HDS is located at one of the Nasmyth foci (the so-called Optical Nasmyth) of the Subaru Telescope. Image rotators, calibration lamps (CAL), image slicer (IS), and an atmospheric dispersion compensator (ADC) are located ahead of the slit. In this subsection the HDS system is described following the order along the optical path of the beam. The important parameters of the optical elements and the detector are given in Table . The pre-slit unit elements and functions functions are explained in Section .
|Slit||length||1000~30000 μm (2-60arcsec)|
|width||10~2000 μm (0.02-4arcsec)|
|Filter||1||ND1, ND2, SQ, OG530, KV408|
|2||SC46, SC42, GG495, KV389, KV370|
|shutter||shortest exposure is 1 sec.|
|echelle||31.6 grooves/mm, blaze angle 71.25 degrees|
|cross disperser||red||250 grooves/mm, blaze angle 5.00 degrees|
|blue||400 grooves/mm, blaze angle 4.76 degrees|
|camera system||focal length 770mm|
|detector||EEV CCD 4100 pix × 2048 pix × 2, 13.5 μm/pix|
The length and width of the slit can be continuously changed.
Two filter wheels are mounted just behind the slit. Users select one of six filters (including an open filter) in each wheel.
An entrance shutter is located behind the filters for exposure control. Note that HDS has no shutter in front of the detector. Hartman shutters, used for focusing of the spectrograph, are mounted just behind the entrance shutter.
The light monitor is used to measure the flux-weighted time of the exposition, used for precise bary- and heliocentric time and velocity corrections. In normal operation this subsystem is retracted. See Section for details.
The f/12 light cone is collimated into a 27cm beam with a collimator mirror mounted 3.3m away from the slit. Users select one of the two collimator mirrors, optimized for either the blue or red regions. Note that the collimator unit can slightly shift relative to the direction of the slit for focusing corresponding to the optical path of the filters used.
The beam from the collimator is dispersed by the echelle grating, resulting in a very high-resolution spectrum. The inclination angle of the echelle grating can be continuously changed for adjustment of the spectrum format on the detector.
In order to resolve the overlapping of different orders, the beam from the echelle grating is dispersed by a cross disperser perpendicular to the dispersion direction of the echelle. One of two gratings, optimized for either the blue or red regions, is selected. The two gratings provide different order separation, so users should take this into account in the selection of the cross disperser. Instead of the cross dispersers, a plane mirror can be used for observations with a quite long slit using a narrow-band filter for order selection.
The beam dispersed by the two selected gratings is collected on the detector by the camera system. The camera consists of three corrector lenses, a spherical primary mirror, and a field-flattening lens. Focusing of the camera is accomplished by shifting the position of the detector unit.
A mosaic of two CCDs (EEV 42-80 CCD) is used as the detector. The CCD has 4100 × 2048 pixels with pixel size 13.5 μm. Thus, the effective detector size is 55mm by 55mm (4100 by 4096 pixels), though a 1.1mm gap between the two CCDs exists. The detector is mounted at the camera focus and cooled by a mechanical cooler. The control of the CCD and data acquisition are done by a system called Messia V, developed specifically for the Subaru telescope instruments.