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bHROS Introduction

bHROS is a bench-mounted (R=150,000) high resolution optical spectrograph with a fiber-fed, prism cross-dispersed echelle. In its original guise, HROS was intended to be a Cassegrain-mounted instrument. Instead, bHROS is located in the pier of Gemini South and fed by optical fibers with input and output micro lenses. The fibers are mounted inside a GMOS-South cassette which positions them in the Cassegrain focal plane of the telescope.

bHROS has been installed at Gemini South and underwent commissioning in July 2005. The instrument became available for scientific use in the 2006A semester and has been indefinitely removed from availability beginning with the 2007A semester.

bHROS has 2 modes of operation; the "object-sky" mode uses two fibers corresponding to 0.7" diameter separated by 20", one each for object and sky. Each fiber feeds its own image slicer such that the slits produced by the slicers place object and sky spectra adjacent to each other in the camera focal plane. This mode of operation is envisaged for work in median-to-good seeing, when simultaneous measurement of the sky background is required. In the "object-only" mode only one fiber is used with a projected diameter of 0.9". It feeds its own dedicated image slicer. This mode is envisaged for median-to-relatively poor seeing when the relatively large aperture will maximize throughput or observations of bright targets when sky background is negligible.

The 'slit' area comprises image slicers that produce slices 0.14" wide in the spectral direction. The slit lengths produced by the slicers in the two modes of operation when projected to the camera focal plane are ~6.5" for object-only mode, and ~2.7" for object-sky mode with a slit separation of ~1". A slicer rotation mechanism is available to ensure a 'vertical' slit at each user-specified central wavelength. This is especially important given the very long 'slit' produced by the slicer.

A cross-dispersion system is required to separate the echelle orders on the detector. For bHROS, the cross-dispersion is provided by a set of fused silica prisms. Prism cross-dispersion is used because it is about a factor of two more efficient than grating cross-dispersion. The system provides a minimum order separation of 3.00mm on the camera focal plane which is needed to permit sufficient inter-order spacing for adjacent sky and object spectra or one broad object spectrum. These constraints are met by using two 60° fused silica prisms operating in tandem.

The echelle unit has a ruled length of 408mm and a blaze angle of ~63° and a ruling of 87g/mm. The echelle has 2 degrees of motion to scan the echellogram on the detector in order to configure the spectrograph for a specific wavelength range.

The detector is a single 2048x4608 E2V CCD with 13.5 µm pixels. The CCD mosaic is located at the prime focus of the camera, immediately behind a field flattener lens. Cooling is provided by a closed cycle cooler and cold finger arrangement.

The spectral characteristics of bHROS are summarized below:

Guiding and tip/tilt image motion compensation is achieved using the GMOS-South On-Instrument Wavefront Sensor (OIWFS). 

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Last update April 12, 2007 by Steven Margheim; Randy Grashuis