History of Instrument description (No. 1)

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  • 1 (2022-09-01 (Thu) 03:01:08)

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Contents

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Optical systems†

Optical layout†

Characteristics†

• Semi-Littrow configuration for an echelle grating.
• White pupil configuration between the echelle and a cross disperser.
• Refractive system is adopted, which makes the system smaller than reflective system. Degradation of the transmittance is suppressed to 64-82% by adopting a high-performance anti-reflection coating. Small color aberration is achieved by combining lenses.

Peripheral optics†

• Calibration lamp
  • Krypton and Argon lamps are used for wavelength calibration.
  • Halogen lamp is used for tracing the echelle formats.
• Slit viewer
  • Opical CCD (Bitran corp. BU-51LIR type CCD).
  • Because filters are placed in front of the slit, incident light always passes through the filters before reaching the slit viewer.
• Imaging
  • Rays reflected by the slit surface are divided by a dichroic mirror into optical and near-infrared wavelength ranges.
  • The near-infrared rays reach the NICMOS-3 detector through a collimator mode for the infrared-imaging mode.
  • Objects invisible in the optical wavelength range can be pointed with this infrared imaging mode.

Echelle formats†

• Note that the vertical length covered by spectra increases with a smaller order, i.e., larger wavelength.
• This is the original format, but due to the increased number of bad pixels, some orders are now in different positions or are not suitable for science use.

Diffraction efficiency†

Spot diagrams†

• The panels correspond to the circle positions in the echelle format diagram shown below.

Optical Devices†

Specifications†

Slit†

• The slits are available in three different widths, 257 µm, 515 µm, and 772 µm, respectively, when viewed from the front of the slit. The common length in the spatial direction is 546 µm.
  • Due to technical limitations, for the slit length direction, a 1000 µm hole is first drilled, and a mask with a 546 µm hole is placed behind it.
• Since the slit is tilted 45° in the direction of slit width to the incident light, the slit width appears to be narrower for the incident light by cos 45°.
  • Taking this into account, the effective slit sizes are designed to be 0.5" x 1.5", 1.0" x 1.5", and 1.5" x 1.5", respectively, when installed on the TAO telescope.
• The slit holes are arranged so that one side of the slit length is aligned with the pixel center vertical column.
• The slit is mirror coated. Incident light that does not pass through the slit hole is reflected and guided to the slit viewer or the optical path of the imaging mode.
• Since reflected light is observed in the imaging mode and slit viewer, the image appears as a shadow equivalent to 1000 μm with respect to the slit length direction. Therefore, even the 1.5"x1.5" slit, which is designed to be a square hole, looks like a rectangle.

Filter†

• The filter wheel has six holes, each installed with the IJ, HK, J, H, Ks, or Close filter.
• IJ (I or J) or HK (H or K) filters are used in the spectroscopic mode. Since the slit viewer is an optical camera, these filters are designed to allow some visible light to pass through.
• J, H, and Ks filters are for photometry in the imaging mode. Note that the slit viewer cannot be used when using these filters because they do not allow visible light to pass through.
• Close filter is mainly used to obtain calibration files such as dark images.

Cross disperser†

• The cross disperser changer includes three cross dispersers (CD-J, CD-H, CD-K), plus Mirror and Dark.
• The I and J modes of spectroscopy utilize the common cross disperser CD-J with different incident angles.
• A plane mirror is gold coated for the infrared imaging mode.

Detector†

Characteristics†

†: 2MASS project measurements are quoted.

Linearity†

• TBD.

Gain & Readout Noise†

• The detector gain and readout noise were measured using blackbody furnace imaging data obtained by varying the integration time.
• The measurement method was based on http://spiff.rit.edu/classes/phys445/lectures/gain/gain.html
• Five pairs of data were acquired with the same settings and a 50 x 50 pixel range with few bad pixels was sampled to obtain the mean and variance. In the figure below, the mean of the five pairs is plotted with its standard deviation as the error bars.
• Fitting was performed on data that remained linearity (black circles in the figure below).
• The results showed a gain of 9.4 e-/ADU and a readout noise of 93 e-. The readout noise reported here is for a full CDS exposure like an observer would take.

Quantum Efficiency†

• We have not measured the quantum efficiency, but the values measured by the 2MASS project, which also employs NICMOS-3, are shown in the figure below.
  • Reference: https://old.ipac.caltech.edu/2mass/releases/allsky/doc/sec3_1b1.html

Slit viewer†

• The product BU-51 LIR by Bitran Corp. is used.
• The CCD is Sony ICX825 (1360 x 1024 pixels, 6.45 μm/pixel).
• The quantum efficiency has not been measured, but Bitran Corp. officially publishes the relative quantum efficiency. The figure below shows it scaled to a peak value of 75%, which is the measurement published by several manufacturers.

Mechanical system†

• The whole system, except for the calibration lamps and the slit viewer, is mounted on an optical bench in a cryostat shaped as a box.
• The optical bench consists of a thin aluminum plate and reinforcing plates, which are assembled so as to also make a box structure, named "cold-box".
• The cold-box also works as a radiation field.
• The cold-box is cooled down to ~120 K to reduce the thermal background radiation, while the detector is cooled down to ~80 K to suppress dark current.
• NICE needs no particular alignment process because the optical parts are fixed on the optical bench.

Data acquisition†

• The driving clock patterns for the NICMOS-3 are transferred from a MESSIA-IV CIC board to a "Bias-Box", and finally to the detector.
• The analog signal from the detector is amplified by a "Pre-Amp", and digitized by A/D boards.
• GUI control software, running on a Windows 2000 PC, displays the obtained spectral images, the hardware status, and the observation log in real time.

References†

• Development of a Near-Infrared Echelle Spectrograph "NICE" (Yamamuro et al. 2007, PASJ, 59, 387)
• The University of Tokyo Atacama Observatory 6.5m telescope: update of the Near-Infrared Echelle Spectrograph NICE as a first light instrument (Asano et al. 2020, SPIE, 11447, 6I)