.
I have developed a special ND filter (Local Attenuation Filter) for observing bright near-infrared stars. This filter is a 60mm diameter with a 4mm thickness, on which an attenuation (0.02% transparency) patch with an 8mm diameter is coated. This filter is expected to be installed near the focal plane of telescope, and the flux through this patch is attenuated. Using this filter, we can observe the attenuated bright star together with not affected field stars as reference for relative photometry. This filter has been installed to the IRSF 1.4m telescope and used for the monitoring of NIR bright stars, for example, η Car.
Focal Plane Arrays (FPA) are key items for modern astronomical observations in the near infrared wavelength, but it is very expensive and not easy to get them. Less expensive NIR FPAs with reasonable performance are very important to spread NIR observation extensively. FPA640×512 manufactured by Chunghwa Leading Photonics Tech is a 640×512 InGaAs detector covering the 0.9-1.7 μm wavelength. Since this array is significantly cheaper than the commonly used NIR FPAs in the astronomical observation, it is possible to be a good choice for particular projects which do not need many pixels, if FPA640×512 has acceptable performance for the purpose. We have evaluated one test grade array of FPA640×512 both in the room and low temperature environment. In order to evaluate the characteristics of this FPA in the low temperature environment, we cooled it down by the mechanical refrigerator and confirmed that it works at 100 K. We have found that the dark current reduces exponentially as the FPA temperature decreases, but it hits the bottom at~1000 e−/sec bellow 200 K with the default setting. We are trying to reduce the dark current by optimizing the bias voltage and the current to the MUX circuit. The latest experiments have shown the possibility that the dark current decreases to~200 e−/sec. This value is still higher than that of NIR FPAs used in the scientific observation, but it may be applicable for the particular purpose, for example, FPAs for slit viewer in spectrometers, wave front sensor, and so on.
We present our new optical and near-infrared (NIR) spectrometer for the IRSF 1.4m telescope. The concept
of it is an effective use of photons, and so we have designed it to obtain a spectrum of the 0.4-2.5μm range
simultaneously and have a small number of optical surfaces in order to reduce reflection loss. Light collected by
the telescope is separated into optical (0.45-0.90μm) and NIR (1.0-2.5μm) wavelengths by a dichroic entrance
window, and two spectrometers are prepared, one for the optical wavelengths and another for the NIR. We use a
sapphire prism in the NIR spectrometer, and a diffraction grating in the optical spectrometer. The optical design
is very simple and the number of optical surfaces is 9 for optical and 10 for NIR (not including the telescope
mirrors). A 1024×250 pixels CCD (optical) and a 1024×1024 HgCdTe detector array (NIR) are used. The
spectral resolution will be email@example.comμm and firstname.lastname@example.orgμm with a 1” slit width. A NIR slit viewer with a 3’.5 ×
3’.5 field of view is also mounted. The development of the spectrometer will be complete by March 2013.
We have developed Wide Field Cryogenic Telescope II (WFCT II) which contains a whole telescope-optics together with a detector in a vacuum case for cooling to suppress thermal emission from a telescope. The telescope inside is a Ritchey-Chretien system with an aperture of 220 mm and a focal length of 1540 mm. Light from celestial objects enters the telescope through a window, hits primary and secondary mirrors, passes through a filter, and reaches a detector. Spiders, baffles, and radiation shields are cooled down to ~80 K or lower by a refrigerator. All the optics reach a low temperature by exchanging heat with the radiation shields. A 1024×1024 InSb infrared array detector covers a field of view of 1 square degree with resolution of 3".5/pixel. The detector is also cooled by the refrigerator and is regulated at 29 ± 0.1 K. WFCT II is mounted on a small equatorial mounting whose size is 1 m in height, 1 m in width, and 1.2 m in length along the N-S direction. The main targets are diffuse emissions radiated from hydrogen atoms, molecules, and carbonaceous materials in star formation regions and the Galactic Center. We have started to obtain scientific data at Sutherland, South African Astronomical Observatory since December 2007.
We have developed a control system for infrared array detectors with 16, 32 or more outputs. Our system consists of five boards (clock pattern generator, clock driver, A/D converter, parallel-in, and isolation), and is operated with a Linux (kernel 2.4 or 2.6) PC. It is capable of supplying 24 DC bias voltages and 16 clock voltages,
adjustable between -7.5V and +7.5V and the shortest clock width of 156 ns. One A/D board converts 16 analog array outputs to digital data simultaneously using 16 A/D converters. The rms of A/D conversions for fixed voltages is 2-3 ADU (or 150-200 μV) at a sampling rate of 250 kSPS. The parallel-in board has 32 optically
isolated input channels, and can receive data from 2 A/D boards simultaneously. The maximum data rate to
main memory of PC is 40 MB/sec, corresponding to 20 frames/sec of a 1024×1024 array. Our system is now
utilized for Aladdin 2 (InSb, 1024×1024, 32 outputs) of Wide Field Cryogenic Telescope 2 and SB-774 (Si:As, 320×240, 16 outputs) of 17μm Fabry-Perot spectrometer. The A/D boards have daisy-chain capability for next generation arrays with more than 32 outputs. In the daisy-chain mode, all A/D converters are triggered simultaneously, but the A/D boards make time-delayed data transfer. The parallel-in board receives data sequentially by every 32 A/D converters. When we apply our system for 2048×2048 detectors with 64 outputs, the frame rate will be 5 frames/sec.
We present our high spectral resolution tandem Fabry-Perot (FP) spectrometer for detecting the pure rotational
transition line of molecular Hydrogen S (1) at 17.035 μm. It is designed to be attached to a new dedicated 1
m telescope planned to be put at a dry and high-altitude site. The spectrometer has two sequentially placed
FP units (order 1000 and 99 with finesse >50) consisting of ZnSe etalons and one narrow band filter. We will
be able to obtain high spectral resolution of R=50,000 at 17.035 μm. The ZnSe etalons of 110mm diameter
with >94% reflectance are to be provided from Barr Associates. The interval and tilt of etalons are sensed and
regulated by piezo actuators and newly-developed capacitance sensors, which resolve 100nm in vacuum and 30K
environment. By changing the interval, we change the wavelength of transmission up to 17.2 μm, corresponding
toν = 3000 km/s. We adopt an on-axis catadioptric system, in which the two FP units are placed. The focal
plane detector is a Raytheon SB-774, 320×240 pixel array of Si:As, yielding 9.1 × 6.8 arcmin2 field of view with
1.7 arcsec pixel scale. To suppress the thermal background radiation and dark current of the Si:As detector, the
system is cooled down to 6K at the detector and 35K for the whole optical system by two refrigerators. The
development of spectrometer will be completed in 2007.
We describe a polarimeter for the near-infrared camera SIRIUS mounted on the IRSF 1.4 m telescope in South Africa. The polarimeter, SIRPOL, consists of an achromatic (1-2.5 μm) wave plate rotator unit and a polarizer located upstream of the camera, both of which are at a room temperature. This minimizes the effect of the mirrors in the camera on instrumental polarization. The combination of the polarimeter with the SIRIUS camera enables a deep (J = 19.2 mag, 5σ in one hour) and wide-field (7.7' × 7.7') imaging polarimetry at JHKs simultaneously. The three color near-infrared polarimetry is useful for understanding the properties of dust grains that cause scattering and absorption in various environments (e.g., star forming regions, late-type stars, and galaxies). Using IRSF and SIRPOL, wide-field near-infrared polarization surveys in various star-forming regions are being conducted, starting from 2006, which aim to study both reflection nebulae associated with young stars and interstellar polarizations of background stars. In this contribution, we describe the hardware and software of SIRPOL and report its first results on the telescope.
We developed a near infrared simultaneous three-band (J, H and Ks) camera, SIRIUS. The design of SIRIUS is optimized to deep, large area surveys in the three IR bands. SIRIUS is equipped with three 1024 x 1024 HgCdTe (HAWAII) arrays, providing simultaneous three-band images. SIRIUS has obtained its first light on the UH 2.2 m telescope in August 2000. SIRIUS is now mounted on the IRSF 1.4 m telescope in Sutherland and is dedicated to deep survey in the southern sky from November 2000. On this telescope, SIRIUS provides 7'.8 x 7'.8 field of view with a pixel scale of 0".45 in all bands. The typical limiting magnitudes are J = 19.2 mag, H = 18.6 mag, Ks = 17.3 mag (15 min. integration, S/N = 10 σ). The effective exposure time (30 sec exposure for each frame) in an hour is about 37 minutes (60%) for each band. Both the instrument and the 1.4 m telescope are in operation.
Some results from the near infrared camera SIRIUS are presented. SIRIUS is designed for deep and wide JHKs-bands simultaneous surveys, being equipped with three science-grade HAWAII (1024×1024) arrays. SIRIUS is attached on a dedicated 1.4m telescope (IRSF) at Sutherland observatory in South Africa. The field of view is 7.8'×7.8', the pixel scale is 0.45", and the limiting magnitude is J=19.2, H=18.6, Ks=17.3 (S/N=10σ and 15minutes integration) with the 1.4m telescope. The survey of southern sky began in November 2000. SIRIUS was also used on the University of Hawaii 2.2m telescope at Mauna Kea for three times in August 2000, October 2000, and September 2001. Surveys of several northern sky areas were done. Unbiased deep survey for 6 degree square area of Large Magellanic Cloud (LMC) is one of the key programs with the 1.4m telescope. Several clusters of intermediate mass YSO candidates have been discovered so far. Monitoring surveys of several selected areas of LMC have also been carried out for detection of variable stars. The other main science programs of SIRIUS are deep imaging surveys of star forming regions in our galaxy, brown dwarf surveys in clusters, search for galaxies behind the Milky way (the Zone of Avoidance), and surveys toward the galactic center.